*vim9.txt*	For Vim version 9.2.  Last change: 2026 May 04


		  VIM REFERENCE MANUAL	  by Bram Moolenaar


Vim9 script commands and expressions.			*Vim9* *vim9*

Most expression help is in |eval.txt|.  This file is about the new syntax and
features in Vim9 script.



1.  What is Vim9 script?		|Vim9-script|
2.  Differences				|vim9-differences|
3.  New style functions			|fast-functions|
4.  Types				|vim9-types|
5.  Generic functions			|generic-functions|
6.  Namespace, Import and Export	|vim9script|
7.  Classes and interfaces		|vim9-classes|
8.  Rationale				|vim9-rationale|


------------------------------------------------------------------------------

  NOTE: In this vim9.txt help file, the Vim9 script code blocks beginning
	with `vim9script` (and individual lines starting with `vim9cmd`) are
	Vim9 script syntax highlighted.  Also, they are sourceable, meaning
	you can run them to see what they output.  To source them, use
	`:'<,'>source` (see |:source-range|), which is done by visually
	selecting the line(s) with |V| and typing `:so`.  For example, try it
	on the following Vim9 script: >vim9

		vim9script
		echowindow "Welcome to Vim9 script!"
<
	There are also code examples that should not be sourced - they
	explain concepts that don't require a sourceable example.  Such code
	blocks appear in generic code syntax highlighting, like this: >

		def ThisFunction()          # script-local
		def g:ThatFunction()        # global
		export def Function()       # for import and import autoload

==============================================================================

1. What is Vim9 script?					*Vim9-script*

Vim script has been growing over time, while preserving backwards
compatibility.  That means bad choices from the past often can't be changed
and compatibility with Vi restricts possible solutions.  Execution is quite
slow, each line is parsed every time it is executed.

The main goal of Vim9 script is to drastically improve performance.  This is
accomplished by compiling commands into instructions that can be efficiently
executed.  An increase in execution speed of 10 to 100 times can be expected.

A secondary goal is to avoid Vim-specific constructs and get closer to
commonly used programming languages, such as JavaScript, TypeScript and Java.

The performance improvements can only be achieved by not being 100% backwards
compatible.  For example, making function arguments available in the "a:"
dictionary adds quite a lot of overhead.  In a Vim9 function this dictionary
is not available.  Other differences are more subtle, such as how errors are
handled.

Vim9 script syntax, semantics, and behavior apply in:
- a function defined with the `:def` command
- a script file where the first command is `vim9script`
- an autocommand defined in the context of the above
- a command prefixed with the `vim9cmd` command modifier

When using `:function` in a Vim9 script file the legacy syntax is used, with
the highest |scriptversion|.  However, this can be confusing and is therefore
discouraged.

Vim9 script and legacy Vim script can be mixed.  There is no requirement to
rewrite old scripts, they keep working as before.  You may want to use a few
`:def` functions for code that needs to be fast.

:vim9[cmd] {cmd}				*:vim9* *:vim9cmd*
		Evaluate and execute {cmd} using Vim9 script syntax,
		semantics, and behavior.  Useful when typing a command,
		in a `:function`, or a legacy Vim script.

		The following short example shows how a legacy Vim script
		command and a :vim9cmd (so Vim9 script context) may appear
		similar, though may differ not just syntactically, but also
		semantically and behaviorally. >vim

		  call popup_notification('entrée'[5:]
		    \ ->str2list()->string(), #{time: 7000})
		  vim9cmd popup_notification('entrée'[5 :]
		      ->str2list()->string(), {time: 7000})
<
		 Notes: 1) The reason for the different output is Vim9 script
			   uses character indexing whereas legacy Vim script
			   uses byte indexing - see |vim9-string-index|.
			2) Syntax is different too.  In Vim9 script:
			  - The space in "[5 :]" is mandatory (see
			    |vim9-white-space|).
			  - Line continuation with "\" is not required.
			  - The "#" (to avoid putting quotes around dictionary
			    keys) is neither required nor allowed - see |#{}|.

						*E1164*
		`:vim9cmd` cannot stand alone; it must be followed by a command.

:leg[acy] {cmd}					*:leg* *:legacy*
		Evaluate and execute {cmd} using legacy Vim script syntax,
		semantics, and behavior.  It is only applicable in a Vim9
		script or a `:def` function.  Using an equivalent script to
		the one, above (see its notes for why the output differs): >vim9

		  vim9script
		  # Legacy context - so, this creates a popup with [769, 101]
		  legacy call popup_notification('entrée'[5:]
		    \ ->str2list()->string(), #{time: 7000})
		  # Vim9 script context - so, this creates a pop up with [101]
		  popup_notification('entrée'[5 :]
		      ->str2list()->string(), {time: 7000})
<
		Vim9 script script-local variables may be used by prefixing
		"s:", like in legacy Vim script.  This example shows the
		difference in syntax: "k" for the script-local variable in
		Vim9 script, "s:k" in the legacy Vim script context. >vim9

		  vim9script
		  var k: string = "Okay"
		  echo k
		  legacy echo s:k
<						*E1189*
		Using `:legacy` is not allowed in compiled Vim9 script
		control flow contexts. For example: >vim9

		  vim9script
		  def F_1189()
		    if v:version == 900
		    # E1189: Cannot use :legacy with this command: endif
		    legacy endif
		  enddef
		  F_1189()
<						*E1234*
		`:legacy` cannot stand alone; it must be followed by a command.


==============================================================================

2. Differences from legacy Vim script			*vim9-differences*

Overview ~
							*E1146*
Brief summary of the differences you will most often encounter when using Vim9
script and `:def` functions; details are below:
- Comments start with #, not ": >
	echo "hello"   # comment
- Using a backslash for line continuation is hardly ever needed: >
	echo "hello "
	     .. yourName
	     .. ", how are you?"
- White space is required in many places to improve readability,
  see |vim9-white-space|.
- Assign values without `:let` *E1126* , declare variables with `:var`: >
	var count = 0
	count += 3
- Constants can be declared with `:final` and `:const`: >
	final matches = []		  # add to the list later
	const names = ['Betty', 'Peter']  # cannot be changed
- `:final` cannot be used as an abbreviation of `:finally`.
- Variables and functions are script-local by default.
- Functions are declared with argument types and return type: >
	def CallMe(count: number, message: string): bool
- Call functions without `:call`: >
	writefile(['done'], 'file.txt')
- You cannot use old Ex commands:
	`:Print`
	`:append`
	`:change`
	`:d`  directly followed by 'd' or 'p'.
	`:insert`
	`:k`
	`:mode`
	`:open`
	`:s`  with only flags
	`:t`
	`:xit`
- Some commands, especially those used for flow control, cannot be shortened.
  E.g., `:throw` cannot be written as `:th`.  *vim9-no-shorten*
- You cannot use curly-braces names.
- A range before a command must be prefixed with a colon: >
	:%s/this/that
- Executing a register with "@r" does not work, you can prepend a colon or use
  `:exe`: >
	:exe @a
- Unless mentioned specifically, the highest |scriptversion| is used.
- When defining an expression mapping, the expression will be evaluated in the
  context of the script where it was defined.
- When indexing a string the index is counted in characters, not bytes:
  |vim9-string-index|
- Some possibly unexpected differences: |vim9-gotchas|.


Comments starting with # ~

In legacy Vim script comments start with double quote.  In Vim9 script
comments start with #. >
	# declarations
	var count = 0  # number of occurrences

The reason is that a double quote can also be the start of a string. In many
places, especially halfway through an expression with a line break, it's hard
to tell what the meaning is, since both a string and a comment can be followed
by arbitrary text.  To avoid confusion only # comments are recognized.  This
is the same as in shell scripts and Python programs.

In Vi # is a command to list text with numbers.  In Vim9 script you can use
`:number` for that. >
	:101 number

To improve readability there must be a space between a command and the #
that starts a comment: >
	var name = value # comment
	var name = value# error!
<							*E1170*
Do not start a comment with #{, it looks like the legacy dictionary literal
and produces an error where this might be confusing.  #{{ or #{{{ are OK,
these can be used to start a fold.

When starting to read a script file Vim doesn't know it is |Vim9| script until
the `vim9script` command is found.  Until that point you would need to use
legacy comments: >
	" legacy comment
	vim9script
	# Vim9 comment

That looks ugly, better put `vim9script` in the very first line: >
	vim9script
	# Vim9 comment

In legacy Vim script # is also used for the alternate file name.  In Vim9
script you need to use %% instead.  Instead of ## use %%% (stands for all
arguments).


Vim9 functions ~
							*E1099*
A function defined with `:def` is compiled.  Execution is many times faster,
often 10 to 100 times.

Many errors are already found when compiling, before the function is executed.
The syntax is strict, to enforce code that is easy to read and understand.

Compilation is done when any of these is encountered:
- the first time the function is called
- when the `:defcompile` command is encountered in the script after the
  function was defined
- `:disassemble` is used for the function.
- a function that is compiled calls the function or uses it as a function
  reference (so that the argument and return types can be checked)
						*E1091* *E1191*
If compilation fails it is not tried again on the next call, instead this
error is given: "E1091: Function is not compiled: {name}".
Compilation will fail when encountering a user command that has not been
created yet.  In this case you can call `execute()` to invoke it at runtime. >
	def MyFunc()
	  execute('DefinedLater')
	enddef

`:def` has no options like `:function` does: "range", "abort", "dict" or
"closure".  A `:def` function always aborts on an error (unless `:silent!` was
used for the command or the error was caught a `:try` block), does not get a
range passed, cannot be a "dict" function, and can always be a closure.
						*vim9-no-dict-function* *E1182*
You can use a Vim9 Class (|Vim9-class|) instead of a "dict function".
You can also pass the dictionary explicitly: >
	def DictFunc(self: dict<any>, arg: string)
	   echo self[arg]
	enddef
	var ad = {item: 'value', func: DictFunc}
	ad.func(ad, 'item')

You can call a legacy dict function though: >
	func Legacy() dict
	  echo self.value
	endfunc
	def CallLegacy()
	  var d = {func: Legacy, value: 'text'}
	  d.func()
	enddef

The argument types and return type need to be specified.  The "any" type can
be used, type checking will then be done at runtime, like with legacy
functions.
							*E1106*
Arguments are accessed by name, without "a:", just like any other language.
There is no "a:" dictionary or "a:000" list.
			*vim9-variable-arguments* *E1055* *E1160* *E1180*
Variable arguments are defined as the last argument, with a name and have a
list type, similar to TypeScript.  For example, a list of numbers: >
	def MyFunc(...itemlist: list<number>)
	   for item in itemlist
	     ...

When a function argument is optional (it has a default value) passing `v:none`
as the argument results in using the default value.  This is useful when you
want to specify a value for an argument that comes after an argument that
should use its default value.  Example: >
	def MyFunc(one = 'one', last = 'last')
	  ...
	enddef
	MyFunc(v:none, 'LAST')  # first argument uses default value 'one'
<
					*vim9-ignored-argument* *E1181*
The argument "_" (an underscore) can be used to ignore the argument.  This is
most useful in callbacks where you don't need it, but do need to give an
argument to match the call.  E.g. when using map() two arguments are passed,
the key and the value, to ignore the key: >
	map(numberList, (_, v) => v * 2)
There is no error for using the "_" argument multiple times.  No type needs to
be given.


Functions and variables are script-local by default ~
							*vim9-scopes*
When using `:function` or `:def` to specify a new function at the script level
in a Vim9 script, the function is local to the script.  Like prefixing "s:" in
legacy script.  To define a global function or variable the "g:" prefix must
be used.  For functions in a script that is to be imported and in an autoload
script "export" needs to be used for those to be used elsewhere. >
	def ThisFunction()          # script-local
	def g:ThatFunction()        # global
	export def Function()       # for import and import autoload
<						*E1075*
When using `:function` or `:def` to specify a nested function inside a `:def`
function and no namespace was given, this nested function is local to the code
block it is defined in.  It cannot be used in `function()` with a string
argument, pass the function reference itself: >
	def Outer()
	  def Inner()
	    echo 'inner'
	  enddef
	  var Fok = function(Inner)     # OK
	  var Fbad = function('Inner')  # does not work

Detail: this is because "Inner" will actually become a function reference to a
function with a generated name.

It is not possible to define a script-local function in a function.  You can
define a local function and assign it to a script-local Funcref (it must have
been declared at the script level).  It is possible to define a global
function by using the "g:" prefix.

When referring to a function and no "s:" or "g:" prefix is used, Vim will
search for the function:
- in the function scope, in block scopes
- in the script scope

Imported functions are found with the prefix from the `:import` command.

Since a script-local function reference can be used without "s:" the name must
start with an upper case letter even when using the "s:" prefix.  In legacy
script "s:funcref" could be used, because it could not be referred to with
"funcref".  In Vim9 script it can, therefore "s:Funcref" must be used to avoid
that the name interferes with builtin functions.
						*vim9-s-namespace* *E1268*
The use of the "s:" prefix is not supported at the Vim9 script level.  All
functions and variables without a prefix are script-local.

In :def functions the use of "s:" depends on the script: Script-local
variables and functions in a legacy script do use "s:", while in a Vim9 script
they do not use "s:".  This matches what you see in the rest of the file.

In legacy functions the use of "s:" for script items is required, as before.
No matter if the script is Vim9 or legacy.

In all cases the function must be defined before used.  That is when it is
called, when `:defcompile` causes it to be compiled, or when code that calls
it is being compiled (to figure out the return type).

The result is that functions and variables without a namespace can usually be
found in the script, either defined there or imported.  Global functions and
variables could be defined anywhere (good luck finding out where!  You can
often see where it was last set using |:verbose|).
							*E1102*
Global functions can still be defined and deleted at nearly any time.  In
Vim9 script script-local functions are defined once when the script is sourced
and cannot be deleted or replaced by itself (it can be by reloading the
script).

When compiling a function and a function call is encountered for a function
that is not (yet) defined, the |FuncUndefined| autocommand is not triggered.
You can use an autoload function if needed, or call a legacy function and have
|FuncUndefined| triggered there.


Reloading a Vim9 script clears functions and variables by default ~
						*vim9-reload* *E1149* *E1150*
When loading a legacy Vim script a second time nothing is removed, the
commands will replace existing variables and functions, create new ones, and
leave removed things hanging around.

When loading a Vim9 script a second time all existing script-local functions
and variables are deleted, thus you start with a clean slate.  This is useful
if you are developing a plugin and want to try a new version.  If you renamed
something you don't have to worry about the old name still hanging around.

If you do want to keep items, use: >
	vim9script noclear

You want to use this in scripts that use a `finish` command to bail out at
some point when loaded again.  E.g. when a buffer local option is set to a
function, the function does not need to be defined more than once: >
	vim9script noclear
	setlocal completefunc=SomeFunc
	if exists('*SomeFunc')
	  finish
	endif
	def SomeFunc()
	....


Variable declarations with :var, :final and :const ~
				*vim9-declaration* *:var* *E1079*
				*E1017* *E1020* *E1054* *E1087* *E1124*
Local variables need to be declared with `:var`.  Local constants need to be
declared with `:final` or `:const`.  We refer to both as "variables" in this
section.

Variables can be local to a script, function or code block: >
	vim9script
	var script_var = 123
	def SomeFunc()
	  var func_var = script_var
	  if cond
	    var block_var = func_var
	  ...

The variables are only visible in the block where they are defined and nested
blocks.  Once the block ends the variable is no longer accessible: >
	if cond
	   var inner = 5
	else
	   var inner = 0
	endif
	echo inner  # Error!

The declaration must be done earlier: >
	var inner: number
	if cond
	   inner = 5
	else
	   inner = 0
	endif
	echo inner

Although this is shorter and faster for simple values: >
	var inner = 0
	if cond
	   inner = 5
	endif
	echo inner
<							*E1025* *E1128*
To intentionally hide a variable from code that follows, a block can be
used: >
	{
	   var temp = 'temp'
	   ...
	}
	echo temp  # Error!

This is especially useful in a user command: >
	command -range Rename {
		 var save = @a
		 @a = 'some expression'
		 echo 'do something with ' .. @a
		 @a = save
	    }

And with autocommands: >
   au BufWritePre *.go {
		 var save = winsaveview()
		 silent! exe ':%! some formatting command'
		 winrestview(save)
	   }

Although using a :def function probably works better.

				*E1022* *E1103* *E1130* *E1131* *E1133*
				*E1134*
Declaring a variable with a type but without an initializer will initialize to
false (for bool), empty (for string, list, dict, etc.) or zero (for number,
any, etc.).  This matters especially when using the "any" type, the value will
default to the number zero.  For example, when declaring a list, items can be
added: >
	var myList: list<number>
	myList->add(7)

Initializing a variable to a null value, e.g. `null_list`, differs from not
initializing the variable.  This throws an error: >
	var myList = null_list
	myList->add(7)  # E1130: Cannot add to null list

<						*E1016* *E1052* *E1066*
In Vim9 script `:let` cannot be used.  An existing variable is assigned to
without any command.  The same for global, window, tab, buffer and Vim
variables, because they are not really declared.  Those can also be deleted
with `:unlet`.
							*E1065*
You cannot use `:va` to declare a variable, it must be written with the full
name `:var`.  Just to make sure it is easy to read.
							*E1178*
`:lockvar` does not work on local variables.  Use `:const` and `:final`
instead.

The `exists()` and `exists_compiled()` functions do not work on local variables
or arguments.
				*E1006* *E1041* *E1167* *E1168* *E1213*
Variables, functions and function arguments cannot shadow previously defined
or imported variables and functions in the same script file.
Variables may shadow Ex commands, rename the variable if needed.

Global variables must be prefixed with "g:", also at the script level. >
	vim9script
	var script_local = 'text'
	g:global = 'value'
	var Funcref = g:ThatFunction

Global functions must be prefixed with "g:": >
	vim9script
	def g:GlobalFunc(): string
	  return 'text'
	enddef
	echo g:GlobalFunc()
The "g:" prefix is not needed for auto-load functions.

					*vim9-function-defined-later*
Although global functions can be called without the "g:" prefix, they must
exist when compiled.  By adding the "g:" prefix the function can be defined
later.  Example: >
	def CallPluginFunc()
	  if exists('g:loaded_plugin')
	    g:PluginFunc()
	  endif
	enddef

If you do it like this, you get an error at compile time that "PluginFunc"
does not exist, even when "g:loaded_plugin" does not exist: >
	def CallPluginFunc()
	  if exists('g:loaded_plugin')
	    PluginFunc()   # Error - function not found
	  endif
	enddef

You can use exists_compiled() to avoid the error, but then the function would
not be called, even when "g:loaded_plugin" is defined later: >
	def CallPluginFunc()
	  if exists_compiled('g:loaded_plugin')
	    PluginFunc()   # Function may never be called
	  endif
	enddef

Since `&opt = value` is now assigning a value to option "opt", ":&" cannot be
used to repeat a `:substitute` command.
							*vim9-unpack-ignore*
For an unpack assignment the underscore can be used to ignore a list item,
similar to how a function argument can be ignored: >
	[a, _, c] = theList
To ignore any remaining items: >
	[a, b; _] = longList
<							*E1163* *E1080*
Declaring more than one variable at a time, using the unpack notation, is
possible.  Each variable can have a type or infer it from the value: >
	var [v1: number, v2] = GetValues()
Use this only when there is a list with values, declaring one variable per
line is much easier to read and change later.


Constants ~
						*vim9-const* *vim9-final*
How constants work varies between languages.  Some consider a variable that
can't be assigned another value a constant.  JavaScript is an example.  Others
also make the value immutable, thus when a constant uses a list, the list
cannot be changed.  In Vim9 we can use both.
							*E1021* *E1307*
`:const` is used for making both the variable and the value a constant.  Use
this for composite structures that you want to make sure will not be modified.
Example: >
	const myList = [1, 2]
	myList = [3, 4]		# Error!
	myList[0] = 9		# Error!
	myList->add(3)		# Error!
<							*:final* *E1125*
`:final` is used for making only the variable a constant, the value can be
changed.  This is well known from Java.  Example: >
	final myList = [1, 2]
	myList = [3, 4]		# Error!
	myList[0] = 9		# OK
	myList->add(3)		# OK

It is common to write constants as ALL_CAPS, but you don't have to.

The constant only applies to the value itself, not what it refers to. >
	final females = ["Mary"]
	const NAMES = [["John", "Peter"], females]
	NAMES[0] = ["Jack"]     # Error!
	NAMES[0][0] = "Jack"    # Error!
	NAMES[1] = ["Emma"]     # Error!
	NAMES[1][0] = "Emma"    # OK, now females[0] == "Emma"


Omitting :call and :eval ~
							*E1190*
Functions can be called without `:call`: >
	writefile(lines, 'file')
Using `:call` is still possible, but this is discouraged.

A method call without `eval` is possible, so long as the start is an
identifier or can't be an Ex command.  For a function either "(" or "->" must
be following, without a line break.  Examples: >
	myList->add(123)
	g:myList->add(123)
	[1, 2, 3]->Process()
	{a: 1, b: 2}->Process()
	"foobar"->Process()
	("foobar")->Process()
	'foobar'->Process()
	('foobar')->Process()

In the rare case there is ambiguity between a function name and an Ex command,
prepend ":" to make clear you want to use the Ex command.  For example, there
is both the `:substitute` command and the `substitute()` function.  When the
line starts with `substitute(` this will use the function. Prepend a colon to
use the command instead: >
	:substitute(pattern (replacement (

If the expression starts with "!" this is interpreted as a shell command, not
negation of a condition.  Thus this is a shell command: >
	!shellCommand->something
Put the expression in parentheses to use the "!" for negation: >
	(!expression)->Method()

Note that while variables need to be defined before they can be used,
functions can be called before being defined.  This is required to allow
for cyclic dependencies between functions.  It is slightly less efficient,
since the function has to be looked up by name.  And a typo in the function
name will only be found when the function is called.


Omitting function() ~

A user defined function can be used as a function reference in an expression
without `function()`. The argument types and return type will then be checked.
The function must already have been defined. >

	var Funcref = MyFunction

When using `function()` the resulting type is "func", a function with any
number of arguments and any return type (including void).  The function can be
defined later if the argument is in quotes.


Lambda using => instead of -> ~
							*vim9-lambda*
In legacy script there can be confusion between using "->" for a method call
and for a lambda.  Also, when a "{" is found the parser needs to figure out if
it is the start of a lambda or a dictionary, which is now more complicated
because of the use of argument types.

To avoid these problems Vim9 script uses a different syntax for a lambda,
which is similar to JavaScript: >
	var Lambda = (arg) => expression
	var Lambda = (arg): type => expression
<							*E1157*
No line break is allowed in the arguments of a lambda up to and including the
"=>" (so that Vim can tell the difference between an expression in parentheses
and lambda arguments).  This is OK: >
	filter(list, (k, v) =>
			v > 0)
This does not work: >
	filter(list, (k, v)
			=> v > 0)
This also does not work: >
	filter(list, (k,
			v) => v > 0)
But you can use a backslash to concatenate the lines before parsing: >
	filter(list, (k,
		\	v)
		\	=> v > 0)
<					*vim9-lambda-arguments* *E1172*
In legacy script a lambda could be called with any number of extra arguments,
there was no way to warn for not using them.  In Vim9 script the number of
arguments must match.  If you do want to accept any arguments, or any further
arguments, use "..._", which makes the function accept
|vim9-variable-arguments|.  Example: >
	var Callback = (..._) => 'anything'
	echo Callback(1, 2, 3)  # displays "anything"

<						*inline-function* *E1171*
Additionally, a lambda can contain statements in {}: >
	var Lambda = (arg) => {
		g:was_called = 'yes'
		return expression
	    }
This can be useful for a timer, for example: >
	var count = 0
	var timer = timer_start(500, (_) => {
		 count += 1
		 echom 'Handler called ' .. count
	     }, {repeat: 3})

The ending "}" must be at the start of a line.  It can be followed by other
characters, e.g.: >
	var d = mapnew(dict, (k, v): string => {
	     return 'value'
	   })
No command can follow the "{", only a comment can be used there.

						*command-block* *E1026*
The block can also be used for defining a user command.  Inside the block Vim9
syntax will be used.

This is an example of using here-docs: >
    com SomeCommand {
        g:someVar =<< trim eval END
          ccc
          ddd
        END
      }

If the statements include a dictionary, its closing bracket must not be
written at the start of a line.  Otherwise, it would be parsed as the end of
the block.  This does not work: >
	command NewCommand {
	     g:mydict = {
	       'key': 'value',
	       }  # ERROR: will be recognized as the end of the block
	   }
Put the '}' after the last item to avoid this: >
	command NewCommand {
	     g:mydict = {
	       'key': 'value' }
	   }

Rationale: The "}" cannot be after a command because it would require parsing
the commands to find it.  For consistency with that no command can follow the
"{".  Unfortunately this means using "() => {  command  }" does not work, line
breaks are always required.

							*vim9-curly*
To avoid the "{" of a dictionary literal to be recognized as a statement block
wrap it in parentheses: >
	var Lambda = (arg) => ({key: 42})

Also when confused with the start of a command block: >
	({
	    key: value
	 })->method()


Automatic line continuation ~
					*vim9-line-continuation* *E1097*
In many cases it is obvious that an expression continues on the next line.  In
those cases there is no need to prefix the line with a backslash (see
|line-continuation|).  For example, when a list spans multiple lines: >
	var mylist = [
		'one',
		'two',
		]
And when a dict spans multiple lines: >
	var mydict = {
		one: 1,
		two: 2,
		}
With a function call: >
	var result = Func(
			arg1,
			arg2
			)

For binary operators in expressions not in [], {} or () a line break is
possible just before or after the operator.  For example: >
	var text = lead
		   .. middle
		   .. end
	var total = start +
		    end -
		    correction
	var result = positive
			? PosFunc(arg)
			: NegFunc(arg)

For a method call using "->" and a member using a dot, a line break is allowed
before it: >
	var result = GetBuilder()
			->BuilderSetWidth(333)
			->BuilderSetHeight(777)
			->BuilderBuild()
	var result = MyDict
			.member

For commands that have an argument that is a list of commands, the | character
at the start of the line indicates line continuation: >
	autocmd BufNewFile *.match if condition
		|   echo 'match'
		| endif

Note that this means that in heredoc the first line cannot start with a bar: >
	var lines =<< trim END
	   | this doesn't work
	END
Either use an empty line at the start or do not use heredoc.  Or temporarily
add the "C" flag to 'cpoptions': >
	set cpo+=C
	var lines =<< trim END
	   | this works
	END
	set cpo-=C
If the heredoc is inside a function 'cpoptions' must be set before :def and
restored after the :enddef.

In places where line continuation with a backslash is still needed, such as
splitting up a long Ex command, comments can start with '#\ ': >
	syn region Text
	      \ start='foo'
	      #\ comment
	      \ end='bar'
Like with legacy script '"\ ' is used.  This is also needed when line
continuation is used without a backslash and a line starts with a bar: >
	au CursorHold * echom 'BEFORE bar'
	      #\ some comment
	      | echom 'AFTER bar'
<
							*E1050*
To make it possible for the operator at the start of the line to be
recognized, it is required to put a colon before a range.  This example will
add "start" and "print": >
	var result = start
	+ print
Like this: >
	var result = start + print

This will assign "start" and print a line: >
	var result = start
	:+ print

After the range an Ex command must follow.  Without the colon you can call a
function without `:call`, but after a range you do need it: >
	MyFunc()
	:% call MyFunc()

Note that the colon is not required for the |+cmd| argument: >
	edit +6 fname

It is also possible to split a function header over multiple lines, in between
arguments: >
	def MyFunc(
		text: string,
		separator = '-'
		): string

Since a continuation line cannot be easily recognized the parsing of commands
has been made stricter.  E.g., because of the error in the first line, the
second line is seen as a separate command: >
	popup_create(some invalid expression, {
	   exit_cb: Func})
Now "exit_cb: Func})" is actually a valid command: save any changes to the
file "_cb: Func})" and exit.  To avoid this kind of mistake in Vim9 script
there must be white space between most command names and the argument.
*E1144*

However, the argument of a command that is a command won't be recognized.  For
example, after "windo echo expr" a line break inside "expr" will not be seen.


Notes:
- "enddef" cannot be used at the start of a continuation line, it ends the
  current function.
- No line break is allowed in the LHS of an assignment.  Specifically when
  unpacking a list |:let-unpack|. This is OK: >
	[var1, var2] =
		Func()
<  This does not work: >
	[var1,
	    var2] =
		Func()
- No line break is allowed in between arguments of an `:echo`, `:execute` and
  similar commands.  This is OK: >
	echo [1,
		2] [3,
			4]
<  This does not work: >
	echo [1, 2]
		[3, 4]
- In some cases it is difficult for Vim to parse a command, especially when
  commands are used as an argument to another command, such as `:windo`.  In
  those cases the line continuation with a backslash has to be used.


White space ~
	*vim9-white-space* *E1004* *E1068* *E1069* *E1074* *E1127* *E1202*
Vim9 script enforces proper use of white space.  This is no longer allowed: >
	var name=234	# Error!
	var name= 234	# Error!
	var name =234	# Error!
There must be white space before and after the "=": >
	var name = 234	# OK
White space must also be put before the # that starts a comment after a
command: >
	var name = 234# Error!
	var name = 234 # OK

White space is required around most operators.

White space is required in a sublist (list slice) around the ":", except at
the start and end: >
	otherlist = mylist[v : count]	# v:count has a different meaning
	otherlist = mylist[:]		# make a copy of the List
	otherlist = mylist[v :]
	otherlist = mylist[: v]

White space is not allowed:
- Between a function name and the "(": >
	Func (arg)	   # Error!
	Func
	     \ (arg)	   # Error!
	Func
	      (arg)	   # Error!
	Func(arg)	   # OK
	Func(
	      arg)	   # OK
	Func(
	      arg	   # OK
	      )
<							*E1205*
White space is not allowed in a `:set` command between the option name and a
following "&", "!", "<", "=", "+=", "-=" or "^=".


No curly braces expansion ~

|curly-braces-names| cannot be used.


Command modifiers are not ignored ~
								*E1176*
Using a command modifier for a command that does not use it gives an error.
								*E1082*
Also, using a command modifier without a following command is now an error.


Dictionary literals ~
						*vim9-literal-dict* *E1014*
Traditionally Vim has supported dictionary literals with a {} syntax: >
	let dict = {'key': value}

Later it became clear that using a simple text key is very common, thus
literal dictionaries were introduced in a backwards compatible way: >
	let dict = #{key: value}

However, this #{} syntax is unlike any existing language.  As it turns out
that using a literal key is much more common than using an expression, and
considering that JavaScript uses this syntax, using the {} form for dictionary
literals is considered a much more useful syntax.  In Vim9 script the {} form
uses literal keys: >
	var dict = {key: value}

This works for alphanumeric characters, underscore and dash.  If you want to
use another character, use a single or double quoted string: >
	var dict = {'key with space': value}
	var dict = {"key\twith\ttabs": value}
	var dict = {'': value}			# empty key
<							*E1139*
In case the key needs to be an expression, square brackets can be used, just
like in JavaScript: >
	var dict = {["key" .. nr]: value}

The key type can be string, number, bool or float.  Other types result in an
error.  Without using [] the value is used as a string, keeping leading zeros.
An expression given with [] is evaluated and then converted to a string.
Leading zeros will then be dropped: >
	var dict = {000123: 'without', [000456]: 'with'}
	echo dict
	{'456': 'with', '000123': 'without'}
A float only works inside [] because the dot is not accepted otherwise: >
	var dict = {[00.013]: 'float'}
	echo dict
	{'0.013': 'float'}


No :xit, :t, :k, :append, :change or :insert ~
							*E1100*
These commands are too easily confused with local variable names.
Instead of `:x` or `:xit` you can use `:exit`.
Instead of `:t` you can use `:copy`.
Instead of `:k` you can use `:mark`.


Comparators ~

The 'ignorecase' option is not used for comparators that use strings.
Thus "=~" works like "=~#".

"is" and "isnot" (|expr-is| and |expr-isnot|) when used on strings now return
false.  In legacy script they just compare the strings, in |Vim9| script they
check identity, and strings are copied when used, thus two strings are never
the same (this might change someday if strings are not copied but reference
counted).


Abort after error ~

In legacy script, when an error is encountered, Vim continues to execute
following lines.  This can lead to a long sequence of errors and need to type
CTRL-C to stop it.  In Vim9 script execution of commands stops at the first
error.  Example: >
	vim9script
	var x = does-not-exist
	echo 'not executed'


For loop ~
							*E1254*
The loop variable must not be declared yet: >
	var i = 1
	for i in [1, 2, 3]   # Error!

It is possible to use a global variable though: >
	g:i = 1
	for g:i in [1, 2, 3]
	  echo g:i
	endfor

Legacy Vim script has some tricks to make a for loop over a list handle
deleting items at the current or previous item.  In Vim9 script it just uses
the index, if items are deleted then items in the list will be skipped.
Example legacy script: >
	let l = [1, 2, 3, 4]
	for i in l
	   echo i
	   call remove(l, index(l, i))
	endfor
Would echo:
	1
	2
	3
	4
In compiled Vim9 script you get:
	1
	3
Generally, you should not change the list that is iterated over.  Make a copy
first if needed.
When looping over a list of lists, the nested lists can be changed.  The loop
variable is "final", it cannot be changed but what its value can be changed.
							*E1306*
The depth of loops, :for and :while loops added together, cannot exceed 10.


Conditions and expressions ~
						*vim9-boolean*
Conditions and expressions are mostly working like they do in other languages.
Some values are different from legacy Vim script:
	value		legacy Vim script	Vim9 script ~
	0		falsy			falsy
	1		truthy			truthy
	99		truthy			Error!
	"0"		falsy			Error!
	"99"		truthy			Error!
	"text"		falsy			Error!

For the "??" operator and when using "!" then there is no error, every value
is either falsy or truthy.  This is mostly like JavaScript, except that an
empty list and dict is falsy:

	type		truthy when ~
	bool		true, v:true or 1
	number		non-zero
	float		non-zero
	string		non-empty
	blob		non-empty
	list		non-empty (different from JavaScript)
	tuple		non-empty (different from JavaScript)
	dictionary	non-empty (different from JavaScript)
	func		when there is a function name
	special		true or v:true
	job		when not NULL
	channel		when not NULL
	class		not applicable
	object		when not NULL
	enum		not applicable
	enum value	always
	typealias	not applicable

The boolean operators "||" and "&&" expect the values to be boolean, zero or
one: >
	1 || false   == true
	0 || 1       == true
	0 || false   == false
	1 && true    == true
	0 && 1       == false
	8 || 0	     Error!
	'yes' && 0   Error!
	[] || 99     Error!

When using "!" for inverting, there is no error for using any type and the
result is a boolean.  "!!" can be used to turn any value into boolean: >
	!'yes'			== false
	!![]			== false
	!![1, 2, 3]		== true

When using "`.."` for string concatenation arguments of simple types are
always converted to string: >
	'hello ' .. 123  == 'hello 123'
	'hello ' .. v:true  == 'hello true'

Simple types are Number, Float, Special and Bool.  For other types |string()|
should be used.
			*false* *true* *null* *null_blob* *null_channel*
			*null_class* *null_dict* *null_function* *null_job*
			*null_list* *null_object* *null_partial* *null_string*
			*E1034*
In Vim9 script one can use the following predefined values: >
	true
	false
	null
	null_blob
	null_channel
	null_class
	null_dict
	null_function
	null_job
	null_list
	null_tuple
	null_object
	null_partial
	null_string
`true` is the same as `v:true`, `false` the same as `v:false`, `null` the same
as `v:null`.

While `null` has the type "special", the other "null_" values have the type
indicated by their name.  Quite often a null value is handled the same as an
empty value, but not always.  The values can be useful to clear a script-local
variable, since they cannot be deleted with `:unlet`.  E.g.: >
	var theJob = job_start(...)
	# let the job do its work
	theJob = null_job

The values can also be useful as the default value for an argument: >
	def MyFunc(b: blob = null_blob)
	    # Note: compare against null, not null_blob,
	    #       to distinguish the default value from an empty blob.
	    if b == null
	        # b argument was not given
See |null-compare| for more information about testing against null.

It is possible to compare `null`  with any value, this will not give a type
error.  However, comparing `null` with a number, float or bool will always
result in `false`.  This is different from legacy script, where comparing
`null` with zero or `false` would return `true`.
							*vim9-false-true*
When converting a boolean to a string `false` and `true` are used, not
`v:false` and `v:true` like in legacy script.  `v:none` has no `none`
replacement, it has no equivalent in other languages.
							*vim9-string-index*
Indexing a string with [idx] or taking a slice with [idx : idx] uses character
indexes instead of byte indexes.  Composing characters are included.
Example: >
	echo 'bár'[1]
In legacy script this results in the character 0xc3 (an illegal byte), in Vim9
script this results in the string 'á'.
A negative index is counting from the end, "[-1]" is the last character.
To exclude the last character use |slice()|.
To count composing characters separately use |strcharpart()|.
If the index is out of range then an empty string results.

In legacy script "++var" and "--var" would be silently accepted and have no
effect.  This is an error in Vim9 script.

Numbers starting with zero are not considered to be octal, only numbers
starting with "0o" are octal: "0o744". |scriptversion-4|


What to watch out for ~
							*vim9-gotchas*
Vim9 was designed to be closer to often used programming languages, but at the
same time tries to support the legacy Vim commands.  Some compromises had to
be made.  Here is a summary of what might be unexpected.

Ex command ranges need to be prefixed with a colon. >
	->		  legacy Vim: shifts the previous line to the right
	->func()	  Vim9: method call in a continuation line
	:->		  Vim9: shifts the previous line to the right

	%s/a/b		  legacy Vim: substitute on all lines
	x = alongname
	     % another	  Vim9: modulo operator in a continuation line
	:%s/a/b		  Vim9: substitute on all lines
	't		  legacy Vim: jump to mark t
	'text'->func()	  Vim9: method call
	:'t		  Vim9: jump to mark t

Some Ex commands can be confused with assignments in Vim9 script: >
	g:name = value    # assignment
	:g:pattern:cmd	  # :global command

To avoid confusion between a `:global` or `:substitute` command and an
expression or assignment, a few separators cannot be used when these commands
are abbreviated to a single character: ':', '-' and '.'. >
	g:pattern:cmd	  # invalid command - ERROR
	s:pattern:repl	  # invalid command - ERROR
	g-pattern-cmd	  # invalid command - ERROR
	s-pattern-repl	  # invalid command - ERROR
	g.pattern.cmd	  # invalid command - ERROR
	s.pattern.repl	  # invalid command - ERROR

Also, there cannot be a space between the command and the separator: >
	g /pattern/cmd	  # invalid command - ERROR
	s /pattern/repl	  # invalid command - ERROR

Functions defined with `:def` compile the whole function.  Legacy functions
can bail out, and the following lines are not parsed: >
	func Maybe()
	  if !has('feature')
	    return
	  endif
	  use-feature
	endfunc
Vim9 functions are compiled as a whole: >
	def Maybe()
	  if !has('feature')
	    return
	  endif
	  use-feature  # May give a compilation error
	enddef
For a workaround, split it in two functions: >
	func Maybe()
	  if has('feature')
	    call MaybeInner()
	  endif
	endfunc
	if has('feature')
	  def MaybeInner()
	    use-feature
	  enddef
	endif
Or put the unsupported code inside an `if` with a constant expression that
evaluates to false: >
	def Maybe()
	  if has('feature')
	    use-feature
	  endif
	enddef
The `exists_compiled()` function can also be used for this.
							*vim9-user-command*
Another side effect of compiling a function is that the presence of a user
command is checked at compile time.  If the user command is defined later an
error will result.  This works: >
	command -nargs=1 MyCommand echom <q-args>
	def Works()
	  MyCommand 123
	enddef
This will give an error for "MyCommand" not being defined: >
	def Works()
	  command -nargs=1 MyCommand echom <q-args>
	  MyCommand 123
	enddef
A workaround is to invoke the command indirectly with `:execute`: >
	def Works()
	  command -nargs=1 MyCommand echom <q-args>
	  execute 'MyCommand 123'
	enddef

Note that for unrecognized commands there is no check for "|" and a following
command.  This will give an error for missing `endif`: >
	def Maybe()
	  if has('feature') | use-feature | endif
	enddef

Other differences ~

Patterns are used like 'magic' is set, unless explicitly overruled.
The 'edcompatible' option value is not used.
The 'gdefault' option value is not used.

You may also find this wiki useful.  It was written by an early adopter of
Vim9 script: https://github.com/lacygoill/wiki/blob/master/vim/vim9.md

							*:++* *:--*
The ++ and -- commands have been added.  They are very similar to adding or
subtracting one: >
		++var
		var += 1
		--var
		var -= 1

Using ++var or --var in an expression is not supported yet.


==============================================================================

3. New style functions					*fast-functions*

							*:def*
:def[!] {name}([arguments])[: {return-type}]
			Define a new function by the name {name}.  The body of
			the function follows in the next lines, until the
			matching `:enddef`.
							*E1073*
			The {name} cannot be reused at the script-local level: >vim9

			  vim9script
			  def F_1073()
			  enddef
			  def F_1073() # E1073: Name already defined: <SNR>...
			  enddef
<							*E1011*
			The {name} must be less than 100 bytes long.

							*E1077*
			{arguments} is a sequence of zero or more argument
			declarations.  There are three forms:
				{name}: {type}
				{name} = {value}
				{name}: {type} = {value}
			The first form is a mandatory argument.  So, the
			declaration must provide a type.  Example: >vim9

			  vim9script
			  def F_1077(x): void
			      # E1077: Missing argument type for x
			  enddef
<
			For the second form, because the declaration does not
			specify it, Vim infers the type.  For both second and
			third forms, a default {value} applies when the
			caller omits it.  Examples: >vim9

			  vim9script
			  def SecondForm(arg = "Hi"): void
			      echo $'2. arg is a "{arg->typename()}" type ' ..
			           $'and the default value of arg is "{arg}"'
			  enddef
			  SecondForm()
			  def ThirdForm(arg2: number = 9): void
			      echo $'3. default value of arg2 is {arg2}'
			  enddef
			  ThirdForm()
<							*E1123*
			Arguments in a builtin function called in a `:def`
			function must have commas between arguments: >vim9

			  vim9script
			  def F_1123(a: number, b: number): void
			      echo max(a b)
			      # E1123: Missing comma before argument: b)
			  enddef
			  F_1123(1, 2)
<							*E1003* *E1027* *E1096*
			The type of value used with `:return` must match
			{return-type}.  When {return-type} is omitted or is
			"void" the function is not allowed to return
			anything.  Examples: >vim9

			  vim9script
			  def F_1003(): bool
			      return  # E1003: Missing return value
			  enddef
			  F_1003()
< >vim9
			  vim9script
			  def F_1027(): bool
			      echo false  # E1027: Missing return statement
			  enddef
			  F_1027()
< >vim9
			  vim9script
			  def F_1096(): void
			      return false  # E1096: Returning a value ...
			  enddef
			  F_1096()
<							*E1056* *E1059*
			When ": {return-type}" is specified, {return-type}
			cannot be omitted (leaving a hanging colon).  The ": "
			also cannot be preceded by white space.  Examples: >vim

			  def F_1056():
                              # E1056: Expected a type:
			  enddef
			  def F_1059() : bool
                              # E1059: No white space allowed before colon:...
			  enddef
<
			The function will be compiled into instructions when
			called or when either `:defcompile` or `:disassemble` is
			used.  (For an example, see |:disassemble|.)  Syntax
			and type errors will be produced at that time.

							*E1058*
			It is possible to nest `:def` inside another `:def` or
			`:function` only up to 49 levels deep.  At 50 or more
			levels, it is a |E1058| error.

							*E1117*
			[!] is allowed only in legacy Vim script because it
			permits function redefinition (as with `:function`!).
			In Vim9 script, ! is not allowed because script-local
			functions cannot be deleted or redefined, though they
			can be removed by reloading the script.  Also, nested
			functions cannot use ! for redefinition.  Examples: >vim

			  " Legacy Vim script :def! example
			  def! LegacyFunc()
			      echo "def! is allowed in a legacy Vim script"
			  enddef
			  call LegacyFunc()
< >vim9
			  vim9script
			  def Func()
			      def! InnerFunc()
			          # E1117: Cannot use ! with nested :def
			      enddef
			  enddef
			  Func()
< >vim9
			  vim9script
			  def! F_477(): void  # E477: No ! allowed
			  enddef
< >vim9
			  vim9script
			  def F_1084(): void
			  enddef
			  delfunction! F_1084
			  # E1084: Cannot delete Vim9 script function F_1084
<
			Note: The generic error *E1028* ("Compiling :def
			function failed") indicates an undeterminable error
			during compilation.  If reproducible, it may be
			reported at https://github.com/vim/vim/issues as
			it could represent a gap in Vim's error reporting.

					*:enddef* *E1057* *E1152* *E1173*
:enddef			End of a function defined with `:def`.  It should be on
			a line by itself.  Examples: >vim9

			  vim9script
			  def MyFunc()
			  echo 'Do Something' | enddef
			  # E1057: Missing :enddef
< >vim9
			  vim9script
			  def F_1173()
			  enddef echo 'X'
			  # E1173: Text found after enddef: echo 'X'
< >vim9
			  vim9script
			  def F_1152()
			      function X()
			      enddef  # E1152: Mismatched enddef
			  enddef
<
You may also find this wiki useful.  It was written by an early adopter of
Vim9 script: https://github.com/lacygoill/wiki/blob/master/vim/vim9.md

If the script the `:def` function is defined in is Vim9 script, script-local
variables must be accessed without using the "s:" prefix.  They must be
defined before the function is compiled and there is no way to avoid errors
(e.g., by using |exists()|) to conditionally skip undeclared variables.
For example: >vim9

	vim9script
	def MyVim9def()
	    echo unus       # Echoes 1
	    # echo s:unus   # This would be E1268 (Cannot use s: in Vim9)
	    if exists('duo')
	        # echo duo  # This would be E1001 (Variable not found: duo)
	    endif
	enddef
	var unus: number = 1
	MyVim9def()         # MyVim9def is compiled ("duo" does not exist yet)
	var duo: number = 2
<
If the script the `:def` function is defined in is legacy Vim script,
script-local variables may be accessed with or without the "s:" prefix.
However, using "s:" may defer variable resolution to runtime, avoiding
compilation errors for variables that may not exist yet, as this example
explains: >vim

	" legacy Vim script
	def! MyLegacyDef(): void
	    echo [unus, s:unus]   # Echoes [1, 1]
	    # (If uncommented) First sourcing of 'echo s:duo' is E121 and
	    # causes a compilation error; subsequent sourcing echoes 2:
	    # echo s:duo
	    if exists("s:duo")
	        # First sourcing: skips echo; subsequent sourcing: echoes 2
	        echo s:duo
	    endif
	    if exists("duo")
	        # (If uncommented) First sourcing of 'echo duo' is E1001 and
	        # causes a compilation error; subsequent sourcing echoes 2:
	        # echo duo
	    endif
	enddef
	let s:unus = 1
	call MyLegacyDef()  " Calls MyLegacyDef() and compiles if not already
	let s:duo = 2
<							*E1269*
Script-local variables in a Vim9 script must be declared at the script
level.  They cannot be created in a `:def` function and may not be declared
in a legacy function with the "s:" prefix.  For example: >vim9

	vim9script
	function F_1269()
	    let s:i_wish = v:true
	endfunction
	F_1269()
	# E1269: Cannot create a Vim9 script variable in a function: s:i_wish
<
						*:defc* *:defcompile*
:defc[ompile]		Compile functions and classes (|class-compile|)
			defined in the current script that were not compiled
			yet.  This will report any errors found during
			compilation.

			Example: When the three lines (up to and including
			`enddef`) are sourced, there is no error because the
			Vim9 `:def` function is not compiled.  However, if all
			four lines are sourced, compilation fails: >vim9

			  vim9script
			  def F_1027(): string
			  enddef
			  defcompile F_1027  # E1027: Missing return statement

:defc[ompile] MyClass	Compile all methods in a class.  (See |:disassemble|
			for an example.)

:defc[ompile] {func}
:defc[ompile] debug {func}
:defc[ompile] profile {func}
			Compile function {func}, if needed.  Use "debug" and
			"profile" to specify the compilation mode.
			This will report any errors found during compilation.
			{func} can also be "ClassName.functionName" to
			compile a function or method in a class.
			{func} can also be "ClassName" to compile all
			functions and methods in a class.

						*:disa* *:disassemble*
:disa[ssemble] {func}	Show the instructions generated for {func}.
			This is for debugging and testing.
			If {func} is not found, error *E1061* occurs.
			{func} can also be "ClassName.functionName" to
			disassemble a function in a class.
			The following example demonstrates using `:defcompile`
			with a |class| and `:disassemble` with a
			"ClassName.functionName" (positioning the cursor on
			the last line of the visually sourced script): >vim9

			  vim9script
			  class Line
			      var lnum: number
			      def new(this.lnum)
			      enddef
			      def SetLnum()
			          cursor(this.lnum, 52)
			      enddef
			  endclass
			  defcompile Line
			  disassemble Line.SetLnum
			  var vlast: Line = Line.new(line("'>"))
			  vlast.SetLnum()  # Cursor is positioned here->_

:disa[ssemble] profile {func}
			Like `:disassemble` but with the instructions used for
			profiling.

:disa[ssemble] debug {func}
			Like `:disassemble` but with the instructions used for
			debugging.

  Note: For command line completion of {func}, script-local functions
	are shown with their <SNR>.  Depending on options, including
	|wildmenumode()|, completion may work with "s:", "<S", or the function
	name directly.  (For example, in Vim started with |-u| NONE, ":disa s:"
	and |c_CTRL-E| lists script-local function names.)


Limitations ~

Variables local to `:def` functions are not visible to string evaluation.
The following example shows that the script-local constant "SCRIPT_LOCAL" is
visible whereas the function-local constant "DEF_LOCAL" is not: >vim9

	vim9script
	const SCRIPT_LOCAL = ['A', 'script-local', 'list']
	def MapList(scope: string): list<string>
	    const DEF_LOCAL: list<string> = ['A', 'def-local', 'list']
	    if scope == 'script local'
	        return [1]->map('SCRIPT_LOCAL[v:val]')
	    else
	        return [1]->map('DEF_LOCAL[v:val]')
	    endif
	enddef
	echo 'script local'->MapList()	# Echoes ['script-local']
	echo 'def local'->MapList()	# E121: Undefined variable: DEF_LOCAL
<
The map argument is a string expression, which is evaluated without the
function scope.  Instead, in Vim9 script, use a lambda: >vim9

	vim9script
	def MapList(): list<string>
	    const DEF_LOCAL: list<string> = ['A', 'def-local', 'list']
	    return [1]->map((_, v) => DEF_LOCAL[v])
	enddef
	echo MapList()			# Echoes ['def-local']
<
For commands that are not compiled, such as `:edit`, |backtick-expansion| can
be used and it can use the local scope.  Example: >vim9

	vim9script
	def EditNewBlah()
	    var fname: string = 'blah.txt'
	    split
	    edit `=fname`
	enddef
	EditNewBlah()  # A new split is created as buffer 'blah.txt'
<
Closures defined in a loop can either share a variable or each have their own
copy, depending on where the variable is declared.  With a variable declared
outside the loop, all closures reference the same shared variable.
The following example demonstrates the consequences, with the "outloop"
variable existing only once: >vim9

	vim9script
	var flist: list<func>
	def ClosureEg(n: number): void
	    var outloop: number = 0  # outloop is declared outside the loop!
	    for i in range(n)
	        outloop = i
	        flist[i] = (): number => outloop  # Closures ref the same var
	    endfor
	    echo range(n)->map((i, _) => flist[i]())
	enddef
	ClosureEg(4)  # Echoes [3, 3, 3, 3]
<
All closures put in the list refer to the same instance, which, in the end,
is 3.

However, when the variable is declared inside the loop, each closure gets its
own copy, as shown in this example: >vim9

	vim9script
	var flist: list<func>
	def ClosureEg(n: number): void
	    for i in range(n)
	        var inloop: number = i  # inloop is declared inside the loop
	        flist[i] = (): number => inloop  # Closures ref each inloop
	    endfor
	    echo range(n)->map((i, _) => flist[i]())
	enddef
	ClosureEg(4)  # Echoes [0, 1, 2, 3]

Another way to have a separate context for each closure is to call a
function to define it: >vim9

	vim9script
	def GetClosure(i: number): func
	    var infunc: number = i
	    return (): number => infunc
	enddef
	var flist: list<func>
	def ClosureEg(n: number): void
	    for i in range(n)
	        flist[i] = GetClosure(i)
	    endfor
	    echo range(n)->map((i, _) => flist[i]())
	enddef
	ClosureEg(4)  # Echoes [0, 1, 2, 3]
<							*E1271*
A closure must be compiled in the context that it is defined in, so that
variables in that context can be found.  This mostly happens correctly,
except when a function is marked for debugging with `:breakadd` after it was
compiled.  Make sure to define the breakpoint before compiling the outer
function.
							*E1248*
In some situations, such as when a Vim9 closure which captures local variables
is converted to a string and then executed, an error occurs.  This happens
because the string execution context cannot access the local variables from
the original context where the closure was defined.  For example: >vim9

	vim9script
	def F_1248(): void
	    var n: number
	    var F: func = () => {
	        n += 1
	    }
	    try
	        execute printf("call %s()", F)
	    catch
	        echo v:exception
	    endtry
	enddef
	F_1248()  # Vim(call):E1248: Closure called from invalid context

In Vim9 script, a loop variable is invalid after the loop is closed.
For example, this timer will echo 0 to 2 on separate lines.  However, if
the variable "n" is used after the `:endfor`, that is an |E121| error: >vim9

	vim9script
	for n in range(3)
	    var nr: number = n
	    timer_start(1000 * n, (_) => {
	        echowindow nr
	    })
	endfor
	try
	    echowindow n
	catch
	    echo v:exception
	endtry
<
	  Note: Using `:echowindow` is useful in a timer because messages go
		into a popup and will not interfere with what the user is
		doing when it triggers.


Converting a :function to a :def~
					*convert_legacy_function_to_vim9*
					*convert_:function_to_:def*
There are many changes that need to be made to convert a `:function` to
a `:def` function.  The following are some of them:

- Change `let` used to declare variables to one of `var`, `const`, or `final`,
  and remove the "s:" from each |script-variable|.
- Change `func` or `function` to `def`.
- Change `endfunc` or `endfunction` to `enddef`.
- Add the applicable type (or "any") to each function argument.
- Remove "a:" from each |function-argument|.
- Remove inapplicable options such as |:func-range|, |:func-abort|,
  |:func-dict|, and |:func-closure|.
- If the function returns something, add the return type.  (Ideally, add
  "void" if it does not return anything.)
- Remove line continuation backslashes from places they are not required.
- Remove `let` for assigning values to |g:|, |b:|, |w:|, |t:|, or |l:| variables.
- Rewrite |lambda| expressions in Vim9 script syntax (see |vim9-lambda|).
- Change comments to start with # (preceded by white space) instead of ".
- Insert white space in expressions where required (see |vim9-white-space|).
- Change "." used for string concatenation to " .. ".  (Alternatively, use
  an |interpolated-string|.)

The following legacy Vim script and Vim9 script examples demonstrate all
those differences.  First, legacy Vim script: >vim

	let s:lnum=0
	function Leg8(arg) abort
	    let l:pre=['Result',
	      \': ']
	    let b:arg=a:arg
	    let s:lnum+=2
	    let b:arg*=4
	    let l:result={pre->join(pre,'')}(l:pre)
	    return l:result.(b:arg+s:lnum)"no space before comment
	endfunction
	call Leg8(10)->popup_notification(#{time: 3000})" Pops up 'Result: 42'

The equivalent in Vim9 script: >vim9

	vim9script
	var lnum: number
	def Vim9(arg: number): string
	    final pre = ['Result',
	      ': ']
	    b:arg = arg
	    lnum += 2
	    b:arg *= 4
	    const RESULT: string = ((lpre) => join(lpre, ''))(pre)
	    return RESULT .. (b:arg + lnum) # space required before # comment
	enddef
	Vim9(10)->popup_notification({time: 3000}) # Pops up 'Result: 42'

<	Note: This example also demonstrates (outside the `:def` function):
		- Removing "#" from the legacy |#{}| - see |vim9-literal-dict|
		- Omitting `:call` (allowed, though unnecessary in Vim9 script)


Calling a :def function in an expr option ~
							*expr-option-function*
The value of a few options, such as 'foldexpr', is an expression that is
evaluated to get a value.  The evaluation can have quite a bit of overhead.
One way to minimize the overhead, and also to keep the option value simple,
is to define a compiled function and set the option to call it without
arguments.  For example: >vim9

	vim9script
	def MyFoldFunc(): string
	    # This matches start of line (^), followed by a digit, a full stop
	    # a space or tab, an uppercase character, with an empty next line
	    return getline(v:lnum) =~ '^[[:digit:]]\.[[:blank:]][[:upper:]]'
	        && getline(v:lnum + 1)->empty() ? '>1' : '1'
	enddef
	set foldexpr=MyFoldFunc()
	set foldmethod=expr
	norm! zM
<
  Warning: This script creates and applies folds at the "Heading 1" level of
	   this vim9.txt help buffer.  (You can use |zR|, in Normal mode, to
	   open all the folds after sourcing the script.)


==============================================================================

4. Types					*vim9-types*

The following types, each shown with its corresponding internal |v:t_TYPE|
variable, are supported:

	number					|v:t_number|
	string					|v:t_string|
	func					|v:t_func|
	func: {type}				|v:t_func|
	func({type}, ...)			|v:t_func|
	func({type}, ...): {type}		|v:t_func|
	list<{type}>				|v:t_list|
	dict<{type}>				|v:t_dict|
	float					|v:t_float|
	bool					|v:t_bool|
	none					|v:t_none|
	job					|v:t_job|
	channel					|v:t_channel|
	blob					|v:t_blob|
	class					|v:t_class|
	object					|v:t_object|
	typealias				|v:t_typealias|
	enum					|v:t_enum|
	enumvalue				|v:t_enumvalue|
	tuple<{type}>				|v:t_tuple|
	tuple<{type}, {type}, ...>		|v:t_tuple|
	tuple<...list<{type}>>			|v:t_tuple|
	tuple<{type}, ...list<{type}>>		|v:t_tuple|
	void

						 *E1031* *E1186*
These types can be used in declarations, though no simple value can have the
"void" type.  Trying to use a void as a value results in an error.  Examples: >vim9

	vim9script
	def NoReturnValue(): void
	enddef
	try
	    const X: any = NoReturnValue()
	catch
	    echo v:exception	  # E1031: Cannot use void value
	    try
	        echo NoReturnValue()
	    catch
	        echo v:exception  # E1186: Expression does not result in a ...
	    endtry
	endtry
<						*E1008* *E1009* *E1010* *E1012*
Ill-formed declarations and mismatching types result in errors.  The following
are examples of errors E1008, E1009, E1010, and E1012: >vim9

	vim9cmd var l: list
	vim9cmd var l: list<number
	vim9cmd var l: list<invalidtype>
	vim9cmd var l: list<number> = ['42']
<
There is no array type.  Instead, use either a list or a tuple.  Those types
may also be literals (constants).  In the following example, [5, 6] is a list
literal and (7, ) a tuple literal.  The echoed list is a list literal too: >vim9

	vim9script
	var l: list<number> = [1, 2]
	var t: tuple<...list<number>> = (3, 4)
	echo [l, t, [5, 6], (7, )]
<
							*tuple-type*
A tuple type may be declared in the following ways:
tuple<number>			a tuple with a single item of type |Number|
tuple<number, string>		a tuple with two items of type |Number| and
				|String|
tuple<number, float, bool>	a tuple with three items of type |Number|,
				|Float| and |Boolean|
tuple<...list<number>>		a variadic tuple with zero or more items of
				type |Number|
tuple<number, ...list<string>>	a tuple with an item of type |Number| followed
				by zero or more items of type |String|

Examples: >
    var myTuple: tuple<number> = (20,)
    var myTuple: tuple<number, string> = (30, 'vim')
    var myTuple: tuple<number, float, bool> = (40, 1.1, true)
    var myTuple: tuple<...list<string>> = ('a', 'b', 'c')
    var myTuple: tuple<number, ...list<string>> = (3, 'a', 'b', 'c')
<
						*variadic-tuple* *E1539*
A variadic tuple has zero or more items of the same type.  The type of a
variadic tuple must end with a list type.  Examples: >
    var myTuple: tuple<...list<number>> = (1, 2, 3)
    var myTuple: tuple<...list<string>> = ('a', 'b', 'c')
    var myTuple: tuple<...list<bool>> = ()
<
				    *vim9-func-declaration* *E1005* *E1007*
				    *vim9-partial-declaration*
				    *vim9-func-type*
A function (or partial) may be declared in the following ways:
func				any kind of function reference, no type
				checking for arguments or return value
func: void			any number and type of arguments, no return
				value
func: {type}			any number and type of arguments with specific
				return type

func()				function with no argument, does not return a
				value
func(): void			same
func(): {type}			function with no argument and return type

func({type})			function with argument type, does not return
				a value
func({type}): {type}		function with argument type and return type
func(?{type})			function with type of optional argument, does
				not return a value
func(...list<{type}>)		function with type of list for variable number
				of arguments, does not return a value
func({type}, ?{type}, ...list<{type}>): {type}
				function with:
				- type of mandatory argument
				- type of optional argument
				- type of list for variable number of
				  arguments
				- return type

If the return type is "void" the function does not return a value.

The reference can also be a |Partial|, in which case it stores extra arguments
and/or a dictionary, which are not visible to the caller.  Since they are
called in the same way, the declaration is the same.  This interactive example
prompts for a circle's radius and returns its area to two decimal places,
using a partial: >vim9

	vim9script
	def CircleArea(pi: float, radius: float): float
	    return pi * radius->pow(2)
	enddef
	const AREA: func(float): float = CircleArea->function([3.14])
	const RADIUS: float = "Enter a radius value: "->input()->str2float()
	echo $"\nThe area of a circle with a radius of {RADIUS} is " ..
	     $"{AREA(RADIUS)} (π to two d.p.)"
<
						*vim9-typealias-type*
Custom types (|typealias|) can be defined with `:type`.  They must start with
a capital letter (which avoids name clashes with either current or future
builtin types) similar to user functions.  This example creates a list of
perfect squares and reports on |type()| (14, a typealias) and the |typename()|: >vim9

	vim9script
	type Ln = list<number>
	final perfect_squares: Ln = [1, 4, 9, 16, 25]
	echo "Typename (Ln): " ..
	     $"type() is {Ln->type()} and typename() is {Ln->typename()}"
<
						*E1105*
A typealias itself cannot be converted to a string: >vim9

	vim9script
	type Ln = list<number>
	const FAILS: func = (): string => {
	    echo $"{Ln}"  # E1105: Cannot convert typealias to string
	    }
<
				    *vim9-class-type*  *vim9-interface-type*
				    *vim9-object-type*
A |class|, |object|, and |interface| may all be used as types.  The following
interactive example prompts for a float value and returns the area of two
different shapes.  It also reports on the |type()| and |typename()| of the
classes, objects, and interface: >vim9

	vim9script
	interface Shape
	    def InfoArea(): tuple<string, float>
	endinterface
	class Circle implements Shape
	    var radius: float
	    def InfoArea(): tuple<string, float>
	        return ('Circle (π × r²)', 3.141593 * this.radius->pow(2))
	    enddef
	endclass
	class Square implements Shape
	    var side: float
	    def InfoArea(): tuple<string, float>
	        return ('Square (s²)', this.side->pow(2))
	    enddef
	endclass
	const INPUT: float = "Enter a float value: "->input()->str2float()
	echo "\nAreas of shapes:"
	var myCircle: object<Circle> = Circle.new(INPUT)
	var mySquare: object<Square> = Square.new(INPUT)
	final shapes: list<Shape> = [myCircle, mySquare]
	for shape in shapes
	    const [N: string, A: float] = shape.InfoArea()
	    echo $"\t- {N} has area of {A}"
	endfor
	echo "\n\t\ttype()\ttypename()\n\t\t------\t----------"
	echo $"Circle\t\t{Circle->type()}\t{Circle->typename()}"
	echo $"Square\t\t{Square->type()}\t{Square->typename()}"
	echo $"Shape\t\t{Shape->type()}\t{Shape->typename()}"
	echo $"MyCircle\t{myCircle->type()}\t{myCircle->typename()}"
	echo $"MySquare\t{mySquare->type()}\t{mySquare->typename()}"
	echo $"shapes\t\t{shapes->type()}\t{shapes->typename()}"
<
					*vim9-enum-type*  *vim9-enumvalue-type*
An |enum| may be used as a type (|v:t_enum|).  Variables holding enum values
have the enumvalue type (|v:t_enumvalue|) at runtime.  The following
interactive example prompts for a character and returns information about
either a square or a rhombus.  It also reports on the |type()| and |typename()|
of the enum and enumvalue: >vim9

	vim9script
	enum Quad
	    Square('four', 'only'),
	    Rhombus('opposite', 'no')
	    var eq: string
	    var ra: string
	    def string(): string
	        return $"\nA {this.name} has " ..
	               $"{this.eq} equal sides and {this.ra} right angles\n\n"
	    enddef
	endenum
	echo "Rhombus (r) or Square (s)?"
	var myQuad: Quad = getcharstr() =~ '\c^R' ? Quad.Rhombus : Quad.Square
	echo myQuad.string() .. "\ttype()\ttypename()"
	echo $"Quad  \t{Quad->type()}  \t{Quad->typename()}"
	echo $"myQuad\t{myQuad->type()}\t{myQuad->typename()}"
<
	 Notes: This script uses builtin method "string()" (|object-string()|).
		The typename() of Quad and myQuad are the same ("enum<Quad>")
		whereas the type() is distinguished (myQuad returns 16,
		enumvalue, whereas Quad returns 15, enum).

Variable types and type casting	~
							*variable-types*
Variables declared in Vim9 script or in a `:def` function have a type, either
specified explicitly or inferred from the initialization.

Global, buffer, window and tab page variables do not have a specific type.
Consequently, their values may change at any time, possibly changing the type.
Therefore, in compiled code, the "any" type is assumed.

This can be a problem when stricter typing is desired, for example, when
declaring a list: >
	var l: list<number> = [1, b:two]
Since Vim doesn't know the type of "b:two", the expression becomes list<any>.
A runtime check verifies the list matches the declared type before assignment.

							*type-casting*
To get more specific type checking, use type casting.  This checks the
variable's type before building the list, rather than checking whether
list items match the declared type.  For example: >
	var l: list<number> = [1, <number>b:two]
<
So, here the type cast checks whether "b:two" is a number and gives an error
if it isn't.

The difference is demonstrated in the following example.  With funcref
variable "NTC", Vim infers the expression type "[1, b:two]" as list<any>, then
verifies whether it can be assigned to the list<number> return type.  With
funcref variable "TC", the type cast means Vim first checks whether "b:two" is
a <number> type: >vim9

	vim9script
	b:two = '2'
	const NTC: func = (): list<number> => {
	    return [1, b:two]
	    }
	disassemble NTC  # 3 CHECKTYPE list<number> stack [-1]
	try
	    NTC()
	catch
	    echo v:exception .. "\n\n"  # expected list<number> but...
	endtry
	const TC: func = (): list<number> => {
	    return [1, <number>b:two]
	    }
	disassemble TC  # 2 CHECKTYPE number stack [-1]
	try
	    TC()
	catch
	    echo v:exception  # expected number but got string
	endtry
<
	  Note: Notice how the error messages differ, showing when
	        type checking occurs.

						*E1104*
The syntax of a type cast is "<{type}>".  An error occurs if either the
opening "<" (|E121|) or closing ">" (E1104) is omitted.  Also, white space
is not allowed either after the "<" (|E15|) or before the ">" (|E1068|), which
avoids ambiguity with smaller-than and greater-than operators.

Although a type casting forces explicit type checking, it neither changes the
value of, nor the type of, a variable.  If you need to alter the type, use a
function such as |string()| to convert to a string, or |str2nr()| to convert a
string to a number.

If type casting is applied to a chained expression, it must be compatible with
the final result.  Examples: >vim9

	vim9script
	# These type casts work
	echo <list<any>>[3, 2, 1]->extend(['Go!'])
	echo <string>[3, 2, 1]->extend(['Go!'])->string()
	echo <tuple<...list<number>>>[3, 2, 1]->list2tuple()
	# This type cast fails
	echo <number>[3, 2, 1]->extend(['Go!'])->string()
<
						*E1272*
If a type is used in a context where types are not expected you can get
E1272.  For example: >
	:vim9cmd echo islocked('x: string')
<  Note: This must be executed from Vim's command line, not sourced.

						*E1363*
If a type is incomplete, such as when an object's class is unknown, E1363
results.  For example: >vim9

	vim9script
	var E1363 = null_class.member  # E1363: Incomplete type
<
Another null object-related error is |E1360|: >vim9

	vim9script
	var obj = null_object
	var E1360 = obj.MyMethod()  # E1360: Using a null object
<

Type inference ~
							*type-inference*
Declaring types explicitly provides many benefits, including targeted type
checking and clearer error messages.  Nonetheless, Vim often can infer types
automatically when they are omitted.  For example, each of these variables'
types are inferred, with the |type()| and |typename()| echoed showing those
inferred types: >vim9

	vim9script
	echo "\t type()\t typename()"
	var b = true    | echo $"{b} \t {b->type()} \t {b->typename()}"
	var f = 4.2     | echo $"{f} \t {f->type()} \t {f->typename()}"
	var l = [1, 2]  | echo $"{l} \t {l->type()} \t {l->typename()}"
	var n = 42      | echo $"{n} \t {n->type()} \t {n->typename()}"
	var s = 'yes'   | echo $"{s} \t {s->type()} \t {s->typename()}"
	var t = (42, )  | echo $"{t} \t {t->type()} \t {t->typename()}"
<
The type of a list, tuple, or dictionary is inferred from the common type of
its values.  When the values are all the same type, that type is used.
If there is a mix of types, the "any" type is used.  In the following example,
the echoed |typename()| for each literal demonstrates these points: >vim9

	vim9script
	echo [1, 2]->typename()				 # list<number>
	echo [1, 'x']->typename()			 # list<any>
	echo {ints: [1, 2], bools: [false]}->typename()	 # dict<list<any>>
	echo (true, false)->typename()			 # tuple<bool, bool>
<
The common type of function references, when they do not all have the same
number of arguments, is indicated with "(...)", meaning the number of
arguments is unequal.  This script demonstrates a "list<func(...): void>": >vim9

	vim9script
	def Foo(x: bool): void
	enddef
	def Bar(x: bool, y: bool): void
	enddef
	var funclist = [Foo, Bar]
	echo funclist->typename()
<
Script-local variables in a Vim9 script are type checked.  The type is
also checked for variables declared in a legacy function.  For example: >vim9

	vim9script
	var my_local = (1, 2)
	function Legacy()
	    let b:legacy = [1, 2]
	endfunction
	Legacy()
	echo $"{my_local} is type {my_local->type()} ({my_local->typename()})"
	echo $"{b:legacy} is type {b:legacy->type()} ({b:legacy->typename()})"
<
							*E1013*
When a type is declared for a List, Tuple, or Dictionary, the type is attached
to it.  Similarly, if a type is not declared, the type Vim infers is attached.
In either case, if an expression attempts to change the type, E1013 results.
This example has its type inferred and demonstrates E1013: >vim9

	vim9script
	var lb = [true, true]	# Two bools, so Vim infers list<bool> type
	echo lb->typename()	# Echoes list<bool>
	lb->extend([0])		# E1013 Argument 2: type mismatch, ...
<
If you want a permissive list, either explicitly use <any> or declare an
empty list initially (or both, i.e., `list<any> = []`).  Examples: >vim9

	vim9script
	final la: list<any> = []
	echo la->extend(['two', 1])
	final le = []
	echo le->extend(la)
<
Similarly for a permissive dictionary: >vim9

	vim9script
	final da: dict<any> = {}
	echo da->extend({2: 2, 1: 'One'})
	final de = {}
	echo de->extend(da)->string()
<
And, although tuples themselves are immutable, permissive tuple concatenation
can be achieved with either "any" or an empty tuple: >vim9

	vim9script
	var t_any: tuple<...list<any>> = (3, '2')
	t_any = t_any + (true, )
	echo t_any
	var t_dec_empty = ()
	t_dec_empty = t_dec_empty + (3, '2', true)
	echo t_dec_empty
<
If a list literal or dictionary literal is not bound to a variable, its type
may change, as this example shows: >vim9

	vim9script
	echo [3, 2, 1]->typename()			 # list<number>
	echo [3, 2, 1]->extend(['Zero'])->typename()	 # list<any>
	echo {1: ['One']}->typename()			 # dict<list<string>>
	echo {1: ['One']}->extend({2: [2]})->typename()	 # dict<list<any>>
<

Stricter type checking ~
						*type-checking*
In legacy Vim script, where a number was expected, a string would be
automatically converted to a number.  This was convenient for an actual number
such as "123", but leads to unexpected problems (and no error message) if the
string doesn't start with a number.  Quite often this leads to hard-to-find
bugs.  For example, in legacy Vim script this echoes "1": >vim

	echo 123 == '123'
<
However, if an unintended space is included, "0" is echoed: >vim

	echo 123 == ' 123'
<
						*E1206*
In Vim9 script this has been made stricter.  In most places it works just as
before if the value used matches the expected type.  For example, in both
legacy Vim script and Vim9 script trying to use anything other than a
dictionary when it is required: >vim

	echo [8, 9]->keys()
	vim9cmd echo [8, 9]->keys()	 # E1206: Dictionary required
<
						*E1023* *E1024* *E1029* *E1030*
						*E1174* *E1175* *E1210* *E1212*
However, sometimes there will be an error in Vim9 script, which breaks
backwards compatibility.  The following examples illustrate various places
this happens.  The legacy Vim script behavior, which does not fail, is shown
first.  It is followed by the error that occurs if the same command is used
in Vim9 script.

  - Using a number (except 0 or 1) where a bool is expected: >vim

	echo v:version ? v:true : v:false
	vim9cmd echo v:version ? true : false  # E1023: Using a Number as a...
<
  - Using a number where a string is expected: >vim

	echo filter([1, 2], 0)
	vim9cmd echo filter([1, 2], 0)  # E1024: Using a Number as a String
<
  - Not using a number where a number is expected: >vim

	" In this example, Vim script treats v:false as 0
	function Not1029()
	  let b:l = [42] | unlet b:l[v:false]
	endfunction
	call Not1029() | echo b:l
< >vim9
	vim9script
	def E1029(): void
	  b:l = [42] | unlet b:l[false]
	enddef
	E1029()  # E1029: Expected number but got bool
<
  - Using a string as a number: >vim

	let b:l = [42] | unlet b:l['#'] | echo b:l
	vim9cmd b:l = [42] | vim9cmd unlet b:l['#']  # E1030: Using a string...
<
  - Not using a string where an argument requires a string: >vim9

	echo substitute('Hallo', 'a', 'e', v:true)
	vim9cmd echo substitute('Hallo', 'a', 'e', true)  # E1174: String...
<
  - Using an empty string in an argument that requires a non-empty string: >vim9

	echo exepath('')
	vim9cmd echo exepath('') # E1175: Non-empty string required for arg...
<
  - Not using a number when it is required: >vim

	echo gettabinfo('a')
	vim9cmd echo gettabinfo('a')  # E1210: Number required for argument 1
<
  - Not using a bool when it is required: >vim

	echo char2nr('¡', 2)
	vim9cmd echo char2nr('¡', 2)  # E1212: Bool required for argument 2
<
  - Not using a number when a number is required (|E521|): >vim

	let &laststatus='2'
	vim9cmd &laststatus = '2'
<
  - Not using a string when a string is required (|E928|): >vim

	let &langmenu = 42
	vim9cmd &langmenu = 42  # E928: String required
<
  - Comparing a |Special| with 'is' fails in some instances (|E1037|, |E1072|): >vim

	" 1 is echoed because these are both true
	echo v:null is v:null && v:none is v:none
	" 0 is echoed because all these expressions are false
	echo v:none is v:null || v:none is 8 || v:true is v:none
	" All these are errors in Vim9 script
	vim9cmd echo v:null is v:null # E1037: Cannot use 'is' with special
	vim9cmd echo v:none is v:none # E1037: Cannot use 'is' with special
	vim9cmd echo v:none is v:null # E1037: Cannot use 'is' with special
	vim9cmd echo v:none is 8      # E1072: Cannot compare special with numb
	vim9cmd echo v:true is v:none # E1072: Cannot compare bool with special
<
	  Note: Although the last two Vim9 script examples above error using
		`v:none`, they return `false` using `null` (which is the same
		as `v:null` - see |v:null|): >vim9

			vim9script
			echo null is 8		# false
			echo true is null	# false
<
  - Using a string where a bool is required (|E1135|): >vim

	echo '42' ? v:true : v:false
	vim9cmd echo '42' ? true : false  # E1135: Using a String as a Bool
<
  - Using a bool as a number (|E1138|): >vim

	let &laststatus=v:true
	vim9cmd &laststatus = true
<
One consequence is that the item type of a list or dict given to |map()| must
not change when its type is either declared or inferred.  For example, this
gives an error in Vim9 script, whereas in legacy Vim script it is allowed: >vim

	" legacy Vim script changes s:mylist to ['item 0', 'item 1']
	let s:mylist = [0, 1]
	call map(s:mylist, {i -> $"item {i}"})
	echo s:mylist
< >vim9
	vim9script
	var mylist = [0, 1]                # Vim infers mylist is list<number>
	map(mylist, (i, _) => $"item {i}") # E1012: type mismatch...
<
The error occurs because `map()` tries to modify the list elements to strings,
which conflicts with the declared type.

Use |mapnew()| instead.  It creates a new list, and Vim infers its type if it
is not specified.  Inferred and declared types are shown in this example: >vim9

	vim9script
	var mylist = [0, 1]
	var infer = mylist->mapnew((i, _) => $"item {i}")
	echo [infer, infer->typename()]
	var declare: list<string> = mylist->mapnew((i, _) => $"item {i}")
	echo [declare, declare->typename()]
<
The key concept here is, variables with declared or inferred types cannot
have the types of the elements within their containers change.  However, type
"changes" are allowed for either:
  - a container literal (not bound to a variable), or
  - a container where |copy()| or |deepcopy()| is used in method chaining.
Both are demonstrated in this example: >vim9

	vim9script
	# list literal
	echo [1, 2]->map((_, v) => $"#{v}")
	echo [1, 2]->map((_, v) => $"#{v}")->typename()
	# deepcopy() in a method chain
	var mylist = [1, 2]
	echo mylist->deepcopy()->map((_, v) => $"#{v}")
	echo mylist->deepcopy()->map((_, v) => $"#{v}")->typename()
	echo mylist
<
The reasoning behind this is, when a type is either declared or inferred
and the list is passed around and changed, the declaration/inference must
always hold so that you can rely on the type to match the declared/inferred
type.  For either a list literal or a fully copied list, that type safety is
not needed because the original list is unchanged (as "echo mylist" shows,
above).

If the item type was not declared or determined to be "<any>", it will not
change, even if all items later become the same type.  However, when `mapnew()`
is used, inference means that the new list will reflect the type(s) present.
For example: >vim9

	vim9script
	# list<any>
	var mylist = [1, '2']			# mixed types, i.e., list<any>
	echo (mylist, mylist->typename())	# ([1, '2'], 'list<any>')
	mylist->map((_, v) => $"item {v}")	# all items are now strings
	echo (mylist, mylist->typename())	# both strings, but list<any>
	# mapnew()
	var newlist = mylist->mapnew((_, v) => v)
	echo (newlist, newlist->typename())	# newlist is a list<string>
<
Using |extend()| and |extendnew()| is similar, i.e., a list literal may use
the former, so, this is okay: >vim9

	vim9cmd echo [1, 2]->extend(['3'])	# [1, 2, 3]
<
whereas, this is not: >vim9

	vim9script
	var mylist: list<number> = [1, 2]
	echo mylist->extend(['3'])	   # E1013: Argument 2: type mismatch
<
Using |extendnew()| is needed for extending an existing typed list, except
where the extension matches the list's type (or it is "any").  For example,
first extending with an element of the same type, then extending with a
different type: >vim9

	vim9script
	var mylist: list<number> = [1, 2]
	mylist->extend([3])
	echo mylist->extendnew(['4'])	   # [1, 2, 3, '4']

<								*E1158*
Using |flatten()| is not allowed in Vim9 script, because it is intended
always to change the type.  This even applies to a list literal
(unlike |map()| and |extend()|).  Instead, use |flattennew()|: >vim9

	vim9cmd [1, [2, 3]]->flatten()		# E1158: Cannot use flatten
	vim9cmd echo [1, [2, 3]]->flattennew()	# [1, 2, 3]
<
Assigning to a funcref with specified arguments (see |vim9-func-declaration|)
involves strict type checking of the arguments.  For example, this works: >vim9

	vim9script
	var F_name_age: func(string, number): string
	F_name_age = (n: string, a: number): string => $"Name: {n}, Age: {a}"
	echo F_name_age('Bob', 42)
<
whereas this fails with error |E1012| (type mismatch): >vim9

	vim9script
	var F_name_age: func(string, number): string
	F_name_age = (n: string, a: string): string => $"Name: {n}, Age: {a}"
<
If there is a variable number of arguments they must have the same type, as in
this example: >vim9

	vim9script
	var Fproduct: func(...list<number>): number
	Fproduct = (...v: list<number>): number => reduce(v, (a, b) => a * b)
	echo Fproduct(3, 2, 4)  # Echoes 24
<
And <any> may be used to accommodate mixed types: >vim9

	vim9script
	var FlatSort: func(...list<any>): any
	FlatSort = (...v: list<any>) => flattennew(v)->sort('n')
	echo FlatSort(true, [[[5, 3], 2], 4])  # Echoes [true, 2, 3, 4, 5]
<
	  Note: Using <any> in a lambda does not avoid type checking of the
		funcref.  It remains constrained by the declared funcref's
		type and, as these examples show, a runtime or compiling error
		occurs when the types mismatch: >vim9

		    vim9script
		    var FuncSN: func(string): number
		    FuncSN = (v: any): number => v->str2nr()
		    echo FuncSN('162')->nr2char()  # Echoes ¢
		    echo FuncSN(162)->nr2char())   # E1013 (runtime error)
< >vim9
		    vim9script
		    var FuncSN: func(string): number
		    FuncSN = (v: any): number => v->str2nr()
		    def FuncSNfail(): void
		        echo FuncSN('162')->nr2char()  # No echo because ...
		        echo FuncSN(162)->nr2char()    # Error while compiling
		    enddef
		    FuncSNfail()
<
When the funcref has no arguments specified, there is no type checking.  This
example shows FlexArgs has a string argument the first time and a list the
following time: >vim9

	vim9script
	var FlexArgs: func: string
	FlexArgs = (s: string): string => $"It's countdown time {s}..."
	echo FlexArgs("everyone")
	FlexArgs = (...values: list<string>): string => join(values, ', ')
	echo FlexArgs('3', '2', '1', 'GO!')
<
				*E1211* *E1217* *E1218* *E1219* *E1220*
				*E1221* *E1222* *E1223* *E1224* *E1225*
				*E1226* *E1228* *E1235* *E1238* *E1251*
				*E1253* *E1256* *E1297* *E1298* *E1301*
				*E1528* *E1529* *E1530* *E1531* *E1534*
Types are checked for most builtin functions to make it easier to spot
mistakes.  The following one-line |:vim9| commands, calling builtin functions,
demonstrate many of those type-checking errors: >vim9

	vim9 9->list2blob()               # E1211: List required for argume...
	vim9 9->ch_close()                # E1217: Channel or Job required ...
	vim9 9->job_info()                # E1218: Job required for argumen...
	vim9 [9]->cos()                   # E1219: Float or Number required...
	vim9 {}->remove([])               # E1220: String or Number require...
	vim9 null_channel->ch_evalraw(9)  # E1221: String or Blob required ...
	vim9 9->col()                     # E1222: String or List required ...
	vim9 9->complete_add()            # E1223: String or Dictionary req...
	vim9 setbufline(9, 9, {})         # E1224: String, Number or List r...
	vim9 9->count(9)                  # E1225: String, List, Tuple or D...
	vim9 9->add(9)                    # E1226: List or Blob required fo...
	vim9 9->remove(9)                 # E1228: List, Dictionary, or Blo...
	vim9 getcharstr('9')              # E1235: Bool or number required ...
	vim9 9->blob2list()               # E1238: Blob required for argume...
	vim9 9->filter(9)                 # E1251: List, Tuple, Dictionary,...
	vim9 9->reverse()                 # E1253: String, List, Tuple or B...
	vim9 9->call(9)                   # E1256: String or Function requi...
	vim9 null_dict->winrestview()     # E1297: Non-NULL Dictionary requ...
	vim9 {}->prop_add_list(null_list) # E1298: Non-NULL List required f...
	vim9 {}->repeat(9)                # E1301: String, Number, List, Tu...
	vim9 9->index(9)                  # E1528: List or Tuple or Blob re...
	vim9 9->join()                    # E1529: List or Tuple required f...
	vim9 9->max()                     # E1530: List or Tuple or Diction...
	vim9 9->get(9)                    # E1531: Argument of get() must b...
	vim9 9->tuple2list()              # E1534: Tuple required for argum...
<
Reserved for future use:			*E1227* *E1250* *E1252*
	E1227: List or Dictionary required for argument %d
	E1250: Argument of %s must be a List, String, Dictionary or Blob
	E1252: String, List or Blob required for argument %d


Categories of variables, defaults and null handling ~
					*variable-categories* *null-variables*
There are three categories of variables:
	primitive	number, float, boolean
	container	string, blob, list, tuple, dict
	specialized	function, job, channel, user-defined-object

When declaring a variable without an initializer, an explicit type must be
provided.  Each category has different default initialization semantics.

Primitives default to type-specific values.  All primitives are empty but do
not equal `null`: >vim9

    vim9script
    var n: number | echo [n, n->empty(), n == null]  # [0, 1, false]
    var f: float  | echo [f, f->empty(), f == null]  # [0.0, 1, false]
    var b: bool   | echo [b, b->empty(), b == null]  # [false, 1, false]
<
Containers default to an empty container.  Only an empty string equals `null`: >vim9

    vim9script
    var s: string       | echo [s, s->empty(), s == null]  # ['', 1, true]
    var z: blob         | echo [z, z->empty(), z == null]  # [0z, 1, false]
    var l: list<string> | echo [l, l->empty(), l == null]  # [[], 1, false]
    var t: tuple<any>   | echo [t, t->empty(), t == null]  # [(), 1, false]
    var d: dict<number> | echo [d, d->empty(), d == null]  # [{}, 1, false]
<
Specialized types default to equaling `null`: >vim9

    vim9script
    var F: func    | echo [F, F == null]  # [function(''), true]
    var j: job     | echo [j, j == null]  # ['no process', true]
    var c: channel | echo [c, c == null]  # ['channel fail', true]
    class Class
    endclass
    var o: Class   | echo [o, o == null]  # [object of [unknown], true]
    enum Enum
    endenum
    var e: Enum    | echo [e, e == null]  # [object of [unknown], true]
<
	  Note: See |empty()| for explanations of empty job, empty channel, and
		empty object types.

Vim does not have a familiar null value.  Instead, it has various null_<type>
predefined values including |null_string|, |null_list|, and |null_job|.
Primitives do not have a null_<type>.  Typical use cases for null_<type> are:
    - to clear a variable and release its resources,
    - as a default for a parameter in a function definition (for an example,
      see |null_blob|), or
    - assigned to a container or specialized variable to set it to null
      for later comparison (for an example, see |null-compare|).

For a specialized variable, like `job`, null_<type> is used to clear the
resources.  For example: >vim9

	vim9script
	var mydate: list<string>
	def Date(channel: channel, msg: string): void
	    mydate->add(msg)
	enddef
	var myjob = job_start([&shell, &shellcmdflag, 'date'], {out_cb: Date})
	echo [myjob, myjob->job_status()]
	sleep 2
	echo $"The date and time is {mydate->join('')}"
	echo [myjob, myjob->job_status()]
	myjob = null_job  # Clear the variable; release the job's resources.
	echo myjob
<
For a container variable, resources may also be cleared by assigning an
empty container to the variable.  For example: >vim9

	vim9script
	var perfect: list<number> = [1, 4]
	perfect->extend([9, 16, 25])
	perfect = []
	echo perfect

Using an empty container, rather than null_<type>, to clear a container
variable may avoid null complications - see |null-anomalies|.

The initialization semantics of container variables and specialized variables
differ.  For containers:
  - An uninitialized container defaults to empty but does not equal `null`
    (except for a uninitialized string).
  - A container initialized to [], (), {}, "", or 0z is empty but does not
    equal `null`.
  - A container initialized as null_<type> defaults to empty and equals `null`.

In the following example, the uninitialized list ("lu") and [] initialized
list ("li") are equivalent and indistinguishable whereas "ln" is a null
container, which is similar to, but not equivalent to, an empty container
(see |null-anomalies|). >vim9

	vim9script
	# uninitialized: empty container, not null
	var lu: list<any>
	echo ['lu', $"empty={lu->empty()}", $"null={lu == null}"]
	# initialized: empty container, not null
	var li: list<any> = []
	echo ['li', $"empty={li->empty()}", $"null={li == null}"]
	# initialized: empty container, null
	var ln: list<any> = null_list
	echo ['ln', $"empty={ln->empty()}", $"null={ln == null}"]
<
Specialized variables default to equaling null.  These job initializations
are equivalent and indistinguishable: >vim9

	vim9script
	var j1: job
	var j2: job = null_job
	var j3 = null_job
	echo (j1 == j2) == (j2 == j3)  # true (equivalent, indistinguishable)
<
When a list, tuple, or dict is declared, if the item type is not specified
it cannot be inferred.  Consequently, the item type defaults to "any": >vim9

	vim9script
	var [t1, t2] = [(), null_tuple]
	echo $'t1 is {t1->typename()} and t2 is {t2->typename()} too'
<
Tuples and functions (or partials) may be declared in various ways.
See |tuple-type|, |variadic-tuple|, and |vim9-func-declaration|.

						*null-compare*
For familiar null compare semantics, where an empty container is not equal to
a null container, do not use null_<type> in a comparison.  That is because,
in Vim9 script, although null_<type> == `null`, comparing an:

  - empty container to `null` is `false`, but
  - empty container to null_<type> is `true`.

So, compare against `null`, not null_<type>.  For example: >vim9

	vim9script
	var bonds: dict<list<string>> = {g: ['007', '008'], o: ['007', '009']}
	def Search(query: string): list<string>
	    return query == "\r" ? null_list : bonds->get(query, [])
	enddef
	echo "Goldfinger (g) or Octopussy (o)?: "
	const C: string = getcharstr()
	var result: list<string> = C->Search()
	if result == null  # <<< DO NOT USE null_list HERE!
	    echo "Error: Nothing was entered"
	else
	    echo result->empty() ? $"No matches for '{C}'" : $"{result}"
	endif
<
	  NOTE: Using "result == null_list" instead of "result == null" would
		fail to distinguish the error (nothing entered) and the valid
		(nothing matched) result because [] == null_list whereas
		[] != null.

Conceptually, think of the null_<type> construct as a hybrid/bridge between
the general `null` and typed `empty` containers, having properties of both.
In the following section there are details about comparison results.

					*null-details* *null-anomalies*
This section describes issues about using null and null_<type>; included below
are the enumerated results of null comparisons.  In some cases, if familiar
with vim9 null semantics, the programmer may choose to use null_<type> in
comparisons and/or other situations.

Elsewhere in the documentation it says, "often a null value is handled the
same as an empty value, but not always".  For example, you cannot add to a
null container: >vim9

	vim9script
	var le: list<any> = []
	le->add('Okay')		# le is now ['Okay']
	var ln = null_list
	ln->add("E1130")	# E1130: Cannot add to null list
<
As explained in |null-compare|, there is a non-transitive relationship among
`null`, null_<type> containers, and `empty`.  To recap, for example: >vim9

	vim9cmd echo (null_dict == {}, null_dict == null, {} != null)
<
The exception is an uninitialized string.  It is equal to `null` (and is the
same instance as `null_string`).  The 'is' operator (|expr-is|) may be used to
determine whether a string is uninitialized: >vim9

	vim9script
	var s: string
	echo s == null_string		# true
	echo s is null_string		# true (the same instance)
	echo s == null			# true (unexpected, perhaps)
	echo s is null			# false (not the same instance)
<
However, don't do the same for the other containers because, when evaluated
against their applicable null_<type> with 'is', they return `false`: >vim9

	vim9script
	var d: dict<any>
	echo d == null_dict		# true
	echo d is null_dict		# false (not the same instance)
	echo d == null			# false (as expected)
	echo d is null			# false (not the same instance)
<
The key distinction here is an uninitialized string is implemented as
`null_string`, while an uninitialized list, dict, tuple, or blob is
implemented as an empty container ([], {}, (), and 0z respectively).
So, those uninitialized types are equal to, but not the same instance as,
their null_<type> counterparts, as this example shows: >vim9

	vim9script
	var t: tuple<any>
	echo t == null_tuple		# true
	echo t is null_tuple		# false

However, a variable initialized to the null_<type> is equal not only to the
null_<type>, it is also equal to null.  For example: >vim9

	vim9script
	var t: tuple<any> = null_tuple
	echo t == null_tuple		# true
	echo t is null_tuple		# true
	echo t == null			# true
<
An uninitialized container variable is not equal to null, except for an
uninitialized string, which is explained in an example, above.  So, these
all echo `true`: >vim9

	vim9script
	var b: blob		| echo b != null
	var d: dict<any>	| echo d != null
	var l: list<any>	| echo l != null
	var t: tuple<any>	| echo t != null
	var s: string		| echo s == null

An uninitialized specialized variable is equal to null so these all echo `true`: >vim9

	vim9script
	var c: channel		| echo c == null
	var F: func		| echo F == null
	var j: job		| echo j == null
	class Class
	endclass
	var nc: Class		| echo nc == null
	enum Enum
	endenum
	var ne: Enum		| echo ne == null
<
	  Note: the specialized variables, like job, default to null and
		no specialized variable has a corresponding empty value.

A container variable initialized to empty equals null_<type>, so these are all
`true`: >vim9

	vim9script
	var s: string = ""     | echo s == null_string
	var b: blob = 0z       | echo b == null_blob
	var l: list<any> = []  | echo l == null_list
	var t: tuple<any> = () | echo t == null_tuple
	var d: dict<any> = {}  | echo d == null_dict
<
However, a container variable initialized to empty does not equal null, so
these are all `true`: >vim9

	vim9script
	var s: string = ""     | echo s != null
	var b: blob = 0z       | echo b != null
	var l: list<any> = []  | echo l != null
	var t: tuple<any> = () | echo t != null
	var d: dict<any> = {}  | echo d != null
<

==============================================================================

						*generic-functions*
5. Generic functions

A generic function allows using the same function with different type
arguments, while retaining type checking for arguments and the return value.
This provides type safety and code reusability.


Declaration~
						*generic-function-declaration*
						*E1553* *E1554*
The type variables for a generic function are declared as its type parameters
within angle brackets ("<" and ">"), directly after the function name.
Multiple type parameters are separated by commas: >

	def[!] {funcname}<{type} [, {types}]>([arguments])[: {return-type}]
	    {function body}
	enddef
<						*generic-function-example*
These type parameters may then be used, like any other type, within the
function signature and its body.  The following example combines two lists
into a list of tuples: >vim9

	vim9script
	def Zip<T, U>(first: list<T>, second: list<U>): list<tuple<T, U>>
	    const LEN: number = ([first->len(), second->len()])->min()
	    final result: list<tuple<T, U>> = []
	    for i in range(LEN)
	        result->add((first[i], second[i]))
	    endfor
	    return result
	enddef
	var n: list<number> = [61, 62, 63]
	var s: list<string> = ['a', 'b', 'c']
	echo $"Zip example #1: {Zip<number, string>(n, s)}"
	echo $"Zip example #2: {Zip<string, number>(s, n)}"
<
						*type-variable-naming* *E1552*
						*type-parameter-naming*
As in the preceding example, the convention is to use a single capital letter
for a name (e.g., T, U, A, etc.).  Although they may comprise more than one
letter, names must start with a capital letter.  In this example, "Ok" is
valid whereas "n" is not: >vim9

	vim9script
	def MyFail<Ok, n>(): void
	enddef
	# E1552: Type variable name must start with an uppercase letter: n...
<
						*E1558* *E1560*
A function must be declared and used either as a generic function or as a
regular function, but not both.  The following Vim9 scripts demonstrate these
errors: >vim9

	vim9script
	My1558<number>()
	# E1558: Unknown generic function: My1558
< >vim9
	vim9script
	def My1560(): void
	enddef
	My1560<string>()
	# E1560: Not a generic function: My1560
<
						*E1561*
Type parameter names must not clash with other identifiers: >vim9

	vim9script
	def My1561<D, E, D>(): D
	enddef
	# E1561: Duplicate type variable name: D

	vim9script
	enum E
	  Yes, No
	endenum
	def My1041<E>(): E
	enddef
	# E0141: Redefining script item "E"
<

Calling a generic function~
						*generic-function-call*
To call a generic function, specify the concrete types in "<" and ">"
between the function name and the argument list: >

	MyFunc<number, string, list<number>>()
<
	NOTE: There are several working examples in this section, which may
	      be sourced, including |generic-function-example|.

					*E1555* *E1556* *E1557* *E1559*
The number of passed type arguments to the function must match the number
of its declared type parameters.  An empty type list is not allowed.
Examples: >vim9

	vim9script
	def My1555<>(): void
	enddef
	# E1555: Empty type list specified for generic function ...
< >vim9
	vim9script
	def My1556<T>(): void
	enddef
	My1556<bool, bool>()
	# E1556: Too many types specified for generic function ...
< >vim9
	vim9script
	def My1557<T, U>(): void
	enddef
	My1557<bool>()
	# E1557: Not enough types specified for generic function ...
< >vim9
	vim9script
	def My1559<T>(): T
	enddef
	My1559()
	# Vim(eval):E1559: Type arguments missing for generic function ...
<
Any Vim9 type (|vim9-types|) can be used as a concrete type in a generic
function.

Spaces are not allowed:
- Between the function name and "<" (|E1068|)
- Between ">" and the opening "(" (|E1068|), or
- Within the "<" and ">", except where required after the comma separating
  the types (|E1202|).

A generic function can be exported and imported like a regular function.
See |:export| and |:import|.

A generic function can be defined inside another regular or generic function.
Example: >vim9
	vim9script
	def Outer(): void
	  # Returns either the first item of a list or a default value
	  def FirstOrDefault<T, U>(lst: list<T>, default: U): any
	    return lst->len() > 0 ? lst[0] : default
	  enddef
	  echo FirstOrDefault<string, bool>(['B', 'C'], false)	# echos B
	  echo FirstOrDefault<number, number>([], 42)		# echos 42
	enddef
	Outer()
<

Using a type variable as a type argument ~

A type variable may also be passed as a type argument.  For example: >vim9

	vim9script
	# T is declared as a type parameter
	# It is used for the 'value' parameter and the return type
	def Id<T>(value: T): T
	    return value
	enddef
	# U is declared as a type parameter
	# It is used for the 'value' parameter and the return type
	def CallId<U>(value: U): U
	    # U is a type variable passed/used as a type argument
	    return Id<U>(value)
	enddef
	echo CallId<string>('I am') .. ' ' .. CallId<number>(42)

This is useful for complex data structures like dictionaries of lists or,
as in the following example, lists of dictionaries: >vim9

	vim9script
	def Flatten<T>(x: list<list<T>>): list<T>
	    final result: list<T> = []
	    for inner in x
	        result->extend(inner)
	    endfor
	    return result
	enddef
	const ENGLISH: list<dict<string>> = [{1: 'one'}, {2: 'two'}]
	const MANDARIN: list<dict<string>> = [{1: '壹'}, {2: '贰'}]
	const ARABIC_N: list<dict<number>> = [{1: 1}, {2: 2}]
	echo Flatten<dict<string>>([ENGLISH, MANDARIN])
	echo Flatten<dict<any>>([ENGLISH, ARABIC_N])
<
In "Flatten<T>", "T" is a declared type parameter.  Everywhere else in
the function, "T" is a type variable referencing that type parameter.


Generic class method~

A Vim9 class method can be a generic function: >vim9

	vim9script
	class Config
	    var settings: dict<any>
	    def Get<T>(key: string): T
	        return this.settings[key]
	    enddef
	endclass
	var c: Config = Config.new({timeout: 30, debug: true})
	echo c.Get<number>('timeout')
	echo c.Get<bool>('debug')
<
						*E1432* *E1433* *E1434*
A generic class method in a base class can be overridden by a generic method
in a child class.  The number of type variables must match between both
methods.  A concrete class method cannot be overridden by a generic method,
and vice versa.


Generic function reference~

A function reference (|Funcref|) can be a generic function.  This allows for
creating factories of functions that operate on specific types: >vim9

	vim9script
	# Match a specified character in a string or the decimal value of the
	# character in a list.  Note: '*' is decimal 42 (U+002A)
	var c: string = "*"
	var char_dec: tuple<string, string> = (c, c->char2nr()->string())
	def Matcher<T>(pattern: string): func(T): bool
	    return (value: T): bool => match(value, pattern) >= 0
	enddef
	var StringMatch = Matcher<string>(char_dec[0])
	echo "*+"->StringMatch()			    # true (has *)
	echo ",-"->StringMatch()			    # false
	var ListMatch = Matcher<list<number>>(char_dec[1])
	echo [42, 43]->ListMatch()			    # true (has 42)
	echo [44, 45]->ListMatch()			    # false
<

Compiling and Disassembling Generic functions~

The |:defcompile| command can be used to compile a generic function with a
specific list of concrete types: >

	defcompile MyFunc<number, list<number>, dict<string>>
<
The |:disassemble| command can be used to list the instructions generated for
a generic function: >

	disassemble MyFunc<string, dict<string>>
	disassemble MyFunc<number, list<blob>>
<

Limitations and Future Work~

Currently, Vim does not support:
   - Type inference for type variables: All types must be explicitly specified
     when calling a generic function.
   - Type constraints: It's not possible to restrict a type variable to a
     specific class or interface (e.g., `T extends SomeInterface`).
   - Default type arguments: Providing a default type for a type parameter
     when not explicitly specified.

==============================================================================

6. Namespace, Import and Export
				*vim9script* *vim9-export* *vim9-import*

A Vim9 script can be written to be imported.  This means that some items are
intentionally exported, made available to other scripts.  When the exporting
script is imported in another script, these exported items can then be used in
that script.  All the other items remain script-local in the exporting script
and cannot be accessed by the importing script.

This mechanism exists for writing a script that can be sourced (imported) by
other scripts, while making sure these other scripts only have access to what
you want them to.  This also avoids using the global namespace, which has a
risk of name collisions.  For example when you have two plugins with similar
functionality.

You can cheat by using the global namespace explicitly.  That should be done
only for things that really are global.


Namespace ~
							*vim9-namespace*
To recognize a file that can be imported the `vim9script` statement must
appear as the first statement in the file (see |vim9-mix| for an exception).
It tells Vim to interpret the script in its own namespace, instead of the
global namespace.  If a file starts with: >
	vim9script
	var myvar = 'yes'
Then "myvar" will only exist in this file.  While without `vim9script` it would
be available as `g:myvar` from any other script and function.
							*E1101*
The variables at the file level are very much like the script-local "s:"
variables in legacy Vim script, but the "s:" is omitted.  And they cannot be
deleted.

In Vim9 script the global "g:" namespace can still be used as before.  And the
"w:", "b:" and "t:" namespaces.  These have in common that variables are not
declared, have no specific type and they can be deleted.  *E1304*

A side effect of `:vim9script` is that the 'cpoptions' option is set to the
Vim default value, like with: >
	:set cpo&vim
One of the effects is that |line-continuation| is always enabled.
The original value of 'cpoptions' is restored at the end of the script, while
flags added or removed in the script are also added to or removed from the
original value to get the same effect.  The order of flags may change.
In the |vimrc| file sourced on startup this does not happen.

							*vim9-mix*
There is one way to use both legacy and Vim9 syntax in one script file: >vim9

	" _legacy Vim script_ comments here
	if !has('vim9script')
	   " _legacy Vim script_ comments/commands here
	   finish
	endif
	vim9script
	# _Vim9 script_ commands/commands from here onwards
	echowindow $"has('vim9script') == {has('vim9script')}"
<
This allows for writing a script that takes advantage of the Vim9 script
syntax if possible, and prevents the vim9script command from throwing an
error if used in a version of Vim without 'vim9script'.

Note that Vim9 syntax changed before Vim 9 so that scripts using the current
syntax (such as "import from" instead of "import") might throw errors.
To prevent these, a safer check may be |v:version| >= 900 instead (because
"has('vim9script')" will return `v:true` back to Vim 8.2 with patch 3965).
Sometimes it is prudent to cut off even later.  Vim9 script's feature set
continues to grow so, for example, if tuples are used (introduced in Vim 9.1
patch 1232), a better condition is: >vim9

	if !has('patch-9.1.1232')
	  echowindow $"Fail: Vim does not have patch 9.1.1232"
	  finish
	endif
	vim9script
	echowindow $"Pass: version {v:versionlong}.  Continuing ..."
<
Whichever vim-mix condition is used, it only works in one of two ways:
    1. The "if" statement evaluates to false, the commands up to `endif` are
       skipped and `vim9script` is then the first command actually executed.
    2. The "if" statement evaluates to true, the commands up to `endif` are
       executed and `finish` bails out before reaching `vim9script`.


Export ~
							*:export* *:exp*
Exporting an item can be written as: >
	export const EXPORTED_CONST = 1234
	export var someValue = ...
	export final someValue = ...
	export const someValue = ...
	export def MyFunc() ...
	export class MyClass ...
	export interface MyClass ...
	export enum MyEnum ...
<							*E1043* *E1044*
As this suggests, only constants, variables, `:def` functions, classes,
interfaces and enums can be exported.

							*E1042*
`:export` can only be used in Vim9 script, at the script level.


Import ~
				*:import* *:imp* *E1094* *E1047* *E1262*
				*E1048* *E1049* *E1053* *E1071* *E1088* *E1236*
The exported items can be imported in another script.  The import syntax has
two forms.  The simple form: >
	import {filename}
<
Where {filename} is an expression that must evaluate to a string.  In this
form the filename should end in ".vim" and the portion before ".vim" will
become the script local name of the namespace.  For example: >
	import "myscript.vim"
<
This makes each exported item in "myscript.vim" available as "myscript.item".
						*:import-as* *E1257* *E1261*
In case the name is long or ambiguous, this form can be used to specify
another name: >
	import {longfilename} as {name}
<
In this form {name} becomes a specific script local name for the imported
namespace.  Therefore {name} must consist of letters, digits and '_', like
|internal-variables|.  The {longfilename} expression must evaluate to any
filename.  For example: >
	import "thatscript.vim.v2" as that
<						*E1060* *E1258* *E1259* *E1260*
Then you can use "that.item", etc.  You are free to choose the name "that".
Use something that will be recognized as referring to the imported script.
Avoid command names, command modifiers and builtin function names, because the
name will shadow them.  It's better not to start the name with a capital
letter, since it can then also shadow global user commands and functions.
Also, you cannot use the name for something else in the script, such as a
function or variable name.

In case the dot in the name is undesired, a local reference can be made for a
function: >
	var LongFunc = that.LongFuncName

This also works for constants: >
	const MAXLEN = that.MAX_LEN_OF_NAME

This does not work for variables, since the value would be copied once and
when changing the variable the copy will change, not the original variable.
You will need to use the full name, with the dot.

`:import` can not be used in a function.  Imported items are intended to exist
at the script level and only imported once.

The script name after `import` can be:
- A relative path, starting "." or "..".  This finds a file relative to the
  location of the script file itself.  This is useful to split up a large
  plugin into several files.
- An absolute path, starting with "/" on Unix or "D:/" on MS-Windows.  This
  will rarely be used.
- A path not being relative or absolute.  This will be found in the
  "import" subdirectories of 'runtimepath' entries.  The name will usually be
  longer and unique, to avoid loading the wrong file.
  Note that "after/import" is not used.

If the name does not end in ".vim" then the use of "as name" is required.

Once a Vim9 script file has been imported, the result is cached and used the
next time the same script is imported.  It will not be read again.

It is not allowed to import the same script twice, also when using two
different "as" names.

When using the imported name the dot and the item name must be in the same
line, there can be no line break: >
	echo that.
		name   # Error!
	echo that
		.name  # Error!
<						*import-map*
When you've imported a function from one script into a Vim9 script you can
refer to the imported function in a mapping by prefixing it with |<SID>|: >
	noremap <silent> ,a :call <SID>name.Function()<CR>

When the mapping is defined "<SID>name." will be replaced with <SNR> and the
script ID of the imported script.
An even simpler solution is using |<ScriptCmd>|: >
	noremap ,a <ScriptCmd>name.Function()<CR>

Note that this does not work for variables, only for functions.

					    *import-legacy* *legacy-import*
`:import` can also be used in legacy Vim script.  The imported namespace still
becomes script-local, even when the "s:" prefix is not given.  For example: >
	import "myfile.vim"
	call s:myfile.MyFunc()

And using the "as name" form: >
	import "otherfile.vim9script" as that
	call s:that.OtherFunc()

However, the namespace cannot be resolved on its own: >
	import "that.vim"
	echo s:that
	" ERROR: E1060: Expected dot after name: s:that
<
This also affects the use of |<SID>| in the legacy mapping context.  Since
|<SID>| is only a valid prefix for a function and NOT for a namespace, you
cannot use it to scope a function in a script local namespace.  Instead of
prefixing the function with |<SID>| you should use |<ScriptCmd>|.  For example:
>
	noremap ,a <ScriptCmd>:call s:that.OtherFunc()<CR>
<
							*:import-cycle*
The `import` commands are executed when encountered.  If script A imports
script B, and B (directly or indirectly) imports A, this will be skipped over.
At this point items in A after "import B" will not have been processed and
defined yet.  Therefore cyclic imports can exist and not result in an error
directly, but may result in an error for items in A after "import B" not being
defined.  This does not apply to autoload imports, see the next section.


Importing an autoload script ~
					*vim9-autoload* *import-autoload*
For optimal startup speed, loading scripts should be postponed until they are
actually needed.  Using the autoload mechanism is recommended:
							*E1264*
     1. In the plugin, define user commands, functions and/or mappings
        referring to items imported from an autoload script. >

	import autoload 'for/search.vim'
	command -nargs=1 SearchForStuff search.Stuff(<f-args>)

<        This goes in .../plugin/anyname.vim.  "anyname.vim" can be freely
        chosen.  The "SearchForStuff" command is now available to the user.

        The "autoload" argument to `:import` means that the script is not
        loaded until one of the items is actually used.  The script will be
        found under the "autoload" directory in 'runtimepath' instead of the
        "import" directory.  Alternatively, either a relative or absolute
        name can be used - see below.

     2. In the autoload script put the bulk of the code. >

	vim9script
	export def Stuff(arg: string): void
	  ...

<       This goes in .../autoload/for/search.vim.

       Putting the "search.vim" script under the "/autoload/for/" directory
       has the effect that "for#search#" will be prefixed to every exported
       item.  The prefix is obtained from the file name, just as you would
       add it manually in a legacy autoload script.  Thus the exported
       function can be found with "for#search#Stuff", but you would normally
       use `import autoload` and not use the prefix (which has the side effect
       of loading the autoload script when compiling a function that
       encounters this name).

       You can split up the functionality and import other scripts from the
       autoload script as you like.  This way you can share code between
       plugins.

Searching for the autoload script in all entries in 'runtimepath' can be a bit
slow.  If the plugin knows where the script is located, quite often a relative
path can be used.  This avoids the search and should be quite a bit faster.
Another advantage is that the script name does not need to be unique.  Also,
an absolute path is possible.  Examples: >
	import autoload '../lib/implement.vim'
	import autoload MyScriptsDir .. '/lib/implement.vim'

For defining a mapping that uses the imported autoload script the special key
|<ScriptCmd>| is useful.  It allows for a command in a mapping to use the
script context of where the mapping was defined.

When compiling a `:def` function and a function in an autoload script is
encountered, the script is not loaded until the `:def` function is called.
This also means you get any errors only at runtime, since the argument and
return types are not known yet.  If you would use the name with '#' characters
then the autoload script IS loaded.

Be careful to not refer to an item in an autoload script that does trigger
loading it unintentionally.  For example, when setting an option that takes a
function name, make sure to use a string, not a function reference: >
	import autoload 'qftf.vim'
	&quickfixtextfunc = 'qftf.Func'  # autoload script NOT loaded
	&quickfixtextfunc = qftf.Func    # autoload script IS loaded
On the other hand, it can be useful to load the script early, at a time when
any errors should be given.

For testing the |test_override()| function can be used to have the
`import autoload` load the script right away, so that the items and types can
be checked without waiting for them to be actually used: >
	test_override('autoload', 1)
Reset it later with: >
	test_override('autoload', 0)
Or: >
	test_override('ALL', 0)


==============================================================================

7. Classes and interfaces				*vim9-classes*

In legacy Vim script, a Dictionary could be used as a kind-of object by adding
members that are functions.  However, this is quite inefficient and requires
the writer to do the work of making sure all the objects have the right
members.  See |Dictionary-function|.

In |Vim9| script you can have classes, objects, interfaces, and enums like
in most popular object-oriented programming languages.  Since this is a lot
of functionality, it is located in a separate help file: |vim9class.txt|.


==============================================================================

8. Rationale						*vim9-rationale*

The :def command ~

Plugin writers have asked for much faster Vim script.  Investigations have
shown that keeping the existing semantics of function calls make this close to
impossible, because of the overhead involved with calling a function, setting
up the local function scope and executing lines.  There are many details that
need to be handled, such as error messages and exceptions.  The need to create
a dictionary for a: and l: scopes, the a:000 list and several others add too
much overhead that cannot be avoided.

Therefore the `:def` method to define a new-style function had to be added,
which allows for a function with different semantics.  Most things still work
as before, but some parts do not.  A new way to define a function was
considered the best way to separate the legacy style code from Vim9 style
code.

Using "def" to define a function comes from Python.  Other languages use
"function" which clashes with legacy Vim script.


Type checking ~

When compiling lines of Vim commands into instructions as much as possible
should be done at compile time.  Postponing it to runtime makes the execution
slower and means mistakes are found only later.  For example, when
encountering the "+" character and compiling this into a generic add
instruction, at runtime the instruction would have to inspect the type of the
arguments and decide what kind of addition to do.  And when the type is
dictionary throw an error.  If the types are known to be numbers then an "add
number" instruction can be used, which is faster.  The error can be given at
compile time, no error handling is needed at runtime, since adding two numbers
almost never fails.

    NOTE: As a tangential point, the exception is integer overflow, where the
    result exceeds the maximum integer value.  For example, adding to a 64-bit
    signed integer where the result is greater than 2^63: >vim9

	vim9script
	echo 9223372036854775807 + 1     # -9223372036854775808
	echo 2->pow(63)->float2nr() + 1  # -9223372036854775808
<
The syntax for types, using <type> for compound types, is similar to Java.
It is easy to understand and widely used.  The type names are what were used
in Vim before, with some additions such as "void" and "bool".


Removing clutter and weirdness ~

Once decided that `:def` functions have different syntax than legacy functions,
we are free to add improvements to make the code more familiar for users who
know popular programming languages.  In other words: remove weird things that
only Vim does.

We can also remove clutter, mainly things that were done to make Vim script
backwards compatible with the good old Vi commands.

Examples:
- Drop `:call` for calling a function and `:eval` for evaluating an
  expression.
- Drop using a leading backslash for line continuation, automatically figure
  out where an expression ends.

However, this does require that some things need to change:
- Comments start with # instead of ", to avoid confusing them with strings.
  This is good anyway, it is also used by several popular languages.
- Ex command ranges need to be prefixed with a colon, to avoid confusion with
  expressions (single quote can be a string or a mark, "/" can be divide or a
  search command, etc.).

Goal is to limit the differences.  A good criteria is that when the old syntax
is accidentally used you are very likely to get an error message.


Syntax and semantics from popular languages ~

Script writers have complained that the Vim script syntax is unexpectedly
different from what they are used to.  To reduce this complaint popular
languages are used as an example.  At the same time, we do not want to abandon
the well-known parts of legacy Vim script.

For many things TypeScript is followed.  It's a recent language that is
gaining popularity and has similarities with Vim script.  It also has a
mix of static typing (a variable always has a known value type) and dynamic
typing (a variable can have different types, this changes at runtime).  Since
legacy Vim script is dynamically typed and a lot of existing functionality
(esp. builtin functions) depends on that, while static typing allows for much
faster execution, we need to have this mix in Vim9 script.

There is no intention to completely match TypeScript syntax and semantics.  We
just want to take those parts that we can use for Vim and we expect Vim users
will be happy with.  TypeScript is a complex language with its own history,
advantages and disadvantages.  To get an idea of the disadvantages read the
book: "JavaScript: The Good Parts".  Or find the article "TypeScript: the good
parts" and read the "Things to avoid" section.

People familiar with other languages (Java, Python, etc.) will also find
things in TypeScript that they do not like or do not understand.  We'll try to
avoid those things.

Specific items from TypeScript we avoid:
- Overloading "+", using it both for addition and string concatenation.  This
  goes against legacy Vim script and often leads to mistakes.  For that reason
  we will keep using ".." for string concatenation.  Lua also uses ".." this
  way.  And it allows for conversion to string for more values.
- TypeScript can use an expression like "99 || 'yes'" in a condition, but
  cannot assign the value to a boolean.  That is inconsistent and can be
  annoying.  Vim recognizes an expression with && or || and allows using the
  result as a bool.  The |falsy-operator| was added for the mechanism to use a
  default value.
- TypeScript considers an empty string as Falsy, but an empty list or dict as
  Truthy.  That is inconsistent.  In Vim an empty list and dict are also
  Falsy.
- TypeScript has various "Readonly" types, which have limited usefulness,
  since a type cast can remove the immutable nature.  Vim locks the value,
  which is more flexible, but is only checked at runtime.
- TypeScript has a complicated "import" statement that does not match how the
  Vim import mechanism works.  A much simpler mechanism is used instead, which
  matches that the imported script is only sourced once.


Declarations ~

Legacy Vim script uses `:let` for every assignment, while in Vim9 declarations
are used.  That is different, thus it's good to use a different command:
`:var`.  This is used in many languages.  The semantics might be slightly
different, but it's easily recognized as a declaration.

Using `:const`  for constants is common, but the semantics varies.  Some
languages only make the variable immutable, others also make the value
immutable.  Since "final" is well known from Java for only making the variable
immutable we decided to use that.  And then `:const` can be used for making
both immutable.  This was also used in legacy Vim script and the meaning is
almost the same.

What we end up with is very similar to Dart: >
	:var name	# mutable variable and value
	:final name	# immutable variable, mutable value
	:const name	# immutable variable and value

Since legacy and Vim9 script will be mixed and global variables will be
shared, optional type checking is desirable.  Also, type inference will avoid
the need for specifying the type in many cases.  The TypeScript syntax fits
best for adding types to declarations: >
	var name: string	  # string type is specified
	...
	name = 'John'
	const greeting = 'hello'  # string type is inferred

This is how we put types in a declaration: >
	var mylist: list<string>
	final mylist: list<string> = ['foo']
	def Func(arg1: number, arg2: string): bool

Two alternatives were considered:
    1. Put the type before the name, like Dart: >
	var list<string> mylist
	final list<string> mylist = ['foo']
	def Func(number arg1, string arg2) bool
<    2. Put the type after the variable name, but do not use a colon, like Go: >
	var mylist list<string>
	final mylist list<string> = ['foo']
	def Func(arg1 number, arg2 string) bool

The first is more familiar for anyone used to C or Java.  The second one
doesn't really have an advantage over the first, so let's discard the second.

Since we use type inference the type can be left out when it can be inferred
from the value.  This means that after `var` we don't know if a type or a name
follows.  That makes parsing harder, not only for Vim but also for humans.
Also, it will not be allowed to use a variable name that could be a type name,
using `var string string` is too confusing.

The chosen syntax, using a colon to separate the name from the type, adds
punctuation, but it actually makes it easier to recognize the parts of a
declaration.


Expressions ~

Expression evaluation was already close to what other languages are doing.
Some details are unexpected and can be improved.  For example a boolean
condition would accept a string, convert it to a number and check if the
number is non-zero.  This is unexpected and often leads to mistakes, since
text not starting with a number would be converted to zero, which is
considered false.  Thus using a string for a condition would often not give an
error and be considered false.  That is confusing.

In Vim9 type checking is stricter to avoid mistakes.  Where a condition is
used, e.g. with the `:if` command and the `||` operator, only boolean-like
values are accepted:
	true:  `true`, `v:true`, `1`, `0 < 9`
	false: `false`, `v:false`, `0`, `0 > 9`
Note that the number zero is false and the number one is true.  This is more
permissive than most other languages.  It was done because many builtin
functions return these values, and changing that causes more problems than it
solves.  After using this for a while it turned out to work well.

If you have any type of value and want to use it as a boolean, use the `!!`
operator (see |expr-!|): >vim9

	vim9script
	# The following are all true:
	echo [!!'text', !![1], !!{'x': 1}, !!1, !!1.1]
	# And these are all false:
	echo [!!'', !![], !!{}, !!0, !!0.0]
<
From a language like JavaScript we have this handy construct: >
	GetName() || 'unknown'
However, this conflicts with only allowing a boolean for a condition.
Therefore the "??" operator was added: >
	GetName() ?? 'unknown'
Here you can explicitly express your intention to use the value as-is and not
result in a boolean.  This is called the |falsy-operator|.


Import and Export ~

A problem of legacy Vim script is that by default all functions and variables
are global.  It is possible to make them script-local, but then they are not
available in other scripts.  This defies the concept of a package that only
exports selected items and keeps the rest local.

In Vim9 script a mechanism very similar to the JavaScript import and export
mechanism is supported.  It is a variant to the existing `:source` command
that works like one would expect:
- Instead of making everything global by default, everything is script-local,
  some of these are exported.
- When importing a script the symbols that are imported are explicitly listed,
  avoiding name conflicts and failures if functionality is added later.
- The mechanism allows for writing a big, long script with a very clear API:
  the exported functions, variables and classes.
- By using relative paths loading can be much faster for an import inside of a
  package, no need to search many directories.
- Once an import has been used, its items are cached and loading it again is
  not needed.
- The Vim-specific use of "s:" to make things script-local can be dropped.

When sourcing a Vim9 script (from either a Vim9 script or legacy Vim script),
only the items defined globally can be used, not the exported items.
Alternatives considered:
- All the exported items become available as script-local items.  This makes
  it uncontrollable what items get defined and likely soon leads to trouble.
- Use the exported items and make them global.  Disadvantage is that it's then
  not possible to avoid name clashes in the global namespace.
- Completely disallow sourcing a Vim9 script, require using `:import`.  That
  makes it difficult to use scripts for testing, or sourcing them from the
  command line to try them out.
Note that you CAN also use `:import` in legacy Vim script, see above.


Compiling functions early ~

Functions are compiled when called or when `:defcompile` is used.  Why not
compile them early, so that syntax and type errors are reported early?

The functions can't be compiled right away when encountered, because there may
be forward references to functions defined later.  Consider defining functions
A, B and C, where A calls B, B calls C, and C calls A again.  It's impossible
to reorder the functions to avoid forward references.

An alternative would be to first scan through the file to locate items and
figure out their type, so that forward references are found, and only then
execute the script and compile the functions.  This means the script has to be
parsed twice, which is slower, and some conditions at the script level, such
as checking if a feature is supported, are hard to use.  An attempt was made
to see if it works, but it turned out to be impossible to make work well.

It would be possible to compile all the functions at the end of the script.
The drawback is that if a function never gets called, the overhead of
compiling it counts anyway.  Since startup speed is very important, in most
cases it's better to do it later and accept that syntax and type errors are
only reported then.  In case these errors should be found early, e.g. when
testing, a `:defcompile` command at the end of the script will help out.


Why not use an existing embedded language? ~

Vim supports interfaces to Perl, Python, Lua, Tcl and a few others.  But
these interfaces have never become widely used, for various reasons.  When
Vim9 was designed a decision was made to make these interfaces lower priority
and concentrate on Vim script.

Still, plugin writers may find other languages more familiar, want to use
existing libraries or see a performance benefit.  We encourage plugin authors
to write code in any language and run it as an external process, using jobs
and channels.  We can try to make this easier somehow.

Using an external tool also has disadvantages.  An alternative is to convert
the tool into Vim script.  For that to be possible without too much
translation, and keeping the code fast at the same time, the constructs of the
tool need to be supported.  Since Vim9 script now includes support for
classes, objects, interfaces, and enums, that is increasingly feasible.



 vim:tw=78:ts=8:noet:ft=help:norl:
