Enum.map_every
You're seeing just the function
map_every, go back to Enum module for more information.
Specs
map_every(t(), non_neg_integer(), (element() -> any())) :: list()
Returns a list of results of invoking fun on every nth
element of enumerable, starting with the first element.
The first element is always passed to the given function, unless nth is 0.
The second argument specifying every nth element must be a non-negative
integer.
If nth is 0, then enumerable is directly converted to a list,
without fun being ever applied.
Examples
iex> Enum.map_every(1..10, 2, fn x -> x + 1000 end)
[1001, 2, 1003, 4, 1005, 6, 1007, 8, 1009, 10]
iex> Enum.map_every(1..10, 3, fn x -> x + 1000 end)
[1001, 2, 3, 1004, 5, 6, 1007, 8, 9, 1010]
iex> Enum.map_every(1..5, 0, fn x -> x + 1000 end)
[1, 2, 3, 4, 5]
iex> Enum.map_every([1, 2, 3], 1, fn x -> x + 1000 end)
[1001, 1002, 1003]