Specs

fetch(term :: t(), key()) :: {:ok, value()} | :error

Invoked in order to access the value stored under key in the given term term.

This function should return {:ok, value} where value is the value under key if the key exists in the term, or :error if the key does not exist in the term.

Many of the functions defined in the Access module internally call this function. This function is also used when the square-brackets access syntax (structure[key]) is used: the fetch/2 callback implemented by the module that defines the structure struct is invoked and if it returns {:ok, value} then value is returned, or if it returns :error then nil is returned.

See the Map.fetch/2 and Keyword.fetch/2 implementations for examples of how to implement this callback.