Enumerable.member-question-mark

You're seeing just the function member-question-mark, go back to Enumerable module for more information.

member?(enumerable, element)

member?(t(), term()) :: {:ok, boolean()} | {:error, module()}

Checks if an element exists within the enumerable.

It should return {:ok, boolean} if you can check the membership of a given element in enumerable with ===/2 without traversing the whole of it.

Otherwise it should return {:error, __MODULE__} and a default algorithm built on top of reduce/3 that runs in linear time will be used.

When called outside guards, the in and not in operators work by using this function.