`Enum.sort_by` typespec explanation

[zizheng]

Hi all, I’m looking at the documentation for Enum.sort_by and saw its mapper parameter has this typespec: (element() -> mapped_element), ... when mapped_element: element().

My questions:

• I’ve never seen this syntax before; is there something I can read on this?
• What’s the purpose of mapped_element here? Since element is just any, can’t we just write (element() -> element())?

[idi527]

@zizheng

I’ve never seen this syntax before; is there something I can read on this?

There are several resources available:

• Typespecs reference — Elixir v1.16.0
• https://elixir-lang.org/getting-started/typespecs-and-behaviours.html
• Erlang -- Types and Function Specifications

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• What’s the purpose of mapped_element here? Since element is just any , can’t we just write (element() -> element()) ?

It seems like in earlier versions of the function the type spec was a bit different:

@spec sort_by(t, (element -> mapped_element), (mapped_element, mapped_element -> boolean)) :: list
         when mapped_element: element

where mapped_element specified (probably not intentionally) that the arguments in sorter should be the same.