Enum.min_max_by

You're seeing just the function min_max_by, go back to Enum module for more information.
Link to this function

min_max_by(enumerable, fun, sorter_or_empty_fallback \\ &</2, empty_fallback \\ fn -> raise(Enum.EmptyError) end)

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Specs

min_max_by(
  t(),
  (element() -> any()),
  (element(), element() -> boolean()) | module(),
  (() -> empty_result)
) :: {element(), element()} | empty_result
when empty_result: any()

Returns a tuple with the minimal and the maximal elements in the enumerable as calculated by the given function.

If multiple elements are considered maximal or minimal, the first one that was found is returned.

Examples

iex> Enum.min_max_by(["aaa", "bb", "c"], fn x -> String.length(x) end)
{"c", "aaa"}

iex> Enum.min_max_by(["aaa", "a", "bb", "c", "ccc"], &String.length/1)
{"a", "aaa"}

iex> Enum.min_max_by([], &String.length/1, fn -> {nil, nil} end)
{nil, nil}

The fact this function uses Erlang's term ordering means that the comparison is structural and not semantic. Therefore, if you want to compare structs, most structs provide a "compare" function, such as Date.compare/2, which receives two structs and returns :lt (less-than), :eq (equal to), and :gt (greater-than). If you pass a module as the sorting function, Elixir will automatically use the compare/2 function of said module:

iex> users = [
...>   %{name: "Ellis", birthday: ~D[1943-05-11]},
...>   %{name: "Lovelace", birthday: ~D[1815-12-10]},
...>   %{name: "Turing", birthday: ~D[1912-06-23]}
...> ]
iex> Enum.min_max_by(users, &(&1.birthday), Date)
{
  %{name: "Lovelace", birthday: ~D[1815-12-10]},
  %{name: "Ellis", birthday: ~D[1943-05-11]}
}

Finally, if you don't want to raise on empty enumerables, you can pass the empty fallback:

iex> Enum.min_max_by([], &String.length/1, fn -> nil end)
nil