Enum.find_index

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

find_index(enumerable, fun)

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Specs

find_index(t(), (element() -> any())) :: non_neg_integer() | nil

Similar to find/3, but returns the index (zero-based) of the element instead of the element itself.

Examples

iex> Enum.find_index([2, 4, 6], fn x -> rem(x, 2) == 1 end)
nil

iex> Enum.find_index([2, 3, 4], fn x -> rem(x, 2) == 1 end)
1