A.Enum.find_index

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

find_index(enumerable, fun)

View Source

Specs

find_index(t(val), (val -> as_boolean(term()))) :: non_neg_integer() | nil
when val: value()

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

Mirrors Enum.find_index/2 with higher performance for Aja structures.