List.keyreplace

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

keyreplace(list, key, position, new_tuple)

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Specs

keyreplace([tuple()], any(), non_neg_integer(), tuple()) :: [tuple()]

Receives a list of tuples and if the identified element by key at position exists, it is replaced with new_tuple.

Examples

iex> List.keyreplace([a: 1, b: 2], :a, 0, {:a, 3})
[a: 3, b: 2]

iex> List.keyreplace([a: 1, b: 2], :a, 1, {:a, 3})
[a: 1, b: 2]