Map.replace

You're seeing just the function replace, go back to Map module for more information.

replace(map, key, value)

(since 1.11.0)

replace(map(), key(), value()) :: map()

Puts a value under key only if the key already exists in map.

iex> Map.replace(%{a: 1, b: 2}, :a, 3)
%{a: 3, b: 2}

iex> Map.replace(%{a: 1}, :b, 2)
%{a: 1}