List.keystore

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

keystore(list, key, position, new_tuple)

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Specs

keystore([tuple()], any(), non_neg_integer(), tuple()) :: [tuple(), ...]

Receives a list of tuples and replaces the element identified by key at position with new_tuple.

If the element does not exist, it is added to the end of the list.

Examples

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

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