id is:exact package:base set:included-with-ghc

Identity function.

id x = x

This function might seem useless at first glance, but it can be very useful in a higher order context.

Examples

>>> length $ filter id [True, True, False, True]
3

>>> Just (Just 3) >>= id
Just 3

>>> foldr id 0 [(^3), (*5), (+2)]
1000

The identity morphism. Implementations should satisfy two laws:

• Right identity f . id = f
• Left identity id . f = f

These essentially state that id should "do nothing".