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

Right to left function composition.

(f . g) x = f (g x)

f . id = f = id . f

Examples

>>> map ((*2) . length) [[], [0, 1, 2], [0]]
[0,6,2]

>>> foldr (.) id [(+1), (*3), (^3)] 2
25

>>> let (...) = (.).(.) in ((*2)...(+)) 5 10
30

Morphism composition. Implementations should satisfy the law:

• Associativity f . (g . h) = (f . g) . h

This means that the way morphisms are grouped is irrelevant, so it is unambiguous to write a composition of morphisms as f . g . h, without parentheses.