Headline

Apply two functions, one after another

Description

Function composition is a fundamental operation on functions so that they are applied one after another. That is the composition of f and g when applied to an argument x is the same as first computing the result y of applying g to x and then applying f to y.

Illustration

Function composition in Haskell

Let's assume the following function:

-- Test an Int to be even
even :: Int -> Bool
even x = x `mod` 2 == 0

The test for a number to be odd can be trivially described by composing even and logical negation. In Language:Haskell, function composition is denoted by the infix operator ".". Thus:

-- Test an Int to be odd
odd :: Int -> Bool
odd = not . even

We can always replace function composition by some pattern of function application. For instance, odd could also be expressed like this:

-- Test an Int to be odd
odd' :: Int -> Bool
odd' x = not (even x)

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