# Higher order function

### From HaskellWiki

## Contents |

## 1 Definition

A **higher order function** is a function that takes other functions as arguments or returns a function as result.

## 2 Discussion

The major use is to abstract common behaviour into one place.

### 2.1 Examples

#### 2.1.1 In the libraries

Many functions in the libraries are higher order. The (probably) most commonly given examples arecurry :: ((a,b)->c) -> a->b->c curry f a b = f (a,b) uncurry :: (a->b->c) -> ((a,b)->c) uncurry f (a,b)= f a b

#### 2.1.2 Simple code examples

Rather than writing

doubleList [] = [] doubleList (x:xs) = 2*x : doubleList xs

and

tripleList [] = [] tripleList (x:xs) = 3*x : tripleList xs

we can parameterize out the difference

multList n [] = [] multList n (x:xs) = n*x : multList n xs

and define

tripleList = multList 3 doubleList = multList 2

leading to a less error prone definition of each.

But now, if we had the function

addToList n [] = [] addToList n (x:xs) = n+x : addToList n xs

we could parameterize the difference again

operlist n bop [] = [] operlist n bop (x:xs) = bop n x : operlist n bop xs

and define doubleList as

doubleList = operList 2 (*)

but this ties us into a constant parameters

and we could redefine things as

mapList f [] = [] mapList f (x:xs) = f x : mapList f xs

and define doubleList as

doubleList = mapList (2*)

This higher order function "mapList" can be used in a wide range of areas to simplify code.

It is called#### 2.1.3 Mathematical examples

In mathematics the counterpart to higher order functions are functionals (mapping functions to scalars) and function operators (mapping functions to functions). Typical functionals are the limit of a sequence, or the integral of an interval of a function.

limit :: [Double] -> Double definiteIntegral :: (Double, Double) -> (Double -> Double) -> Double

Typical operators are the indefinite integral, the derivative, the function inverse.

indefiniteIntegral :: Double -> (Double -> Double) -> (Double -> Double) derive :: (Double -> Double) -> (Double -> Double) inverse :: (Double -> Double) -> (Double -> Double)

Here is a numerical approximation:

derive :: Double -> (Double -> Double) -> (Double -> Double) derive eps f x = (f(x+eps) - f(x-eps)) / (2*eps)

## 3 See also

Accumulator recursion where the accumulator is a higher order function is one interesting case of continuation passing style.