Higher order function
A higher order function is a function that takes other functions as arguments or returns a function as result.
The major use is to abstract common behaviour into one place.
In the libraries
Many functions in the libraries are higher order. The (probably) most commonly given examples are
Two other common ones are
curry, uncurry. A possible implementation of these is:
curry :: ((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
curry's first argument must be a function which accepts a pair. It applies that function to its next two arguments.
uncurry is the inverse of
curry. Its first argument must be a function taking two values.
uncurry then applies that function to the components of the pair which is the second argument.
Simple code examples
Rather than writing
doubleList  =  doubleList (x:xs) = 2*x : doubleList xs
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
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
map in Haskell's Prelude.
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 a numerical approximation:
derive :: Double -> (Double -> Double) -> (Double -> Double) derive eps f x = (f(x+eps) - f(x-eps)) / (2*eps)