CODEWITHSUNDEEP
haskell
Yaiyan
Yaiyan

Reputation: 101

Returning a function in Haskell

In Haskell, is it possible to return a function from within a function? I tried searching on Hoogle for:

(a -> b) -> a -> b

but couldn't find any functions that could achieve this.

Upvotes: 1

Views: 1345

Answers (3)

gregorias
gregorias

Reputation: 363

Treating functions as first-class citizens is one of the advantages of Haskell and doesn't require any special syntax.

For example if you want to have a function which will double the result of given function you can write:

doubleFunction f = (\x -> 2 * (f x))

When given a function f, doubleFunction is equal to a function which multiplies (f x) by 2. You can write it using function composition:

doubleFunction f = (* 2) . f

or even shorter:

doubleFunction = ((* 2) .)

doubleFunction is of type:

doubleFunction :: (Num a) => (a -> b) -> a -> b

Upvotes: 3

leftaroundabout
leftaroundabout

Reputation: 120751

Yes, you can, and it's so common and deeply ingrained in the language that you don't even notice it!

For instance, when you write

mean :: Double -> Double -> Double
mean a b = (a + b) / 2

What you're really doing is, you define a function

mean :: Double -> EndoDouble
mean a = meanA
 where meanA b = (a + b) / 2

where the type of meanA,

type EndoDouble = Double -> Double

happens to be a function type.

So, any "multi-parameter function" in Haskell really is a single-parameter function that returns a function! The concept is called Currying, you might have heard about it.

Upvotes: 6

Michael Rawson
Michael Rawson

Reputation: 1975

Yes, you can: functions are values in Haskell, so it can be as simple as

functionIdentity :: (a -> b) -> a -> b
functionIdentity f = f

...which is just a specialised identity function.

You may also "explicitly" return a function by using a lambda if you wish, like const:

const x = \_ -> x

Upvotes: 3

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