Partial application

(Difference between revisions)

Current revision

Partial application in Haskell involves passing less than the full number of arguments to a function that takes multiple arguments.

For example:

add     :: Int -> Int -> Int
add x y = x + y
 
addOne = add 1

In this example,

addOne

is the result of partially applying

add

. It is a new function that takes an integer, adds 1 to it and returns that as the result. The important property here is that the

->

operator is right associative, and function application is left associative, meaning the type signature of add actually looks like this:

add :: Int -> (Int -> Int)

This means that add actually takes one argument and returns a function that takes another argument and returns an Int.

For example:

comp2 :: (a -> b) -> (b -> b -> c) -> (a -> a -> c)
comp2 f g = (\x y -> g (f x) (f y))

Remembering the maxim that: Functions are not partial, you can partially apply a function.

So, is this a partial definition of

comp2'

comp2' f = (\x y -> add (f x) (f y))

Not really, this is the definition of another function. But you can achieve the desired result by partially applying

comp2

, like so:

comp2' f = comp2 f add

@@ Line 12: / Line 12: @@
 </haskell>
-In this example, <hask>addOne</hask> is the result of partially applying <hask>add</hask>. It is a new function that takes and integer, adds 1 to it and returns that as the result.
+In this example, <hask>addOne</hask> is the result of partially applying <hask>add</hask>. It is a new function that takes an integer, adds 1 to it and returns that as the result.
 The important property here is that the <hask>-></hask> operator is right associative, and function application is left associative, meaning the type signature of add actually looks like this: