Currying is the process of transforming a function that takes multiple arguments into a function that takes just a single argument and returns another function if any arguments are still needed.
f :: a -> b -> c
is the curried form of
g :: (a, b) -> c
f = curry g g = uncurry f
In Haskell, all functions are considered curried: That is, all functions in Haskell take just single arguments.This is mostly hidden in notation, and so may not be apparent to a new Haskeller. Let's take the function
div :: Int -> Int -> Int
Int -> Int
You'll notice that the notation for types reflects this: you can read
Int -> Int -> Int
Much of the time, currying can be ignored by the new programmer. The major advantage of considering all functions as curried is theoretical: formal proofs are easier when all functions are treated uniformly (one argument in, one result out). Having said that, there are Haskell idioms and techniques for which you need to understand currying.
Currying provides a convenient way of writing some functions without having to explicitly name them:
- (unsugared:(1+)) is the "increment" function,(+) 1
- is the "double" function,(2*)
- is the "indent" function,("\t"++)
- is the "is-capital-vowel-in-English" function (ignoring the "sometimes Y").(`elem` "AEIOU")
Sometimes it's valuable to think about functions abstractly without specifically giving all their arguments: this is the Pointfree style.
Sometimes half the work of the function can be done looking only at the first argument (but there really is only one argument, remember?): see functional dispatch.