Learn Functional Programming in Haskell/

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Currying

Currying

Discover currying as another technique to write functions with multiple arguments. It also gives us the benefit of partial application.

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Introduction to currying

In the last lesson, we learned how to write functions with multiple arguments. For example, the function dividesTuple checks whether the first argument divides the second argument:

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dividesTuple :: (Int, Int) -> Bool
dividesTuple (a, b) = mod b a == 0
main = print (dividesTuple (3, 6))

There is actually another way to write functions with multiple arguments, by applying a technique called currying – again named after logician Haskell Brooks Curry. The curried version of divides reads like this:

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divides :: Int -> Int -> Bool
divides a b = mod b a == 0
main = print (divides 3 6)

Not a lot has changed in the definition. We primarily dropped parentheses in the type annotation and the equation. This is also true when we call the function on arguments. We no longer need parentheses.

*Divides> dividesTuple (2, 6)
True
*Divides> dividesTuple (3, 5)
False
*Divides> divides 2 6
True
*Divides> divides 3 5
False

Ok, so we saved typing a few characters. Let’s take a closer look at what’s happening with the type annotation to see what other benefits there are.

divides :: Int -> Int -> Bool

The function arrow operator -> associates with the right, which means we can read it like this:

divides :: Int -> (Int -> Bool)

This tells us that divides is a higher order function – a ...