How do you define and use curry and uncurry (Prelude functions) in Haskell?

Question

How can I use curry and uncurry prelude functions in Haskell?

Moreover, why do the following definitions throw an error when loaded?

curry' :: ((a -> b) -> c) -> a -> b -> c
curry' f = \x y -> f (x, y)

uncurry' :: a -> b -> c -> ((a -> b) -> c)
uncurry' f = \(x,y) -> f x y

Answer 1

You're getting errors because your type signatures are wrong, you should be using tuples instead of functions for the a and b arguments:

curry' :: ((a, b) -> c) -> a -> b -> c
uncurry' :: (a -> b -> c) -> ((a, b) -> c)

Also, notice the parentheses I added to uncurry' s type, those are important in this case. What you have is equivalent to

uncurry' :: a -> (b -> (c -> ((a -> b) -> c)))

Which is not the same, this is a function that takes 3 arguments and produces a function instead of a function that takes a 2 argument function and returns a function of one tuple argument.

You could use these functions like

> uncurry (+) (1, 2)
3
> curry fst 1 2
1
> curry snd 1 2
2

(I didn't see any other Prelude functions that take tuples as arguments)

EDIT: At chi's request, here's a more visual explanation of that last sentence:

a -> (b -> (c -> ((a, b) -> c)))

is the type of a function that takes 3 arguments a , b , and c , and returns a function of type (a, b) -> c .

(a -> b -> c) -> ((a, b) -> c)

is the type of a function that takes a single argument a -> b -> c and returns a function (a, b) -> c .