function that input an integer and return a function in Haskell

Question

Totally get lost of the following example that returning a function

getAddFunc :: Int -> (Int -> Int)
getAddFunc x y = x + y
adds3 = getAddFunc 3
fourPlus3 = adds3 4

the function signature Int -> (Int -> Int) told me it accept an Int and function that typed Int -> Int And I am very confused about the function definition x y = x + y, it looks like it took 2 integers x and y rather than just 1 integer! What's the meaning of x and y here? I am very confused.

Can someone help me with this?

does it make more sense after reading this?

Robin Zigmond
– Robin Zigmond

2021-12-01 21:23:44 +00:00
Commented Dec 1, 2021 at 21:23 — Robin Zigmond
– Robin Zigmond, Commented Dec 1, 2021 at 21:23
It's the beauty of haskell

ShamPooSham
– ShamPooSham

2021-12-01 21:32:43 +00:00
Commented Dec 1, 2021 at 21:32 — ShamPooSham
– ShamPooSham, Commented Dec 1, 2021 at 21:32
@ShamPooSham just start that beauty journey for 40 min :)

SLN
– SLN

2021-12-01 21:35:20 +00:00
Commented Dec 1, 2021 at 21:35 — SLN
– SLN, Commented Dec 1, 2021 at 21:35

chi · Accepted Answer · 2021-12-01 21:25:21Z

4

You are looking at a curried function. The code

getAddFunc x y = x + y

is equivalent to

getAddFunc x = \y -> x + y

i.e., taking x as argument and returning a function of y.

This can also be written

getAddFunc = \x -> \y -> x + y

or

getAddFunc = \x y -> x + y

In Haskell, from a technical point of view, every function takes exactly one argument. However, after taking that argument, it can return another function taking as input a "second" argument. This mechanism can also be interpreted as a function taking "two" arguments. This further generalizes to N arguments in a similar way.

answered Dec 1, 2021 at 21:25

chi

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Comments

willeM_ Van Onsem · Accepted Answer · 2021-12-01 21:26:32Z

3

All functions in Haskell take one parameter. Indeed, the function:

getAddFunc x y = x + y

is equivalent to:

getAddFunc x = \y -> x + y

and to:

getAddFunc = \x -> \y -> x + y

You thus construct a function that takes a parameter x, and it maps it to a function that takes a parameter y and will return x + y.

This thus means that getAddFunc 42 is a function that takes a parameter and will add 42 to it.

The (->) type constructor is right associative. Indeed, Int -> (Int -> Int) is the same as Int -> Int -> Int.

answered Dec 1, 2021 at 21:26

willeM_ Van Onsem

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