Question

What is the problem with this code?

addNum :: Int->Int-> Int
addNum a b = a+b

divideby :: ( Int->Int -> Int ) -> Int  ->float
divideby  f  z  =  f /z

I want to take the addNum function as an input to divideby with a divisor and then output the answer. So the divideby function should act as a higher order function.

What is the problem with this code? It gives following error:

*** Expression     : f / z
*** Term           : f
*** Type           : Int -> Int -> Int
*** Does not match : Int

Answer 1

It means exactly what the error says. You have specified that the type of your function is

divideby :: ( Int->Int -> Int ) -> Int  ->float

That means, the first argument must be a function, with the type Int->Int -> Int, and the second input must be an Int, and then it will produce a float. Naturally, Haskell does not know how to divide a function by an Int, which is what you are telling it to do with the expression f / z.

---

In response to the comment: no. That is not a higher-order function, but it is close. Take a look:

addNum :: Int -> Int -> Int
addNum a b = a + b

divideBy :: Int -> Int -> Int -> Int
divideBy a b c = (addNum a b) `div` c

divideBy is a function that takes 3 Ints as input, and produces an Int. Since its inputs are merely Int, it is not a higher-order function. However, you could abstract the use of addNum into an input, thus making it a higher-order function. addNum has type (Int -> Int -> Int), so, in addition to the inputs we have already, we will make that the type of our first input

divideBy :: (Int -> Int -> Int) -> Int -> Int -> Int -> Int
divideBy f a b c = (f a b) `div` c

This is a higher-order function, that also compiles correctly and has meaning. 🙂 You can pass addNum as an input to this function.

ghci> divideBy addNum 2 4 6 -- (2 + 4) / 6
1