Why doesn’t this simple code in Haskell compile?

Question

I’m studying Haskell, and I don’t understand why this code doesn’t compile:

foo :: Int -> Double -> Double
foo a b = a+b

The error message is:

Couldn't match expected type `Double' with actual type `Int'
In the first argument of `(+)', namely `a'
In the expression: a + b
In an equation for `foo': foo a b = a + b

I know the operator '+' is a function with the parameters

(+) :: (Num a) => a -> a -> a

but Int and Double types are instances of Num.

So, why does the compiler not accept this code??

Thank you very much!

Answer 1

The (+) method works with any type, but it requires the two elements to be of the same type. When he identifies that the first element is an Int, he infers that the other will be as well. For its addition to work, it is necessary to convert the elements to the same type.

This is due to Monomorphism Restriction, when you do not specify the type, it will infer the types using standard typification rules, the example is exactly the sum function (+). The (+) :: (Num a) => a -> a -> a , will stay Int -> Int -> Int

Source: https://stackoverflow.com/a/8262122/9916784