Question

In Haskell, why would you define a function with a type constraint:

ghci> :t (==)  
(==) :: (Eq a) => a -> a -> Bool

Rather than defining it so it’s type was:

ghci> :t (==)  
(==) :: Eq -> Eq -> Bool

Answer 1

You wouldn’t do the second version because you’d get a compile error. Eq is not a type, it’s a typeclass. You can’t use a typeclass in places where a type is required.

If you did define your own type MyEq and then define a function == with the type MyEq -> MyEq -> Bool, the expression "hello" == "hello" would be invalid because "hello" is a value of type String and not of type MyEq. Since there is no subtyping in haskell a value couldn’t be of type String and of type MyEq at the same time.

So if you want to define a function that can accept values of different types which meet certain conditions, you need type classes.