Super basic question - but I can't seem to get a clear answer. The below function won't compile:
randomfunc :: a -> a -> b
randomfunc e1 e2
| e1 > 2 && e2 > 2 = "Both greater"
| otherwise = "Not both greater"
main = do
let x = randomfunc 2 1
putStrLn $ show x
I'm confused as to why this won't work. Both parameters are type 'a' (Ints) and the return parameter is type 'b' (String)?
Error:
"Couldn't match expected type ‘b’ with actual type ‘[Char]’"
Not quite. Your function signature indicates: for all types a and b, randomfunc will return something of type b if given two things of type a.
However, randomFunc returns a String ([Char]). And since you compare e1 with 2 each other, you cannot use all a's, only those that can be used with >:
(>) :: Ord a => a -> a -> Bool
Note that e1 > 2 also needs a way to create such an an a from 2:
(> 2) :: (Num a, Ord a) => a -> Bool
So either use a specific type, or make sure that you handle all those constraints correctly:
randomfunc :: Int -> Int -> String
randomFunc :: (Ord a, Num a) => a -> a -> String
Both parameters are type 'a' (Ints) and the return parameter is type 'b' (String)?
In a Haskell type signature, when you write names that begin with a lowercase letter such as a, the compiler implicitly adds forall a. to the beginning of the type. So, this is what the compiler actually sees:
randomfunc :: forall a b. a -> a -> b
The type signature claims that your function will work for whatever ("for all") types a and b the caller throws at you. But this is not true for your function, since it only works on Int and String respectively.
You need to make your type more specific:
randomfunc :: Int -> Int -> String
On the other hand, perhaps you intended to ask the compiler to fill out a and b for you automatically, rather than to claim that it will work for all a and b. In that case, what you are really looking for is the PartialTypeSignatures feature:
{-# LANGUAGE PartialTypeSignatures #-}
randomfunc :: _a -> _a -> _b
