[Haskell-beginners] fonction in a typeclass that does not mention the type variable

[TP]

Peter Jones wrote:

> Actually, the problem is with your `symbolToConstructor' function.  It
> needs to look like this:
> 
> symbolToConstructor :: (Foo a) => String -> a
> 
> Then symbolToInfixLevel can be:
> 
> symbolToInfixLevel :: String -> Int
> 

Sorry, I have not been able to write an example with your proposition. Might 
you give more informations?

In the meantime, I have been able to write a working example proposing two 
variants:

------------------------------
{-# LANGUAGE ScopedTypeVariables #-}

data Exp = Plus Exp Exp | Minus Exp Exp | Atom String
    deriving Show

class Foo a where

    symbolToConstructor :: String -> ( a -> a -> a )
    infixLevel :: a -> Int

    -- The second argument is only used to indicate the type of the
    -- intermediary result (i.e. the type of the result of
    -- symbolToConstructor).
    symbolToInfixLevel :: String -> a -> Int
    symbolToInfixLevel s u =
        infixLevel $ (((symbolToConstructor s) undefined undefined)::a)


instance Foo Exp where

    symbolToConstructor e = case e of
                "+" -> Plus
                "-" -> Minus
    infixLevel e = case e of
        Plus _ _ -> 6
        Minus _ _ -> 7

main = do

-- First variant
print $ infixLevel $ (((symbolToConstructor "+") undefined undefined)::Exp)
print $ infixLevel $ (((symbolToConstructor "-") undefined undefined)::Exp)
-- Second variant
print $ symbolToInfixLevel "+" (undefined::Exp)
print $ symbolToInfixLevel "-" (undefined::Exp)
------------------------------