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

21 Aug 2013

      Hi,

I have a simple test case containing a typeclass Foo with one type variable 
a. The goal is to write once and for all a function symbolToInfixLevel 
function that combines two other functions defined in the typeclass:

-----------------------
class Foo a where

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

    symbolToInfixLevel :: String -> Int
    symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined 
undefined
-----------------------

This yields an error because there is no "a" in the type signature for 
symbolToInfixLevel:

$ runghc test_typeclass_without_typevariable.hs 
test_typeclass_without_typevariable.hs:1:1:
    The class method `symbolToInfixLevel'
    mentions none of the type variables of the class Foo a
    When checking the class method: symbolToInfixLevel :: String -> Int
    In the class declaration for `Foo'

Now, if I define symbolToInfixLevel out of the typeclass:

-----------------------
class Foo a where

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

symbolToInfixLevel :: String -> Int
symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined 
undefined
-----------------------

Now, I obtain:

$ runghc test_typeclass_without_typevariable.hs 
test_typeclass_without_typevariable.hs:7:24:
    No instance for (Foo a0) arising from a use of `infixLevel'
    In the expression: infixLevel
    In the expression:
      infixLevel $ (symbolToConstructor s) undefined undefined
    In an equation for `symbolToInfixLevel':
        symbolToInfixLevel s
          = infixLevel $ (symbolToConstructor s) undefined undefined

How to get rid from this situation?

Thanks in advance,

TP