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

[Peter Jones]

TP <paratribulations at free.fr> writes:
> symbolToInfixLevel :: String -> Int
> symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) 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?

Your `symbolToConstructor' function is too specific for both cases you
describe.  You'd need to change the `String' argument to `a' and put it
back in the type class or write it like this:

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

-- 
Peter Jones, Founder, Devalot.com
Defending the honor of good code