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

21 Aug 2013


      TP  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 

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