Maybe I don't fully understand what you are trying to achieve, but I don't
think what you are trying to do makes sense. infixLevel and
symbolToConstructor will have different implementations for each instance
of Foo, and the compiler cannot possibly know which implementations you
mean. If the implementation of symbolToInfixLevel is independent of the
implementations of infixLevel and symbolToConstructor then it shouldn't
need them in its definition.
Peter
On 21 August 2013 17:39, TP
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
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