Hi! I think the following works: foo = bar undefined where bar :: MyTypeClass a => a -> a bar x = fromString (toString x) The type synonym is crucial in telling the compiler that the argument type (over which the occurrence of toSTring is overloaded) and the result type are the same. Cheers, /kff

Hello, the following works as expected: foo :: MyTypeClass resultType => resultType foo = let x = fromString (toString x) in x Ciao, Wolfgang On Friday, 2003-05-16, 16:35, CEST, Andre Pang wrote:

Hi all, I'm having some trouble trying to get a particular function working.

Let's say I have a type class, and I have two functions -- fromString and toString -- which can convert a string to an instance of that type class, and convert an instance of that type class to a string, respectively. e.g.

module SillyFreeTypeVariables where

class MyTypeClass c where toString :: c -> String fromString :: String -> c

data Instance1 = Instance1 instance MyTypeClass Instance1 where toString _ = "Instance1" fromString "Instance1" = Instance1

data Instance2 = Instance2 instance MyTypeClass Instance2 where toString _ = "Instance2" fromString "Instance2" = Instance2

Now, there's a function foo:

foo :: MyTypeClass c => c

foo generates things things which belong to MyTypeClass. I want to use it as follows:

SillyFreeTypeVariables> foo :: Instance1 Instance1 SillyFreeTypeVariables> foo :: Instance2 Instance2

My problem is: how can foo be written? I've tried:

foo = fromString (toString undefined))

but this gives back an error message:

Ambiguous type variable `a' in the top-level constraint: `MyTypeClass a' arising from use of `toString' at SillyFreeTypeVariables.hs:18

Which is understandable, because the type of '(toString undefined)' is ambiguous, since there's a free type variable there ("toString undefined" doesn't actually return a type, only a type class constraint, which the type checker complains about). What I'd like is that to be able to fix the type variable in MyTypeClass for "undefined", so that it is the same type as the result type of foo, e.g.

foo :: MyTypeClass resultType => resultType foo = fromString (toString (undefined :: resultType))

but I can't seem to do this -- GHC's scoped type variables don't seem to help either, or at least I couldn't get it working. I'm pretty sure that fixing the free type variable would work because if I change foo to look like

foo :: MyTypeClass resultType => resultType foo = fromString (toString (undefined :: Instance1))

then the typechecker is happy and it compiles and runs, although now I can obviously only get foo to return something with a type of Instance1, thus rendering it useless :). I don't want to pass a parameter to foo to get this to work. Is it possible?

Thanks muchly!