Fixing a type variable inside a function

[Wolfgang Jeltsch]

Hello,

the following works as expected:
    foo :: MyTypeClass resultType =3D> resultType
    foo =3D let x =3D 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
> =09toString :: c -> String
> =09fromString :: String -> c
>
>      data Instance1 =3D Instance1
>      instance MyTypeClass Instance1 where
> =09toString _ =3D "Instance1"
> =09fromString "Instance1" =3D Instance1
>
>      data Instance2 =3D Instance2
>      instance MyTypeClass Instance2 where
> =09toString _ =3D "Instance2"
> =09fromString "Instance2" =3D Instance2
>
> Now, there's a function foo:
>
>      foo :: MyTypeClass c =3D> 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 =3D fromString (toString undefined))
>
> but this gives back an error message:
>
>      Ambiguous type variable `a' in the top-level constraint:
> =09`MyTypeClass a'
> =09    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=
=2E
>
>      foo :: MyTypeClass resultType =3D> resultType
>      foo =3D 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 =3D> resultType
>      foo =3D 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!