Coercion among existential types

[Till Mossakowski]

When implementing a tool for a heterogeneous language,
I want to write a coerce function that allows me
to coerce among different existential types:

-- use -fglasgow-exts -package lang
import Dynamic

class (Typeable id, Typeable as) => 
      Syntax id as | id -> as
      where parse :: id -> as

data HetSyntax = forall id as .
          Syntax id as => HetSyntax id as

class (Syntax id as, Typeable env) => 
      StaticAnalysis id as env | id -> as, id -> env
      where analysis :: id -> as -> env
            show_env :: id -> env -> String

data HetEnv = forall id as env .
          StaticAnalysis id as env => HetEnv id as

coerce :: HetSyntax -> HetEnv
coerce (HetSyntax i a) = 
  case fromDynamic (toDyn (i,a)) ::  (StaticAnalysis id as env=> Maybe (id,as))  
of                           
     Just (i1,a1) -> HetEnv i1 a1
     Nothing -> error "No static analysis"

I have encountered the following error message:

Syntax3.hs:23:
    Ambiguous type variable(s) `id', `as', `env'
	in the constraint `StaticAnalysis id as env'
    arising from use of `HetEnv' at Syntax3.hs:23
    In a case alternative: HetEnv i1 a1

It seems that id, as, env are universally quantified.
Is there a way to make them dependent on the types of i and a?

Annotating i and a does not help:
coerce (HetSyntax (i::id) (a::as)) = 
  case fromDynamic (toDyn (i,a)) ::  (StaticAnalysis id as env=> Maybe (id,as))  
of                           
     Just (i1,a1) -> HetEnv i1 a1
     Nothing -> error "No static analysis"

leads to

Syntax3.hs:23:
    Could not deduce (StaticAnalysis id as env)
	from the context (Syntax id as)
    Probable fix:
	Add (StaticAnalysis id as env)
	to the the existential context of a data constructor
    arising from use of `HetEnv' at Syntax3.hs:23
    In a case alternative: HetEnv i1 a1

What I want is that the coercion is successful iff there is an
instance of StaticAnalysis extending the given instance of
Syntax.

Till
-- 
Till Mossakowski                Phone +49-421-218-4683
Dept. of Computer Science       Fax +49-421-218-3054
University of Bremen            till@tzi.de           
P.O.Box 330440, D-28334 Bremen  http://www.tzi.de/~till