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