[project @ 2001-10-25 14:30:43 by simonpj]

[ghc-hetmet.git] / ghc / compiler / typecheck / TcMType.lhs

diff --git a/ghc/compiler/typecheck/TcMType.lhs b/ghc/compiler/typecheck/TcMType.lhs
index 9d27e67..95069c7 100644 (file)
--- a/ghc/compiler/typecheck/TcMType.lhs
+++ b/ghc/compiler/typecheck/TcMType.lhs
@@ -865,6 +865,15 @@ check_source_ty dflags ctxt pred@(ClassP cls tys)
                        other         -> dopt Opt_GlasgowExts               dflags
 
 check_source_ty dflags SigmaCtxt (IParam name ty) = check_arg_type ty
+       -- Implicit parameters only allows in type
+       -- signatures; not in instance decls, superclasses etc
+       -- The reason for not allowing implicit params in instances is a bit subtle
+       -- If we allowed        instance (?x::Int, Eq a) => Foo [a] where ...
+       -- then when we saw (e :: (?x::Int) => t) it would be unclear how to 
+       -- discharge all the potential usas of the ?x in e.   For example, a
+       -- constraint Foo [Int] might come out of e,and applying the
+       -- instance decl would show up two uses of ?x.
+
 check_source_ty dflags TypeCtxt  (NType tc tys)   = mapTc_ check_arg_type tys
 
 -- Catch-all