[project @ 2002-11-18 14:22:01 by simonpj]

----------------------------------
	Ambiguity for implicit parameters
	---------------------------------

	MERGE TO STABLE

Implicit parameters do not cause amiguity. For example, this is OK:

	foo :: (?x :: [a]) => Int -> Int
	foo _ = length ?x

Test is tcrun023.hs

diff --git a/ghc/compiler/typecheck/TcMType.lhs b/ghc/compiler/typecheck/TcMType.lhs
index daf2acf..0364a0e 100644 (file)
--- a/ghc/compiler/typecheck/TcMType.lhs
+++ b/ghc/compiler/typecheck/TcMType.lhs
@@ -50,7 +50,7 @@ import TypeRep                ( Type(..), SourceType(..), TyNote(..),  -- Friend; can see repr
                        ) 
 import TcType          ( TcType, TcThetaType, TcTauType, TcPredType,
                          TcTyVarSet, TcKind, TcTyVar, TyVarDetails(..),
-                         tcEqType, tcCmpPred,
+                         tcEqType, tcCmpPred, isClassPred,
                          tcSplitPhiTy, tcSplitPredTy_maybe, tcSplitAppTy_maybe, 
                          tcSplitTyConApp_maybe, tcSplitForAllTys,
                          tcIsTyVarTy, tcSplitSigmaTy, 
@@ -908,7 +908,14 @@ checkAmbiguity forall_tyvars theta tau_tyvars
   where
     complain pred     = addErrTc (ambigErr pred)
     extended_tau_vars = grow theta tau_tyvars
-    is_ambig pred     = any ambig_var (varSetElems (tyVarsOfPred pred))
+
+       -- Only a *class* predicate can give rise to ambiguity
+       -- An *implicit parameter* cannot.  For example:
+       --      foo :: (?x :: [a]) => Int
+       --      foo = length ?x
+       -- is fine.  The call site will suppply a particular 'x'
+    is_ambig pred     = isClassPred  pred &&
+                       any ambig_var (varSetElems (tyVarsOfPred pred))
 
     ambig_var ct_var  = (ct_var `elem` forall_tyvars) &&
                        not (ct_var `elemVarSet` extended_tau_vars)