From: simonpj <unknown>
Date: Mon, 18 Nov 2002 14:22:01 +0000 (+0000)
Subject: [project @ 2002-11-18 14:22:01 by simonpj]
X-Git-Tag: Approx_11550_changesets_converted~1444
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=7ff78c251654969f9973c087bec3ae98d5e5b606;p=ghc-hetmet.git

[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
--- 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)