X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=95250f8c6d81fb67d5fafaf1e4a08062c9339392;hb=ddf73c04de77994ad138771a7492007d794faf5e;hp=342114b9fbb04e3ab9316409ecab54c8bec2d0fb;hpb=84923cc7de2a93c22a2f72daf9ac863959efae13;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 342114b..95250f8 100644
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -849,10 +849,11 @@ bindIrredsR loc qtvs co_vars reft givens irreds
 		-- The givens can include methods
 		-- See Note [Pruning the givens in an implication constraint]
 
-	   -- If there are no 'givens', then it's safe to 
+	   -- If there are no 'givens' *and* the refinement is empty
+	   -- (the refinement is like more givens), then it's safe to 
 	   -- partition the 'wanteds' by their qtvs, thereby trimming irreds
 	   -- See Note [Freeness and implications]
-	; irreds' <- if null givens'
+	; irreds' <- if null givens' && isEmptyRefinement reft
 	     	     then do
 	     	    	{ let qtv_set = mkVarSet qtvs
 	     	    	      (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
@@ -862,7 +863,8 @@ bindIrredsR loc qtvs co_vars reft givens irreds
 	
 	; let all_tvs = qtvs ++ co_vars	-- Abstract over all these
 	; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds'
-				-- This call does the real work
+			-- This call does the real work
+			-- If irreds' is empty, it does something sensible
 	; extendLIEs implics
 	; return bind } 
 
@@ -875,6 +877,8 @@ makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement
 -- The binding looks like
 --	(ir1, .., irn) = f qtvs givens
 -- where f is (evidence for) the new implication constraint
+--	f :: forall qtvs. {reft} givens => (ir1, .., irn)
+-- qtvs includes coercion variables
 --
 -- This binding must line up the 'rhs' in reduceImplication
 makeImplicationBind loc all_tvs reft
@@ -2256,7 +2260,12 @@ disambiguate extended_defaulting insts
 					-- use [Integer, Double]
 				do { integer_ty <- tcMetaTy integerTyConName
 				   ; checkWiredInTyCon doubleTyCon
-				   ; return [integer_ty, doubleTy] }
+				   ; string_ty <- tcMetaTy stringTyConName
+				   ; ovl_str <- doptM Opt_OverloadedStrings
+				   ; if ovl_str 	-- Add String if -foverloaded-strings
+					then return [integer_ty,doubleTy,string_ty] 
+					else return [integer_ty,doubleTy] }
+
 	; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
 	; mapM_ (disambigGroup default_tys) defaultable_groups  }
   where
@@ -2278,13 +2287,13 @@ disambiguate extended_defaulting insts
 
    defaultable_classes clss 
 	| extended_defaulting = any isInteractiveClass clss
-	| otherwise = all isStandardClass clss && any isNumericClass clss
+	| otherwise = all isStandardClass clss && (any isNumericClass clss || any ((== isStringClassKey) . classKey) clss)
 
  	-- In interactive mode, or with -fextended-default-rules,
 	-- we default Show a to Show () to avoid graututious errors on "show []"
    isInteractiveClass cls 
 	= isNumericClass cls
-	|| (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+	|| (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey, isStringClassKey])
 
 
 disambigGroup :: [Type]			-- The default types
@@ -2359,14 +2368,18 @@ tcSimplifyDeriv orig tyvars theta
 	; wanteds <- newDictBndrsO orig (substTheta tenv theta)
 	; (irreds, _) <- topCheckLoop doc wanteds
 
-	; let (dicts, non_dicts) = partition isDict irreds
-				 	-- Exclude implication consraints
-	; addNoInstanceErrs non_dicts	-- I'm not sure if these can really happen
+	-- Insist that the context of a derived instance declaration
+	-- consists of constraints of form (C a b), where a,b are
+	-- type variables
+	-- NB: the caller will further check the tv_dicts for
+	--     legal instance-declaration form
+	; let (tv_dicts, non_tv_dicts) = partition isTyVarDict irreds
+	; addNoInstanceErrs non_tv_dicts
 
 	; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
-	      simpl_theta = substTheta rev_env (map dictPred dicts)
-		-- This reverse-mapping is a Royal Pain, 
-		-- but the result should mention TyVars not TcTyVars
+	      simpl_theta = substTheta rev_env (map dictPred tv_dicts)
+		-- This reverse-mapping is a pain, but the result
+		-- should mention the original TyVars not TcTyVars
 
 	; return simpl_theta }
   where