X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=451e3b148ee60531a9bb328e2c0c92a6b7b2a15a;hb=14ec68724389d204e9121361e015ab776f348c5e;hp=182a7c50a935f5c49e63d589545e025d305409b1;hpb=2423c249f5ca7785d0ec89eb33e72662da7561c1;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 182a7c5..451e3b1 100644
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -657,7 +657,7 @@ tcSimplifyInfer
 	:: SDoc
 	-> TcTyVarSet		-- fv(T); type vars
 	-> [Inst]		-- Wanted
-	-> TcM ([TcTyVar],	-- Tyvars to quantify (zonked)
+	-> TcM ([TcTyVar],	-- Tyvars to quantify (zonked and quantified)
 		[Inst],		-- Dict Ids that must be bound here (zonked)
 		TcDictBinds)	-- Bindings
 	-- Any free (escaping) Insts are tossed into the environment
@@ -848,6 +848,7 @@ bindIrredsR loc qtvs co_vars reft givens irreds
   | otherwise
   = do	{ let givens' = filter isDict givens
 		-- The givens can include methods
+		-- See Note [Pruning the givens in an implication constraint]
 
 	   -- If there are no 'givens', then it's safe to 
 	   -- partition the 'wanteds' by their qtvs, thereby trimming irreds
@@ -1171,7 +1172,7 @@ tcSimplifyRestricted 	-- Used for restricted binding groups
 	-> [Name]		-- Things bound in this group
 	-> TcTyVarSet		-- Free in the type of the RHSs
 	-> [Inst]		-- Free in the RHSs
-	-> TcM ([TyVar],	-- Tyvars to quantify (zonked)
+	-> TcM ([TyVar],	-- Tyvars to quantify (zonked and quantified)
 		TcDictBinds)	-- Bindings
 	-- tcSimpifyRestricted returns no constraints to
 	-- quantify over; by definition there are none.
@@ -1876,6 +1877,20 @@ We can satisfy the (C Int) from the superclass of D, so we don't want
 to float the (C Int) out, even though it mentions no type variable in
 the constraints!
 
+Note [Pruning the givens in an implication constraint]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we are about to form the implication constraint
+	forall tvs.  Eq a => Ord b
+The (Eq a) cannot contribute to the (Ord b), because it has no access to
+the type variable 'b'.  So we could filter out the (Eq a) from the givens.
+
+Doing so would be a bit tidier, but all the implication constraints get
+simplified away by the optimiser, so it's no great win.   So I don't take
+advantage of that at the moment.
+
+If you do, BE CAREFUL of wobbly type variables.
+
+
 %************************************************************************
 %*									*
 		Avails and AvailHow: the pool of evidence