X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypecheck%2FTcSimplify.lhs;h=d8c31946592fd58dcb4b8cc71b574a81045545df;hb=55d04fc7a6fbd58358f284bd84648dad09046f60;hp=bfaf629d9d2d1876a8e75cdf0526206c70293fc1;hpb=b58e1155b0ec79ec6983c3e9a42880d511b7bc10;p=ghc-hetmet.git

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index bfaf629..d8c3194 100644
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -7,7 +7,8 @@
 
 \begin{code}
 module TcSimplify (
-	tcSimplifyInfer, tcSimplifyInferCheck, tcSimplifyCheck, 
+	tcSimplifyInfer, tcSimplifyInferCheck, tcSimplifyCheck,
+	tcSimplifyRestricted,
 	tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop, 
 
 	tcSimplifyThetas, tcSimplifyCheckThetas,
@@ -323,6 +324,15 @@ URK!  Let's not do this. So this is illegal:
 	f :: Int -> Int
 	f x = x + ?y
 
+There's a nasty corner case when the monomorphism restriction bites:
+
+	f = x + ?y
+
+The argument above suggests that we must generalise over the ?y parameter, 
+but the monomorphism restriction says that we can't.  The current 
+implementation chooses to let the monomorphism restriction 'win' in this
+case, but it's not clear what the Right Thing is.
+
 BOTTOM LINE: you *must* quantify over implicit parameters.
 
 
@@ -479,33 +489,45 @@ tcSimplifyCheck
 		 TcDictBinds)	-- Bindings
 
 tcSimplifyCheck doc qtvs givens wanted_lie
-  = checkLoop doc qtvs givens (lieToList wanted_lie)	`thenTc` \ (frees, binds, irreds) ->
+  = checkLoop doc qtvs givens (lieToList wanted_lie) try `thenTc` \ (frees, binds, irreds) ->
 
 	-- Complain about any irreducible ones
     complainCheck doc givens irreds		`thenNF_Tc_`
 
 	-- Done
     returnTc (mkLIE frees, binds)
+  where
+	-- When checking against a given signature we always reduce
+	-- until we find a match against something given, or can't reduce
+    try qtvs inst | isFree qtvs inst  = Free
+		  | otherwise         = ReduceMe 
 
-checkLoop doc qtvs givens wanteds
-  =   	-- Step 1
+tcSimplifyRestricted doc qtvs givens wanted_lie
+  = checkLoop doc qtvs givens (lieToList wanted_lie) try `thenTc` \ (frees, binds, irreds) ->
+
+	-- Complain about any irreducible ones
+    complainCheck doc givens irreds		`thenNF_Tc_`
+
+	-- Done
+    returnTc (mkLIE frees, binds)
+  where
+    try qtvs inst | not (tyVarsOfInst inst `intersectsVarSet` qtvs) = Free
+	          | otherwise                                       = ReduceMe
+
+checkLoop doc qtvs givens wanteds try_me
+  =		-- Step 1
     zonkTcTyVarsAndFV qtvs		`thenNF_Tc` \ qtvs' ->
     mapNF_Tc zonkInst givens		`thenNF_Tc` \ givens' ->
     mapNF_Tc zonkInst wanteds		`thenNF_Tc` \ wanteds' ->
-    let
-	      -- When checking against a given signature we always reduce
-	      -- until we find a match against something given, or can't reduce
-	try_me inst | isFree qtvs' inst  = Free
-		    | otherwise          = ReduceMe 
-    in
+
 		-- Step 2
-    reduceContext doc try_me givens' wanteds'    `thenTc` \ (no_improvement, frees, binds, irreds) ->
+    reduceContext doc (try_me qtvs') givens' wanteds'		`thenTc` \ (no_improvement, frees, binds, irreds) ->
 	
 		-- Step 3
     if no_improvement then
 	returnTc (frees, binds, irreds)
     else
-	checkLoop doc qtvs givens' (irreds ++ frees)	`thenTc` \ (frees1, binds1, irreds1) ->
+	checkLoop doc qtvs givens' (irreds ++ frees) try_me	`thenTc` \ (frees1, binds1, irreds1) ->
 	returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
 
 complainCheck doc givens irreds