[project @ 2001-04-30 10:50:00 by simonpj]

[ghc-hetmet.git] / ghc / compiler / typecheck / TcSimplify.lhs

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index bfaf629..d8c3194 100644 (file)
--- 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