Ensure the orientation of var-var equalities is correct for instatiation

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

diff --git a/compiler/typecheck/TcTyFuns.lhs b/compiler/typecheck/TcTyFuns.lhs
index bed053d..1e65471 100644 (file)
--- a/compiler/typecheck/TcTyFuns.lhs
+++ b/compiler/typecheck/TcTyFuns.lhs
@@ -37,7 +37,9 @@ import Name
 import Bag
 import Outputable
 import SrcLoc  ( Located(..) )
 import Bag
 import Outputable
 import SrcLoc  ( Located(..) )

+import Util    ( debugIsOn )

 import Maybes
 import Maybes

+import MonadUtils

 import FastString
 
 -- standard
 import FastString
 
 -- standard

@@ -69,21 +71,16 @@ tcUnfoldSynFamInst (TyConApp tycon tys)
   | not (isOpenSynTyCon tycon)     -- unfold *only* _synonym_ family instances
   = return Nothing
   | otherwise
   | not (isOpenSynTyCon tycon)     -- unfold *only* _synonym_ family instances
   = return Nothing
   | otherwise

-  = do { -- we only use the indexing arguments for matching, 
-         -- not the additional ones
-       ; maybeFamInst <- tcLookupFamInst tycon idxTys
+  = do { -- The TyCon might be over-saturated, but that's ok for tcLookupFamInst
+       ; maybeFamInst <- tcLookupFamInst tycon tys

        ; case maybeFamInst of
            Nothing                -> return Nothing
        ; case maybeFamInst of
            Nothing                -> return Nothing

-           Just (rep_tc, rep_tys) -> return $ Just (mkTyConApp rep_tc tys',
-                                                   mkTyConApp coe_tc tys')
+           Just (rep_tc, rep_tys) -> return $ Just (mkTyConApp rep_tc rep_tys,
+                                                   mkTyConApp coe_tc rep_tys)

              where
              where

-               tys'   = rep_tys ++ restTys

                coe_tc = expectJust "TcTyFuns.tcUnfoldSynFamInst" 
                                    (tyConFamilyCoercion_maybe rep_tc)
        }
                coe_tc = expectJust "TcTyFuns.tcUnfoldSynFamInst" 
                                    (tyConFamilyCoercion_maybe rep_tc)
        }

-    where
-        n                = tyConArity tycon
-        (idxTys, restTys) = splitAt n tys

 tcUnfoldSynFamInst _other = return Nothing
 \end{code}
 
 tcUnfoldSynFamInst _other = return Nothing
 \end{code}
 

@@ -231,9 +228,6 @@ tcReduceEqs locals wanteds
 We maintain normalised equalities together with the skolems introduced as
 intermediates during flattening of equalities as well as 
 
 We maintain normalised equalities together with the skolems introduced as
 intermediates during flattening of equalities as well as 
 

-!!!TODO: We probably now can do without the skolem set.  It's not used during
-finalisation in the current code.
-

 \begin{code}
 -- |Configuration of normalised equalities used during solving.
 --
 \begin{code}
 -- |Configuration of normalised equalities used during solving.
 --

@@ -286,7 +280,15 @@ no further propoagation is possible.
 --
 normaliseEqs :: [Inst] -> TcM EqConfig
 normaliseEqs eqs 
 --
 normaliseEqs :: [Inst] -> TcM EqConfig
 normaliseEqs eqs 

-  = do { ASSERTM2( allM isValidWantedEqInst eqs, ppr eqs )
+  = do { if debugIsOn then do { all_unsolved <- allM wantedEqInstIsUnsolved eqs
+                                     ; let msg = ptext (sLit "(This warning is harmless; for Simon & Manuel)")
+                                     ; WARN( not all_unsolved, msg $$ ppr eqs ) return () }
+                     else return ()
+           -- This is just a warning (not an error) because a current
+           -- harmless bug means that we sometimes solve the same
+           -- equality more than once It'll go away with the new
+           -- solver. See Trac #2999 for example
+

        ; traceTc $ ptext (sLit "Entering normaliseEqs")
 
        ; (eqss, skolemss) <- mapAndUnzipM normEqInst eqs
        ; traceTc $ ptext (sLit "Entering normaliseEqs")
 
        ; (eqss, skolemss) <- mapAndUnzipM normEqInst eqs

@@ -303,10 +305,15 @@ normaliseEqs eqs
 --
 normaliseDicts :: Bool -> [Inst] -> TcM EqConfig
 normaliseDicts isWanted insts
 --
 normaliseDicts :: Bool -> [Inst] -> TcM EqConfig
 normaliseDicts isWanted insts

-  = do { traceTc $ ptext (sLit "Entering normaliseDicts") <+>
-                   ptext (if isWanted then sLit "[Wanted]" else sLit "[Local]")
+  = do { traceTc $ hang (ptext (sLit "Entering normaliseDicts") <+>
+                         ptext (if isWanted then sLit "[Wanted] for" 
+                                            else sLit "[Local] for"))
+                     4 (ppr insts)

        ; (insts', eqss, bindss, skolemss) <- mapAndUnzip4M (normDict isWanted) 
                                                            insts
        ; (insts', eqss, bindss, skolemss) <- mapAndUnzip4M (normDict isWanted) 
                                                            insts

+
+       ; traceTc $ hang (ptext (sLit "normaliseDicts returns"))
+                     4 (ppr insts' $$ ppr eqss)

        ; return $ emptyEqConfig { eqs     = concat eqss
                                 , locals  = if isWanted then [] else insts'
                                 , wanteds = if isWanted then insts' else []
        ; return $ emptyEqConfig { eqs     = concat eqss
                                 , locals  = if isWanted then [] else insts'
                                 , wanteds = if isWanted then insts' else []

@@ -347,7 +354,7 @@ finaliseEqsAndDicts (EqConfig { eqs     = eqs
 
          -- Assert that all cotvs of wanted equalities are still unfilled, and
          -- zonk all final insts, to make any improvement visible
 
          -- Assert that all cotvs of wanted equalities are still unfilled, and
          -- zonk all final insts, to make any improvement visible

-       ; ASSERTM2( allM isValidWantedEqInst eqs'', ppr eqs'' )
+       ; ASSERTM2( allM wantedEqInstIsUnsolved eqs'', ppr eqs'' )

        ; zonked_locals  <- zonkInsts locals'
        ; zonked_wanteds <- zonkInsts (eqs'' ++ wanteds')
        ; return (zonked_locals, zonked_wanteds, final_binds, improved)
        ; zonked_locals  <- zonkInsts locals'
        ; zonked_wanteds <- zonkInsts (eqs'' ++ wanteds')
        ; return (zonked_locals, zonked_wanteds, final_binds, improved)

@@ -363,7 +370,7 @@ finaliseEqsAndDicts (EqConfig { eqs     = eqs
 
 A normal equality is a properly oriented equality with associated coercion
 that contains at most one family equality (in its left-hand side) is oriented
 
 A normal equality is a properly oriented equality with associated coercion
 that contains at most one family equality (in its left-hand side) is oriented

-such that it may be used as a reqrite rule.  It has one of the following two 
+such that it may be used as a rewrite rule.  It has one of the following two 

 forms:
 
 (1) co :: F t1..tn ~ t  (family equalities)
 forms:
 
 (1) co :: F t1..tn ~ t  (family equalities)

@@ -376,11 +383,10 @@ Variable equalities fall again in two classes:
 
 The types t, t1, ..., tn may not contain any occurrences of synonym
 families.  Moreover, in Forms (2) & (3), the left-hand side may not occur in
 
 The types t, t1, ..., tn may not contain any occurrences of synonym
 families.  Moreover, in Forms (2) & (3), the left-hand side may not occur in

-the right-hand side, and the relation x > y is an arbitrary, but total order
-on type variables
-
-!!!TODO: We may need to keep track of swapping for error messages (and to
-re-orient on finilisation).
+the right-hand side, and the relation x > y is an (nearly) arbitrary, but
+total order on type variables.  The only restriction that we impose on that
+order is that for x > y, we are happy to instantiate x with y taking into
+account kinds, signature skolems etc (cf, TcUnify.uUnfilledVars).

 
 \begin{code}
 data RewriteInst
 
 \begin{code}
 data RewriteInst

@@ -593,15 +599,14 @@ checkOrientation ty1 ty2 co inst
            ; return []
            }
 
            ; return []
            }
 

-      -- two tvs, left greater => unchanged
+      -- two tvs (distinct tvs, due to previous equation)

     go ty1@(TyVarTy tv1) ty2@(TyVarTy tv2)
     go ty1@(TyVarTy tv1) ty2@(TyVarTy tv2)

-      | tv1 > tv2
-      = mkRewriteVar False tv1 ty2 co
-
-      -- two tvs, right greater => swap
-      | otherwise
-      = do { co' <- mkSymEqInstCo co (ty2, ty1)
-           ; mkRewriteVar True tv2 ty1 co'
+      = do { isBigger <- tv1 `tvIsBigger` tv2
+           ; if isBigger                                      -- left greater
+               then mkRewriteVar False tv1 ty2 co             --   => unchanged
+               else do { co' <- mkSymEqInstCo co (ty2, ty1)   -- right greater
+                       ; mkRewriteVar True tv2 ty1 co'        --   => swap
+                       }

            }
 
       -- only lhs is a tv => unchanged
            }
 
       -- only lhs is a tv => unchanged

@@ -620,6 +625,26 @@ checkOrientation ty1 ty2 co inst
            ; mkRewriteVar True tv2 ty1 co'
            }
 
            ; mkRewriteVar True tv2 ty1 co'
            }
 

+      -- data type constructor application => decompose
+      -- NB: Special cased for efficiency - could be handled as type application
+    go (TyConApp con1 args1) (TyConApp con2 args2)
+      |  con1 == con2
+      && not (isOpenSynTyCon con1)   -- don't match family synonym apps
+      = do { co_args <- mkTyConEqInstCo co con1 (zip args1 args2)
+           ; eqss <- zipWith3M (\ty1 ty2 co -> checkOrientation ty1 ty2 co inst)
+                     args1 args2 co_args
+           ; return $ concat eqss
+           }
+
+      -- function type => decompose
+      -- NB: Special cased for efficiency - could be handled as type application
+    go (FunTy ty1_l ty1_r) (FunTy ty2_l ty2_r)
+      = do { (co_l, co_r) <- mkFunEqInstCo co (ty1_l, ty2_l) (ty1_r, ty2_r)
+           ; eqs_l <- checkOrientation ty1_l ty2_l co_l inst
+           ; eqs_r <- checkOrientation ty1_r ty2_r co_r inst
+           ; return $ eqs_l ++ eqs_r
+           }
+

       -- type applications => decompose
     go ty1 ty2 
       | Just (ty1_l, ty1_r) <- repSplitAppTy_maybe ty1   -- won't split fam apps
       -- type applications => decompose
     go ty1 ty2 
       | Just (ty1_l, ty1_r) <- repSplitAppTy_maybe ty1   -- won't split fam apps

@@ -629,8 +654,6 @@ checkOrientation ty1 ty2 co inst
            ; eqs_r <- checkOrientation ty1_r ty2_r co_r inst
            ; return $ eqs_l ++ eqs_r
            }
            ; eqs_r <- checkOrientation ty1_r ty2_r co_r inst
            ; return $ eqs_l ++ eqs_r
            }

--- !!!TODO: would be more efficient to handle the FunApp and the data
--- constructor application explicitly.

 
       -- inconsistency => type error
     go ty1 ty2
 
       -- inconsistency => type error
     go ty1 ty2

@@ -646,6 +669,55 @@ checkOrientation ty1 ty2 co inst
                                             , rwi_swapped = swapped
                                             }]
 
                                             , rwi_swapped = swapped
                                             }]
 

+    -- if tv1 `tvIsBigger` tv2, we make a rewrite rule tv1 ~> tv2
+    tvIsBigger :: TcTyVar -> TcTyVar -> TcM Bool
+    tvIsBigger tv1 tv2 
+      = isBigger tv1 (tcTyVarDetails tv1) tv2 (tcTyVarDetails tv2)
+      where
+        isBigger tv1 (SkolemTv _)     tv2 (SkolemTv _)
+          = return $ tv1 > tv2
+        isBigger _   (MetaTv _ _)     _   (SkolemTv _)
+          = return True
+        isBigger _   (SkolemTv _)     _   (MetaTv _ _)
+          = return False
+        isBigger tv1 (MetaTv info1 _) tv2 (MetaTv info2 _)
+          -- meta variable meets meta variable 
+          -- => be clever about which of the two to update 
+          --   (from TcUnify.uUnfilledVars minus boxy stuff)
+          = case (info1, info2) of
+              -- Avoid SigTvs if poss
+              (SigTv _, SigTv _)             -> return $ tv1 > tv2
+              (SigTv _, _      ) | k1_sub_k2 -> return False
+              (_,       SigTv _) | k2_sub_k1 -> return True
+
+              (_, _) 
+                | k1_sub_k2 &&
+                  k2_sub_k1    
+                  -> case (nicer_to_update tv1, nicer_to_update tv2) of
+                       (True, False) -> return True
+                       (False, True) -> return False
+                       _             -> return $ tv1 > tv2
+                | k1_sub_k2    -> return False
+                | k2_sub_k1    -> return True
+                | otherwise    -> kind_err >> return True
+              -- Update the variable with least kind info
+              -- See notes on type inference in Kind.lhs
+              -- The "nicer to" part only applies if the two kinds are the same,
+              -- so we can choose which to do.
+          where
+            kind_err = addErrCtxtM (unifyKindCtxt False tv1 (mkTyVarTy tv2)) $
+                       unifyKindMisMatch k1 k2
+
+            k1 = tyVarKind tv1
+            k2 = tyVarKind tv2
+            k1_sub_k2 = k1 `isSubKind` k2
+            k2_sub_k1 = k2 `isSubKind` k1
+
+            nicer_to_update tv = isSystemName (Var.varName tv)
+                -- Try to update sys-y type variables in preference to ones
+                -- gotten (say) by instantiating a polymorphic function with
+                -- a user-written type sig 
+

 flattenType :: Inst     -- context to get location  & name
             -> Type     -- the type to flatten
             -> TcM (Type,           -- the flattened type
 flattenType :: Inst     -- context to get location  & name
             -> Type     -- the type to flatten
             -> TcM (Type,           -- the flattened type

@@ -809,17 +881,23 @@ mkDictBind dict isWanted rewriteCo pred
   where
     loc = tci_loc dict
 
   where
     loc = tci_loc dict
 

--- gamma :: Fam args ~ alpha
--- => alpha :: Fam args ~ alpha, with alpha := Fam args
+-- gamma ::^l Fam args ~ alpha
+-- => gamma ::^w Fam args ~ alpha, with alpha := Fam args & gamma := Fam args

 --    (the update of alpha will not be apparent during propagation, as we
 --    never follow the indirections of meta variables; it will be revealed
 --    when the equality is zonked)
 --    (the update of alpha will not be apparent during propagation, as we
 --    never follow the indirections of meta variables; it will be revealed
 --    when the equality is zonked)

+--
+--  NB: It's crucial to update *both* alpha and gamma, as gamma may already
+--      have escaped into some other coercions during normalisation.
+--

 wantedToLocal :: RewriteInst -> TcM RewriteInst
 wantedToLocal eq@(RewriteFam {rwi_fam   = fam, 
                               rwi_args  = args, 
 wantedToLocal :: RewriteInst -> TcM RewriteInst
 wantedToLocal eq@(RewriteFam {rwi_fam   = fam, 
                               rwi_args  = args, 

-                              rwi_right = alphaTy@(TyVarTy alpha)})
+                              rwi_right = TyVarTy alpha,
+                              rwi_co    = Left gamma})

   = do { writeMetaTyVar alpha (mkTyConApp fam args)
   = do { writeMetaTyVar alpha (mkTyConApp fam args)

-       ; return $ eq {rwi_co = mkGivenCo alphaTy}
+       ; writeMetaTyVar gamma (mkTyConApp fam args)
+       ; return $ eq {rwi_co = mkGivenCo $ mkTyVarTy gamma}

        }
 wantedToLocal _ = panic "TcTyFuns.wantedToLocal"
 \end{code}
        }
 wantedToLocal _ = panic "TcTyFuns.wantedToLocal"
 \end{code}

@@ -873,8 +951,11 @@ mapSubstRules eq eqs
        }
   where
     substRules eq1 eq2
        }
   where
     substRules eq1 eq2

-      = do {   -- try the SubstFam rule
-             optEqs <- applySubstFam eq1 eq2
+      = do {traceTc $ hang (ptext (sLit "Trying subst rules with"))
+                        4 (ppr eq1 $$ ppr eq2)
+
+               -- try the SubstFam rule
+           ; optEqs <- applySubstFam eq1 eq2

            ; case optEqs of
                Just (eqs, skolems) -> return (eqs, [], skolems)
                Nothing             -> do 
            ; case optEqs of
                Just (eqs, skolems) -> return (eqs, [], skolems)
                Nothing             -> do 

@@ -942,20 +1023,26 @@ applySubstFam :: RewriteInst
               -> TcM (Maybe ([RewriteInst], TyVarSet))
 applySubstFam eq1@(RewriteFam {rwi_fam = fam1, rwi_args = args1})
               eq2@(RewriteFam {rwi_fam = fam2, rwi_args = args2})
               -> TcM (Maybe ([RewriteInst], TyVarSet))
 applySubstFam eq1@(RewriteFam {rwi_fam = fam1, rwi_args = args1})
               eq2@(RewriteFam {rwi_fam = fam2, rwi_args = args2})

+
+    -- rule matches => rewrite

   | fam1 == fam2 && tcEqTypes args1 args2 &&
     (isWantedRewriteInst eq2 || not (isWantedRewriteInst eq1))
   | fam1 == fam2 && tcEqTypes args1 args2 &&
     (isWantedRewriteInst eq2 || not (isWantedRewriteInst eq1))

--- !!!TODO: tcEqTypes is insufficient as it does not look through type synonyms
--- !!!Check whether anything breaks by making tcEqTypes look through synonyms.
--- !!!Should be ok and we don't want three type equalities.

   = do { co2' <- mkRightTransEqInstCo co2 co1 (lhs, rhs)
        ; eq2' <- deriveEqInst eq2 lhs rhs co2'
        ; liftM Just $ normEqInst eq2'
        }
   = do { co2' <- mkRightTransEqInstCo co2 co1 (lhs, rhs)
        ; eq2' <- deriveEqInst eq2 lhs rhs co2'
        ; liftM Just $ normEqInst eq2'
        }

+
+    -- rule would match with eq1 and eq2 swapped => put eq2 into todo list
+  | fam1 == fam2 && tcEqTypes args1 args2 &&
+    (isWantedRewriteInst eq1 || not (isWantedRewriteInst eq2))
+  = return $ Just ([eq2], emptyVarSet)
+

   where
     lhs = rwi_right eq1
     rhs = rwi_right eq2
     co1 = eqInstCoType (rwi_co eq1)
     co2 = rwi_co eq2
   where
     lhs = rwi_right eq1
     rhs = rwi_right eq2
     co1 = eqInstCoType (rwi_co eq1)
     co2 = rwi_co eq2

+

 applySubstFam _ _ = return Nothing
 \end{code}
 
 applySubstFam _ _ = return Nothing
 \end{code}
 

@@ -977,17 +1064,26 @@ applySubstVarVar :: RewriteInst
                  -> TcM (Maybe ([RewriteInst], TyVarSet))
 applySubstVarVar eq1@(RewriteVar {rwi_var = tv1})
                  eq2@(RewriteVar {rwi_var = tv2})
                  -> TcM (Maybe ([RewriteInst], TyVarSet))
 applySubstVarVar eq1@(RewriteVar {rwi_var = tv1})
                  eq2@(RewriteVar {rwi_var = tv2})

+
+    -- rule matches => rewrite

   | tv1 == tv2 &&
     (isWantedRewriteInst eq2 || not (isWantedRewriteInst eq1))
   = do { co2' <- mkRightTransEqInstCo co2 co1 (lhs, rhs)
        ; eq2' <- deriveEqInst eq2 lhs rhs co2'
        ; liftM Just $ normEqInst eq2'
        }
   | tv1 == tv2 &&
     (isWantedRewriteInst eq2 || not (isWantedRewriteInst eq1))
   = do { co2' <- mkRightTransEqInstCo co2 co1 (lhs, rhs)
        ; eq2' <- deriveEqInst eq2 lhs rhs co2'
        ; liftM Just $ normEqInst eq2'
        }

+
+    -- rule would match with eq1 and eq2 swapped => put eq2 into todo list
+  | tv1 == tv2 &&
+    (isWantedRewriteInst eq1 || not (isWantedRewriteInst eq2))
+  = return $ Just ([eq2], emptyVarSet)
+

   where
     lhs = rwi_right eq1
     rhs = rwi_right eq2
     co1 = eqInstCoType (rwi_co eq1)
     co2 = rwi_co eq2
   where
     lhs = rwi_right eq1
     rhs = rwi_right eq2
     co1 = eqInstCoType (rwi_co eq1)
     co2 = rwi_co eq2

+

 applySubstVarVar _ _ = return Nothing
 \end{code}
 
 applySubstVarVar _ _ = return Nothing
 \end{code}
 

@@ -1005,6 +1101,8 @@ co2' is returned.  (The equality co1 is not returned as it remain unaltered.)
 
 \begin{code}
 applySubstVarFam :: RewriteInst -> RewriteInst -> TcM (Maybe RewriteInst)
 
 \begin{code}
 applySubstVarFam :: RewriteInst -> RewriteInst -> TcM (Maybe RewriteInst)

+
+  -- rule matches => rewrite

 applySubstVarFam eq1@(RewriteVar {rwi_var = tv1})
                  eq2@(RewriteFam {rwi_fam = fam2, rwi_args = args2})
   | tv1 `elemVarSet` tyVarsOfTypes args2
 applySubstVarFam eq1@(RewriteVar {rwi_var = tv1})
                  eq2@(RewriteFam {rwi_fam = fam2, rwi_args = args2})
   | tv1 `elemVarSet` tyVarsOfTypes args2

@@ -1019,6 +1117,13 @@ applySubstVarFam eq1@(RewriteVar {rwi_var = tv1})
     rhs2 = rwi_right eq2
     co1  = eqInstCoType (rwi_co eq1)
     co2  = rwi_co eq2
     rhs2 = rwi_right eq2
     co1  = eqInstCoType (rwi_co eq1)
     co2  = rwi_co eq2

+
+  -- rule would match with eq1 and eq2 swapped => put eq2 into todo list
+applySubstVarFam (RewriteFam {rwi_args = args1})
+                 eq2@(RewriteVar {rwi_var = tv2})
+  | tv2 `elemVarSet` tyVarsOfTypes args1
+  = return $ Just eq2
+

 applySubstVarFam _ _ = return Nothing
 \end{code}
 
 applySubstVarFam _ _ = return Nothing
 \end{code}
 

@@ -1153,44 +1258,72 @@ Return all remaining wanted equalities.  The Boolean result component is True
 if at least one instantiation of a flexible that is *not* a skolem from
 flattening was performed.
 
 if at least one instantiation of a flexible that is *not* a skolem from
 flattening was performed.
 

+We need to instantiate all flexibles that arose as skolems during flattening
+of wanteds before we instantiate any other flexibles. Consider F delta ~
+alpha, F alpha ~ delta, where alpha is a skolem and delta a free flexible. We
+need to produce F (F delta) ~ delta (and not F (F alpha) ~ alpha). Otherwise,
+we may wrongly claim to having performed an improvement, which can lead to
+non-termination of the combined class-family solver.
+

 \begin{code}
 instantiateAndExtract :: [RewriteInst] -> Bool -> TyVarSet -> TcM ([Inst], Bool)
 instantiateAndExtract eqs localsEmpty skolems
 \begin{code}
 instantiateAndExtract :: [RewriteInst] -> Bool -> TyVarSet -> TcM ([Inst], Bool)
 instantiateAndExtract eqs localsEmpty skolems

-  = do { results <- mapM inst wanteds
-       ; let residuals    = [eq | Left eq <- results]
-             only_skolems = and [tv `elemVarSet` skolems | Right tv <- results]
+  = do { traceTc $ hang (ptext (sLit "instantiateAndExtract:"))
+                     4 (ppr eqs $$ ppr skolems)
+           -- start by *only* instantiating skolem flexibles from flattening
+       ; unflat_wanteds <- liftM catMaybes $ 
+                             mapM (inst (`elemVarSet` skolems)) wanteds
+           -- only afterwards instantiate free flexibles
+       ; residuals <- liftM catMaybes $ mapM (inst (const True)) unflat_wanteds
+       ; let improvement = length residuals < length unflat_wanteds

        ; residuals' <- mapM rewriteInstToInst residuals
        ; residuals' <- mapM rewriteInstToInst residuals

-       ; return (residuals', not only_skolems)
+       ; return (residuals', improvement)

        }
   where
     wanteds      = filter (isWantedCo . rwi_co) eqs
     checkingMode = length eqs > length wanteds || not localsEmpty
                      -- no local equalities or dicts => checking mode
 
        }
   where
     wanteds      = filter (isWantedCo . rwi_co) eqs
     checkingMode = length eqs > length wanteds || not localsEmpty
                      -- no local equalities or dicts => checking mode
 

-    inst eq@(RewriteVar {rwi_var = tv1, rwi_right = ty2, rwi_co = co})
-
-        -- co :: alpha ~ t
-      | isMetaTyVar tv1
-      = doInst (rwi_swapped eq) tv1 ty2 co eq
-
-        -- co :: a ~ alpha
-      | Just tv2 <- tcGetTyVar_maybe ty2
-      , isMetaTyVar tv2
-      = doInst (not $ rwi_swapped eq) tv2 (mkTyVarTy tv1) co eq
+        -- co :: alpha ~ t or co :: a ~ alpha
+    inst mayInst eq@(RewriteVar {rwi_var = tv1, rwi_right = ty2, rwi_co = co})
+      = do { flexi_tv1       <- isFlexible mayInst tv1
+           ; maybe_flexi_tv2 <- isFlexibleTy mayInst ty2
+           ; case (flexi_tv1, maybe_flexi_tv2) of
+               (True, _) 
+                 -> -- co :: alpha ~ t
+                    doInst (rwi_swapped eq) tv1 ty2 co eq
+               (False, Just tv2) 
+                 -> -- co :: a ~ alpha                  
+                    doInst (not $ rwi_swapped eq) tv2 (mkTyVarTy tv1) co eq
+               _ -> return $ Just eq
+           }

 
         -- co :: F args ~ alpha, and we are in checking mode (ie, no locals)
 
         -- co :: F args ~ alpha, and we are in checking mode (ie, no locals)

-    inst eq@(RewriteFam {rwi_fam = fam, rwi_args = args, rwi_right = ty2, 
-                         rwi_co = co})
+    inst mayInst eq@(RewriteFam {rwi_fam = fam, rwi_args = args, 
+                                 rwi_right = ty2, rwi_co = co})

       | Just tv2 <- tcGetTyVar_maybe ty2
       , isMetaTyVar tv2
       | Just tv2 <- tcGetTyVar_maybe ty2
       , isMetaTyVar tv2

-      , checkingMode || tv2 `elemVarSet` skolems
-                        -- !!!TODO: this is too liberal, even if tv2 is in 
+      , mayInst tv2 && (checkingMode || tv2 `elemVarSet` skolems)
+                        -- !!!FIXME: this is too liberal, even if tv2 is in 

                         -- skolems we shouldn't instantiate if tvs occurs 
                         -- in other equalities that may propagate it into the
                         -- environment
       = doInst (not $ rwi_swapped eq) tv2 (mkTyConApp fam args) co eq
 
                         -- skolems we shouldn't instantiate if tvs occurs 
                         -- in other equalities that may propagate it into the
                         -- environment
       = doInst (not $ rwi_swapped eq) tv2 (mkTyConApp fam args) co eq
 

-    inst eq = return $ Left eq
+    inst _mayInst eq = return $ Just eq
+
+    -- tv is a meta var and not filled
+    isFlexible mayInst tv
+      | isMetaTyVar tv && mayInst tv = liftM isFlexi $ readMetaTyVar tv
+      | otherwise                    = return False
+
+    -- type is a tv that is a meta var and not filled
+    isFlexibleTy mayInst ty
+      | Just tv <- tcGetTyVar_maybe ty = do {flexi <- isFlexible mayInst tv
+                                            ; if flexi then return $ Just tv 
+                                                       else return Nothing
+                                            }
+      | otherwise                      = return Nothing

 
     doInst _swapped _tv _ty (Right ty) _eq 
       = pprPanic "TcTyFuns.doInst: local eq: " (ppr ty)
 
     doInst _swapped _tv _ty (Right ty) _eq 
       = pprPanic "TcTyFuns.doInst: local eq: " (ppr ty)

@@ -1199,6 +1332,20 @@ instantiateAndExtract eqs localsEmpty skolems
            ; uMeta swapped tv lookupTV ty cotv
            }
       where
            ; uMeta swapped tv lookupTV ty cotv
            }
       where

+        -- Try to fill in a meta variable.  There is *no* need to consider
+        -- reorienting the underlying equality; `checkOrientation' makes sure
+        -- that we get variable-variable equalities only in the appropriate
+        -- orientation.
+        --
+        uMeta :: Bool                    -- is this a swapped equality?
+              -> TcTyVar                 -- tyvar to instantiate
+              -> LookupTyVarResult       -- lookup result of that tyvar
+              -> TcType                  -- to to instantiate tyvar with
+              -> TcTyVar                 -- coercion tyvar of current equality
+              -> TcM (Maybe RewriteInst) -- returns the original equality if
+                                         -- the tyvar could not be instantiated,
+                                         -- and hence, the equality must be kept
+

         -- meta variable has been filled already
         -- => keep the equality
         uMeta _swapped tv (IndirectTv fill_ty) ty _cotv
         -- meta variable has been filled already
         -- => keep the equality
         uMeta _swapped tv (IndirectTv fill_ty) ty _cotv

@@ -1206,103 +1353,55 @@ instantiateAndExtract eqs localsEmpty skolems
                    ptext (sLit "flexible") <+> ppr tv <+>
                    ptext (sLit "already filled with") <+> ppr fill_ty <+>
                    ptext (sLit "meant to fill with") <+> ppr ty
                    ptext (sLit "flexible") <+> ppr tv <+>
                    ptext (sLit "already filled with") <+> ppr fill_ty <+>
                    ptext (sLit "meant to fill with") <+> ppr ty

-               ; return $ Left eq
+               ; return $ Just eq

                }
 
                }
 

-        -- type variable meets type variable
-        -- => check that tv2 hasn't been updated yet and choose which to update
-        uMeta swapped tv1 (DoneTv details1) (TyVarTy tv2) cotv
-          | tv1 == tv2
-          = panic "TcTyFuns.uMeta: normalisation shouldn't allow x ~ x"
+        -- signature skolem
+        -- => cannot update (retain the equality)!
+        uMeta _swapped _tv (DoneTv (MetaTv (SigTv _) _)) _non_tv_ty _cotv
+          = return $ Just eq

 
 

-          | otherwise
+        -- type variable meets type variable
+        -- => `checkOrientation' already ensures that it is fine to instantiate
+        --    tv1 with tv2, but chase tv2's instantiations if necessary
+        -- NB: tv's instantiations won't alter the orientation in which we
+        --     want to instantiate as they either constitute a family 
+        --     application or are themselves due to a properly oriented
+        --     instantiation
+        uMeta swapped tv1 details1@(DoneTv (MetaTv _ ref)) ty@(TyVarTy tv2) cotv

           = do { lookupTV2 <- lookupTcTyVar tv2
                ; case lookupTV2 of
           = do { lookupTV2 <- lookupTcTyVar tv2
                ; case lookupTV2 of

-                   IndirectTv ty   -> 
-                     uMeta swapped tv1 (DoneTv details1) ty cotv
-                   DoneTv details2 -> 
-                     uMetaVar swapped tv1 details1 tv2 details2 cotv
+                   IndirectTv ty' -> 
+                     uMeta swapped tv1 details1 ty' cotv
+                   DoneTv _       -> 
+                     uMetaInst swapped tv1 ref ty cotv

                }
 
                }
 

-        ------ Beyond this point we know that ty2 is not a type variable
-
-        -- signature skolem meets non-variable type
-        -- => cannot update (retain the equality)!
-        uMeta _swapped _tv (DoneTv (MetaTv (SigTv _) _)) _non_tv_ty _cotv
-          = return $ Left eq
-

         -- updatable meta variable meets non-variable type
         -- => occurs check, monotype check, and kinds match check, then update
         uMeta swapped tv (DoneTv (MetaTv _ ref)) non_tv_ty cotv
         -- updatable meta variable meets non-variable type
         -- => occurs check, monotype check, and kinds match check, then update
         uMeta swapped tv (DoneTv (MetaTv _ ref)) non_tv_ty cotv

+          = uMetaInst swapped tv ref non_tv_ty cotv
+
+        uMeta _ _ _ _ _ = panic "TcTyFuns.uMeta"
+
+        -- We know `tv' can be instantiated; check that `ty' is alright for
+        -- instantiating `tv' with and then do it; otherwise, return the original
+        -- equality.
+        uMetaInst swapped tv ref ty cotv

           = do {   -- occurs + monotype check
           = do {   -- occurs + monotype check

-               ; mb_ty' <- checkTauTvUpdate tv non_tv_ty    
+               ; mb_ty' <- checkTauTvUpdate tv ty    

                              
                ; case mb_ty' of
                    Nothing  -> 
                              
                ; case mb_ty' of
                    Nothing  -> 

-                     -- normalisation shouldn't leave families in non_tv_ty
-                     panic "TcTyFuns.uMeta: unexpected synonym family"
+                     -- there may be a family in non_tv_ty due to an unzonked,
+                     -- but updated skolem for a local equality
+                     return $ Just eq

                    Just ty' ->
                      do { checkUpdateMeta swapped tv ref ty'  -- update meta var
                         ; writeMetaTyVar cotv ty'             -- update co var
                    Just ty' ->
                      do { checkUpdateMeta swapped tv ref ty'  -- update meta var
                         ; writeMetaTyVar cotv ty'             -- update co var

-                        ; return $ Right tv
+                        ; return Nothing

                         }
                }
                         }
                }

-
-        uMeta _ _ _ _ _ = panic "TcTyFuns.uMeta"
-
-        -- uMetaVar: unify two type variables
-        -- meta variable meets skolem 
-        -- => just update
-        uMetaVar swapped tv1 (MetaTv _ ref) tv2 (SkolemTv _) cotv
-          = do { checkUpdateMeta swapped tv1 ref (mkTyVarTy tv2)
-               ; writeMetaTyVar cotv (mkTyVarTy tv2)
-               ; return $ Right tv1
-               }
-
-        -- meta variable meets meta variable 
-        -- => be clever about which of the two to update 
-        --   (from TcUnify.uUnfilledVars minus boxy stuff)
-        uMetaVar swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2) cotv
-          = do { tv <- case (info1, info2) of
-                         -- Avoid SigTvs if poss
-                         (SigTv _, _      ) | k1_sub_k2 -> update_tv2
-                         (_,       SigTv _) | k2_sub_k1 -> update_tv1
-
-                         (_,   _) | k1_sub_k2 -> if k2_sub_k1 && 
-                                                    nicer_to_update_tv1
-                                                 then update_tv1  -- Same kinds
-                                                 else update_tv2
-                                  | k2_sub_k1 -> update_tv1
-                                  | otherwise -> kind_err >> return tv1
-              -- Update the variable with least kind info
-              -- See notes on type inference in Kind.lhs
-              -- The "nicer to" part only applies if the two kinds are the same,
-              -- so we can choose which to do.
-
-               ; writeMetaTyVar cotv (mkTyVarTy tv2)
-               ; return $ Right tv
-               }
-          where
-                -- Kinds should be guaranteed ok at this point
-            update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
-                         >> return tv1
-            update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
-                         >> return tv2
-
-            kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
-                       unifyKindMisMatch k1 k2
-
-            k1 = tyVarKind tv1
-            k2 = tyVarKind tv2
-            k1_sub_k2 = k1 `isSubKind` k2
-            k2_sub_k1 = k2 `isSubKind` k1
-
-            nicer_to_update_tv1 = isSystemName (Var.varName tv1)
-                -- Try to update sys-y type variables in preference to ones
-                -- gotten (say) by instantiating a polymorphic function with
-                -- a user-written type sig 
-
-        uMetaVar _ _ _ _ _ _ = panic "uMetaVar"

 \end{code}
 
 
 \end{code}