Clean up some comments

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

diff --git a/compiler/typecheck/TcTyFuns.lhs b/compiler/typecheck/TcTyFuns.lhs
index bed053d..2237e3f 100644 (file)
--- a/compiler/typecheck/TcTyFuns.lhs
+++ b/compiler/typecheck/TcTyFuns.lhs
@@ -231,9 +231,6 @@ 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 +283,7 @@ 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 { ASSERTM2( allM wantedEqInstIsUnsolved eqs, ppr eqs )

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

@@ -303,10 +300,15 @@ 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 +349,7 @@ 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)

@@ -379,9 +381,6 @@ 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
 
 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).
-

 \begin{code}
 data RewriteInst
   = RewriteVar  -- Form (2) above
 \begin{code}
 data RewriteInst
   = RewriteVar  -- Form (2) above

@@ -809,17 +808,23 @@ 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 +878,11 @@ 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 +950,26 @@ 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 +991,26 @@ 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 +1028,8 @@ 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 +1044,13 @@ 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 +1185,72 @@ 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)

@@ -1206,7 +1266,7 @@ 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
                }
 
         -- type variable meets type variable

@@ -1229,7 +1289,7 @@ instantiateAndExtract eqs localsEmpty skolems
         -- signature skolem meets non-variable type
         -- => cannot update (retain the equality)!
         uMeta _swapped _tv (DoneTv (MetaTv (SigTv _) _)) _non_tv_ty _cotv
         -- signature skolem meets non-variable type
         -- => cannot update (retain the equality)!
         uMeta _swapped _tv (DoneTv (MetaTv (SigTv _) _)) _non_tv_ty _cotv

-          = return $ Left eq
+          = return $ Just eq

 
         -- updatable meta variable meets non-variable type
         -- => occurs check, monotype check, and kinds match check, then update
 
         -- updatable meta variable meets non-variable type
         -- => occurs check, monotype check, and kinds match check, then update

@@ -1239,12 +1299,13 @@ instantiateAndExtract eqs localsEmpty skolems
                              
                ; 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

                         }
                }
 
                         }
                }
 

@@ -1256,38 +1317,36 @@ instantiateAndExtract eqs localsEmpty skolems
         uMetaVar swapped tv1 (MetaTv _ ref) tv2 (SkolemTv _) cotv
           = do { checkUpdateMeta swapped tv1 ref (mkTyVarTy tv2)
                ; writeMetaTyVar cotv (mkTyVarTy tv2)
         uMetaVar swapped tv1 (MetaTv _ ref) tv2 (SkolemTv _) cotv
           = do { checkUpdateMeta swapped tv1 ref (mkTyVarTy tv2)
                ; writeMetaTyVar cotv (mkTyVarTy tv2)

-               ; return $ Right tv1
+               ; return Nothing

                }
 
         -- 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
                }
 
         -- 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
+          = do { 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

               -- 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)
               -- 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
+               ; return Nothing

                }
           where
                 -- Kinds should be guaranteed ok at this point
             update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
                }
           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)
             update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)

-                         >> return tv2

 
             kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
                        unifyKindMisMatch k1 k2
 
             kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
                        unifyKindMisMatch k1 k2