Refactor, improve, and document the deriving mechanism

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

diff --git a/compiler/typecheck/TcTyFuns.lhs b/compiler/typecheck/TcTyFuns.lhs
index b3f97be..0130aa6 100644 (file)
--- a/compiler/typecheck/TcTyFuns.lhs
+++ b/compiler/typecheck/TcTyFuns.lhs
@@ -1,5 +1,11 @@
 
 \begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
 
 module TcTyFuns(
        finalizeEqInst,
@@ -570,7 +576,7 @@ decompInst i@(EqInst {})
                         do { cotvs <- zipWithM (\t1 t2 -> 
                                                 newMetaTyVar TauTv 
                                                              (mkCoKind t1 t2)) 
-                                               tys1' tys2'
+                                               tys1 tys2
                            ; let cos = map TyVarTy cotvs
                            ; writeMetaTyVar old_covar (TyConApp con1 cos)
                            ; return $ map mkWantedCo cotvs
@@ -578,17 +584,19 @@ decompInst i@(EqInst {})
                       -- co_i := Con_i old_co
                       (\old_co -> return $ 
                                     map mkGivenCo $
-                                        mkRightCoercions (length tys1') old_co)
-           ; insts <- zipWithM mkEqInst (zipWith EqPred tys1' tys2') cos
-           ; return (insts, not $ null insts)
+                                        mkRightCoercions (length tys1) old_co)
+           ; insts <- zipWithM mkEqInst (zipWith EqPred tys1 tys2) cos
+           ; traceTc (text "decomp identicalHead" <+> ppr insts) 
+           ; return (insts, not $ null insts) 
            }
       | con1 /= con2 && not (isOpenSynTyCon con1 || isOpenSynTyCon con2)
         -- not matching data constructors (of any flavour) are bad news
       = do { env0 <- tcInitTidyEnv
-           ; let (env1, tidy_ty1)  =  tidyOpenType env0 ty1
-                 (env2, tidy_ty2)  =  tidyOpenType env1 ty2
-                 extra                  = sep [ppr tidy_ty1, char '~', ppr tidy_ty2]
-                 msg            = ptext SLIT("Couldn't match expected type against inferred type")
+           ; let (env1, tidy_ty1) = tidyOpenType env0 ty1
+                 (env2, tidy_ty2) = tidyOpenType env1 ty2
+                 extra                   = sep [ppr tidy_ty1, char '~', ppr tidy_ty2]
+                 msg             = 
+                   ptext SLIT("Unsolvable equality constraint:")
            ; failWithTcM (env2, hang msg 2 extra)
            }
       where