Cover PredTy case in Type.tyFamInsts

[ghc-hetmet.git] / compiler / types / FunDeps.lhs

diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 8f529a4..7f9f050 100644 (file)
--- a/compiler/types/FunDeps.lhs
+++ b/compiler/types/FunDeps.lhs
@@ -29,6 +29,7 @@ import Outputable
 import Util
 import FastString
 
+import Data.List       ( nubBy )
 import Data.Maybe      ( isJust )
 \end{code}
 
@@ -201,8 +202,8 @@ type Equation = (TyVarSet, [(Type, Type)])
 
 pprEquation :: Equation -> SDoc
 pprEquation (qtvs, pairs) 
-  = vcat [ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)),
-         nest 2 (vcat [ ppr t1 <+> ptext SLIT(":=:") <+> ppr t2 | (t1,t2) <- pairs])]
+  = vcat [ptext (sLit "forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)),
+         nest 2 (vcat [ ppr t1 <+> ptext (sLit ":=:") <+> ppr t2 | (t1,t2) <- pairs])]
 \end{code}
 
 Given a bunch of predicates that must hold, such as
@@ -293,7 +294,7 @@ improveOne inst_env pred@(ClassP cls tys, _) preds
        , not (instanceCantMatch inst_tcs trimmed_tcs)
        , eqn <- checkClsFD qtvs fd cls_tvs tys_inst tys
        , let p_inst = (mkClassPred cls tys_inst, 
-                       ptext SLIT("arising from the instance declaration at")
+                       ptext (sLit "arising from the instance declaration at")
                        <+> ppr (getSrcLoc ispec))
        ]
 
@@ -469,7 +470,8 @@ badFunDeps :: [Instance] -> Class
           -> TyVarSet -> [Type]        -- Proposed new instance type
           -> [Instance]
 badFunDeps cls_insts clas ins_tv_set ins_tys 
-  = [ ispec | fd <- fds,       -- fds is often empty
+  = nubBy eq_inst $
+    [ ispec | fd <- fds,       -- fds is often empty, so do this first!
              let trimmed_tcs = trimRoughMatchTcs clas_tvs fd rough_tcs,
              ispec@(Instance { is_tcs = inst_tcs, is_tvs = tvs, 
                                is_tys = tys }) <- cls_insts,
@@ -482,19 +484,26 @@ badFunDeps cls_insts clas ins_tv_set ins_tys
   where
     (clas_tvs, fds) = classTvsFds clas
     rough_tcs = roughMatchTcs ins_tys
+    eq_inst i1 i2 = instanceDFunId i1 == instanceDFunId i2
+       -- An single instance may appear twice in the un-nubbed conflict list
+       -- because it may conflict with more than one fundep.  E.g.
+       --      class C a b c | a -> b, a -> c
+       --      instance C Int Bool Bool
+       --      instance C Int Char Char
+       -- The second instance conflicts with the first by *both* fundeps
 
 trimRoughMatchTcs :: [TyVar] -> FunDep TyVar -> [Maybe Name] -> [Maybe Name]
 -- Computing rough_tcs for a particular fundep
---     class C a b c | a -> b where ... 
+--     class C a b c | a -> b where ...
 -- For each instance .... => C ta tb tc
--- we want to match only on the types ta, tc; so our
+-- we want to match only on the type ta; so our
 -- rough-match thing must similarly be filtered.  
--- Hence, we Nothing-ise the tb type right here
-trimRoughMatchTcs clas_tvs (_,rtvs) mb_tcs
+-- Hence, we Nothing-ise the tb and tc types right here
+trimRoughMatchTcs clas_tvs (ltvs, _) mb_tcs
   = zipWith select clas_tvs mb_tcs
   where
-    select clas_tv mb_tc | clas_tv `elem` rtvs = Nothing
-                        | otherwise           = mb_tc
+    select clas_tv mb_tc | clas_tv `elem` ltvs = mb_tc
+                         | otherwise           = Nothing
 \end{code}