X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypes%2FFunDeps.lhs;h=7f9f0509e2f2a14f58a6d4a817dd9622fa38746b;hb=1ae354e107715a9e3fd4e2d67b61f868c090e4ae;hp=949cac489e3eca9ce861d93008f3a72e00a8786f;hpb=ebc623f8daba6bc027c625d333d28ff2ccd630cf;p=ghc-hetmet.git

diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 949cac4..7f9f050 100644
--- 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}
 
@@ -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}