Add {-# OPTIONS_GHC -w #-} and some blurb to all compiler modules

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

diff --git a/compiler/typecheck/FamInst.lhs b/compiler/typecheck/FamInst.lhs
index fd98fe9..d789c88 100644 (file)
--- a/compiler/typecheck/FamInst.lhs
+++ b/compiler/typecheck/FamInst.lhs
@@ -1,6 +1,13 @@
 The @FamInst@ type: family instance heads
 
 \begin{code}
+{-# OPTIONS_GHC -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/WorkingConventions#Warnings
+-- for details
+
 module FamInst ( 
         checkFamInstConsistency, tcExtendLocalFamInstEnv
     ) where
@@ -180,19 +187,27 @@ checkForConflicts inst_envs famInst
 
        ; let { matches   = lookupFamInstEnvUnify inst_envs fam tys'
             ; conflicts = [ conflictingFamInst
-                          | match@(_, conflictingFamInst) <- matches
-                          , conflicting fam tys' tycon match 
+                          | match@((conflictingFamInst, _), _) <- matches
+                          , conflicting tycon match 
                           ]
             }
        ; unless (null conflicts) $
           conflictInstErr famInst (head conflicts)
        }
   where
-    -- In the case of data/newtype instances, any overlap is a conflict (as
-    -- these instances imply injective type mappings).
-    conflicting _   _    tycon _                 | isAlgTyCon tycon = True
-    conflicting fam tys' tycon (subst, cFamInst) | otherwise     =
-      panic "FamInst.checkForConflicts: overlap check for indexed synonyms is still missing"
+      -- * In the case of data family instances, any overlap is fundamentally a 
+      --   conflict (as these instances imply injective type mappings).
+      -- * In the case of type family instances, overlap is admitted as long as 
+      --   the right-hand sides of the overlapping rules coincide under the
+      --   overlap substitution.  We require that they are syntactically equal;
+      --   anything else would be difficult to test for at this stage.
+    conflicting tycon1 ((famInst2, _), subst) 
+      | isAlgTyCon tycon1 = True
+      | otherwise         = not (rhs1 `tcEqType` rhs2)
+      where
+        tycon2 = famInstTyCon famInst2
+        rhs1   = substTy subst $ synTyConType tycon1
+        rhs2   = substTy subst $ synTyConType tycon2
 
 conflictInstErr famInst conflictingFamInst
   = addFamInstLoc famInst $