Comments and parens only

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

diff --git a/compiler/typecheck/FamInst.lhs b/compiler/typecheck/FamInst.lhs
index 8e8df88..67c526d 100644 (file)
--- a/compiler/typecheck/FamInst.lhs
+++ b/compiler/typecheck/FamInst.lhs
@@ -36,6 +36,16 @@ check whether the instances in the two modules are consistent, *unless* we can
 be certain that the instances of the two modules have already been checked for
 consistency during the compilation of modules that we import.
 
+Why do we need to check?  Consider 
+   module X1 where               module X2 where
+    data T1                        data T2
+    type instance F T1 b = Int     type instance F a T2 = Char
+    f1 :: F T1 a -> Int                    f2 :: Char -> F a T2
+    f1 x = x                       f2 x = x
+
+Now if we import both X1 and X2 we could make (f2 . f1) :: Int -> Char.
+Notice that neither instance is an orphan.
+
 How do we know which pairs of modules have already been checked?  Any pair of
 modules where both modules occur in the `HscTypes.dep_finsts' set (of the
 `HscTypes.Dependencies') of one of our directly imported modules must have
@@ -187,9 +197,9 @@ checkForConflicts inst_envs famInst
           conflictInstErr famInst (head conflicts)
        }
   where
-      -- * In the case of data family instances, any overlap is fundamentally a 
+      -- - 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 
+      -- - 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.