X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FFamInst.lhs;h=5f4b2a3f922a38ac76a0c12c0ecd822b89091320;hb=306fac3f65c2e68e6c320a9db221d126c989fad2;hp=8e8df882d32d3f44a5022cadde3e1ae3f32c9bf8;hpb=f5e36de375d03a0af3858c4222690bbaa5f8a1ee;p=ghc-hetmet.git

diff --git a/compiler/typecheck/FamInst.lhs b/compiler/typecheck/FamInst.lhs
index 8e8df88..5f4b2a3 100644
--- a/compiler/typecheck/FamInst.lhs
+++ b/compiler/typecheck/FamInst.lhs
@@ -20,7 +20,7 @@ import LazyUniqFM
 import FiniteMap
 import FastString
 
-import Maybe
+import Maybes
 import Monad
 \end{code}
 
@@ -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
@@ -107,8 +117,8 @@ checkFamInstConsistency famInstMods directlyImpMods
     -- already loaded in the EPS or they are in the HPT.
     --
     check modInstsEnv (ModulePair m1 m2)
-      = let { instEnv1 = fromJust . lookupModuleEnv modInstsEnv $ m1
-	    ; instEnv2 = fromJust . lookupModuleEnv modInstsEnv $ m2
+      = let { instEnv1 = (expectJust "checkFamInstConsistency") . lookupModuleEnv modInstsEnv $ m1
+	    ; instEnv2 = (expectJust "checkFamInstConsistency") . lookupModuleEnv modInstsEnv $ m2
 	    ; insts1   = famInstEnvElts instEnv1
 	    }
         in
@@ -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.