Replace couple of fromJust with expectJust

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

diff --git a/compiler/typecheck/FamInst.lhs b/compiler/typecheck/FamInst.lhs
index 712ac39..5f4b2a3 100644 (file)
--- a/compiler/typecheck/FamInst.lhs
+++ b/compiler/typecheck/FamInst.lhs
@@ -5,8 +5,6 @@ module FamInst (
         checkFamInstConsistency, tcExtendLocalFamInstEnv
     ) where
 
-#include "HsVersions.h"
-
 import HscTypes
 import FamInstEnv
 import TcMType
@@ -18,10 +16,11 @@ import Name
 import Module
 import SrcLoc
 import Outputable
-import UniqFM
+import LazyUniqFM
 import FiniteMap
+import FastString
 
-import Maybe
+import Maybes
 import Monad
 \end{code}
 
@@ -37,6 +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
@@ -108,8 +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
@@ -174,7 +183,7 @@ checkForConflicts inst_envs famInst
                         Nothing        -> panic "FamInst.checkForConflicts"
                         Just (tc, tys) -> tc `mkTyConApp` tys
              }
-       ; (tvs', _, tau') <- tcInstSkolType FamInstSkol ty
+       ; (_, _, tau') <- tcInstSkolType FamInstSkol ty
 
        ; let (fam, tys') = tcSplitTyConApp tau'
 
@@ -188,9 +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.
@@ -202,11 +211,13 @@ checkForConflicts inst_envs famInst
         rhs1   = substTy subst $ synTyConType tycon1
         rhs2   = substTy subst $ synTyConType tycon2
 
+conflictInstErr :: FamInst -> FamInst -> TcRn ()
 conflictInstErr famInst conflictingFamInst
   = addFamInstLoc famInst $
-    addErr (hang (ptext SLIT("Conflicting family instance declarations:"))
+    addErr (hang (ptext (sLit "Conflicting family instance declarations:"))
               2 (pprFamInsts [famInst, conflictingFamInst]))
 
+addFamInstLoc :: FamInst -> TcRn a -> TcRn a
 addFamInstLoc famInst thing_inside
   = setSrcSpan (mkSrcSpan loc loc) thing_inside
   where