Improve error messages for type functions

[ghc-hetmet.git] / compiler / types / TyCon.lhs

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index dc7e2d3..befc4e6 100644 (file)
--- a/compiler/types/TyCon.lhs
+++ b/compiler/types/TyCon.lhs
@@ -39,6 +39,7 @@ module TyCon(
         isCoercionTyCon, isCoercionTyCon_maybe,
         isForeignTyCon,
 
+       isInjectiveTyCon,
        isDataTyCon, isProductTyCon, isEnumerationTyCon, 
        isNewTyCon, isAbstractTyCon, isOpenTyCon,
         isUnLiftedTyCon,
@@ -815,6 +816,17 @@ isOpenTyCon (SynTyCon {synTcRhs = OpenSynTyCon {}}) = True
 isOpenTyCon (AlgTyCon {algTcRhs = OpenTyCon {}})    = True
 isOpenTyCon _                                      = False
 
+-- | Injective 'TyCon's can be decomposed, so that
+--     T ty1 ~ T ty2  =>  ty1 ~ ty2
+isInjectiveTyCon :: TyCon -> Bool
+isInjectiveTyCon tc = not (isSynTyCon tc)
+       -- Ultimately we may have injective associated types
+        -- in which case this test will become more interesting
+       --
+       -- It'd be unusual to call isInjectiveTyCon on a regular H98
+       -- type synonym, because you should probably have expanded it first
+       -- But regardless, it's not injective!
+
 -- | Extract the mapping from 'TyVar' indexes to indexes in the corresponding family
 -- argument lists form an open 'TyCon' of any sort, if the given 'TyCon' is indeed
 -- such a beast and that information is available