Fixes to data type families

[ghc-hetmet.git] / compiler / basicTypes / DataCon.lhs

diff --git a/compiler/basicTypes/DataCon.lhs b/compiler/basicTypes/DataCon.lhs
index ee2c685..a3504a6 100644 (file)
--- a/compiler/basicTypes/DataCon.lhs
+++ b/compiler/basicTypes/DataCon.lhs
@@ -246,6 +246,8 @@ data DataCon
                --       The declaration format is held in the TyCon (algTcGadtSyntax)
 
        dcUnivTyVars :: [TyVar],        -- Universally-quantified type vars 
+                                       -- INVARIANT: length matches arity of the dcRepTyCon
+
        dcExTyVars   :: [TyVar],        -- Existentially-quantified type vars 
                -- In general, the dcUnivTyVars are NOT NECESSARILY THE SAME AS THE TYVARS
                -- FOR THE PARENT TyCon. With GADTs the data con might not even have 
@@ -484,20 +486,24 @@ mkDataCon name declared_infix
     real_stricts = map mk_dict_strict_mark theta ++ arg_stricts
 
        -- Example
-       --   data instance T [a] where 
-       --      TI :: forall b. b -> T [Maybe b]
+       --   data instance T (b,c) where 
+       --      TI :: forall e. e -> T (e,e)
+       --
        -- The representation tycon looks like this:
-       --   data :R7T a where 
-       --      TI :: forall b c. (c :=: Maybe b) b -> :R7T c
+       --   data :R7T b c where 
+       --      TI :: forall b1 c1. (b1 ~ c1) => b1 -> :R7T b1 c1
+
     orig_res_ty 
        | Just (fam_tc, fam_tys) <- tyConFamInst_maybe tycon
-       , let fam_subst = zipTopTvSubst (tyConTyVars fam_tc) res_tys
+       , let fam_subst = zipTopTvSubst (tyConTyVars tycon) res_tys
        = mkTyConApp fam_tc (substTys fam_subst fam_tys)
        | otherwise
        = mkTyConApp tycon res_tys
        where
          res_tys = substTyVars (mkTopTvSubst eq_spec) univ_tvs
-               -- In the example above, res_tys is a singleton, (Maybe b)
+               -- In the example above, 
+               --      univ_tvs = [ b1, c1 ]
+               --      res_tys  = [ b1, b1 ]
 
        -- Representation arguments and demands
        -- To do: eliminate duplication with MkId