Remove dataConInstOrigDictsAndArgTys

[ghc-hetmet.git] / compiler / iface / BuildTyCl.lhs

diff --git a/compiler/iface/BuildTyCl.lhs b/compiler/iface/BuildTyCl.lhs
index ef75d7f..296b430 100644 (file)
--- a/compiler/iface/BuildTyCl.lhs
+++ b/compiler/iface/BuildTyCl.lhs
@@ -148,13 +148,16 @@ mkNewTyConRhs tycon_name tycon con
         -- non-recursive newtypes
     all_coercions = True
     tvs    = tyConTyVars tycon
-    rhs_ty = ASSERT(not (null (dataConInstOrigDictsAndArgTys con (mkTyVarTys tvs)))) 
-            -- head (dataConInstOrigArgTys con (mkTyVarTys tvs))
-            head (dataConInstOrigDictsAndArgTys con (mkTyVarTys tvs))
+    inst_con_ty = applyTys (dataConUserType con) (mkTyVarTys tvs)
+    rhs_ty = ASSERT( isFunTy inst_con_ty ) funArgTy inst_con_ty
        -- Instantiate the data con with the 
        -- type variables from the tycon
-       -- NB: a newtype DataCon has no existentials; hence the
-       --     call to dataConInstOrigArgTys has the right type args
+       -- NB: a newtype DataCon has a type that must look like
+       --        forall tvs.  <arg-ty> -> T tvs
+       -- Note that we *can't* use dataConInstOrigArgTys here because
+       -- the newtype arising from   class Foo a => Bar a where {}
+       -- has a single argument (Foo a) that is a *type class*, so
+       -- dataConInstOrigArgTys returns [].
 
     etad_tvs :: [TyVar]        -- Matched lazily, so that mkNewTypeCoercion can
     etad_rhs :: Type   -- return a TyCon without pulling on rhs_ty