Fix a bug in MatchCon, and clarify what dataConInstOrigArgTys does

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

diff --git a/compiler/iface/BuildTyCl.lhs b/compiler/iface/BuildTyCl.lhs
index 333d808..9f35453 100644 (file)
--- a/compiler/iface/BuildTyCl.lhs
+++ b/compiler/iface/BuildTyCl.lhs
@@ -148,6 +148,8 @@ mkNewTyConRhs tycon_name tycon con
     rhs_ty = head (dataConInstOrigArgTys con (mkTyVarTys tvs))
        -- 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
 
     etad_tvs :: [TyVar]        -- Matched lazily, so that mkNewTypeCoercion can
     etad_rhs :: Type   -- return a TyCon without pulling on rhs_ty