[project @ 2000-09-14 13:46:39 by simonpj]

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

diff --git a/ghc/compiler/basicTypes/MkId.lhs b/ghc/compiler/basicTypes/MkId.lhs
index 0bb7540..87b49ef 100644 (file)
--- a/ghc/compiler/basicTypes/MkId.lhs
+++ b/ghc/compiler/basicTypes/MkId.lhs
@@ -258,10 +258,6 @@ mkDataConWrapId data_con
               mkLams tyvars $ mkLams dict_args $ Lam id_arg1 $
               Note (Coerce result_ty (head orig_arg_tys)) (Var id_arg1)
 
-{-     I nuked this because map (:) xs would create a
-       new local lambda for the (:) in core-to-stg.  
-       There isn't a defn for the worker!
-
             | null dict_args && all not_marked_strict strict_marks
             = Var work_id      -- The common case.  Not only is this efficient,
                                -- but it also ensures that the wrapper is replaced
@@ -270,10 +266,16 @@ mkDataConWrapId data_con
                                -- becomes 
                                --              f $w: x
                                -- This is really important in rule matching,
-                               -- which is a bit sad.  (We could match on the wrappers,
+                               -- (We could match on the wrappers,
                                -- but that makes it less likely that rules will match
-                               -- when we bring bits of unfoldings together
--}
+                               -- when we bring bits of unfoldings together.)
+               --
+               -- NB:  because of this special case, (map (:) ys) turns into
+               --      (map $w: ys), and thence into (map (\x xs. $w: x xs) ys)
+               --      in core-to-stg.  The top-level defn for (:) is never used.
+               --      This is somewhat of a bore, but I'm currently leaving it 
+               --      as is, so that there still is a top level curried (:) for
+               --      the interpreter to call.
 
             | otherwise
             = mkLams all_tyvars $ mkLams dict_args $