Add comments, plus fix zapFragileInfo to zap worker info

[ghc-hetmet.git] / compiler / simplCore / SimplEnv.lhs

diff --git a/compiler/simplCore/SimplEnv.lhs b/compiler/simplCore/SimplEnv.lhs
index 040240c..245f313 100644 (file)
--- a/compiler/simplCore/SimplEnv.lhs
+++ b/compiler/simplCore/SimplEnv.lhs
@@ -608,8 +608,12 @@ substLetIdBndr :: SimplEnv -> InBndr       -- Env and binder to transform
               -> (SimplEnv, OutBndr)
 -- C.f. substIdBndr above
 -- Clone Id if necessary, substitute its type
--- Return an Id with completely zapped IdInfo
+-- Return an Id with its fragile info zapped
+--     namely, any info that depends on free variables
 --     [addLetIdInfo, below, will restore its IdInfo]
+--     We want to retain robust info, especially arity and demand info,
+--     so that they are available to occurrences that occur in an
+--     earlier binding of a letrec
 -- Augment the subtitution 
 --     if the unique changed, *or* 
 --     if there's interesting occurrence info
@@ -620,7 +624,8 @@ substLetIdBndr env@(SimplEnv { seInScope = in_scope, seIdSubst = id_subst }) old
   where
     id1           = uniqAway in_scope old_id
     id2    = substIdType env id1
-    -- we want to get rid of any info that's dependent on free variables,
+
+    -- We want to get rid of any info that's dependent on free variables,
     -- but keep other info (like the arity).
     new_id = zapFragileIdInfo id2