From 38beaac7209db3ca80754ad73eed71cbc5d3ddd0 Mon Sep 17 00:00:00 2001
From: Chris Double <chris.double@double.co.nz>
Date: Fri, 30 Nov 2007 00:01:03 +1300
Subject: [PATCH] Infinite left recursion now causes a failed parser rather
 than a call stack error

---
 extra/peg/peg.factor | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/extra/peg/peg.factor b/extra/peg/peg.factor
index 39aacd974e..7fa1fb90e5 100644
--- a/extra/peg/peg.factor
+++ b/extra/peg/peg.factor
@@ -52,7 +52,11 @@ PRIVATE>
   packrat-cache get [
     2dup get-cached dup not-in-cache? [ 
 !      "cache missed: " write over parser-id number>string write " - " write nl ! pick .
-      drop [ (parse) dup ] 2keep put-cached
+      drop 
+      #! Protect against left recursion blowing the callstack
+      #! by storing a failed parse in the cache.
+      [ f ] dipd  [ put-cached ] 2keep
+      [ (parse) dup ] 2keep put-cached
     ] [ 
 !      "cache hit: " write over parser-id number>string write " - " write nl ! pick . 
       2nip