From f9dae95bd0f1434aa56fd8de259325b9742e28e2 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Mon, 19 Oct 2015 09:10:31 -0400
Subject: parsers: Add parse-maybe.

* syntax-highlight/parsers.scm (parse-maybe): New procedure.
---
 syntax-highlight/parsers.scm | 11 +++++++++++
 1 file changed, 11 insertions(+)

(limited to 'syntax-highlight/parsers.scm')

diff --git a/syntax-highlight/parsers.scm b/syntax-highlight/parsers.scm
index 31c17ae..5d08e45 100644
--- a/syntax-highlight/parsers.scm
+++ b/syntax-highlight/parsers.scm
@@ -38,6 +38,7 @@
             parse-both
             parse-any
             parse-each
+            parse-maybe
             parse-many
             parse-string
             parse-char-set
@@ -117,6 +118,16 @@ parser fails without consuming any input if any parser in PARSERS
 fails."
   (fold-right parse-both (parse-return '()) parsers))
 
+(define (parse-maybe parser)
+  "Create a parser that always succeeds, but tries to use PARSER.  If
+PARSER succeeds, its result is returned, otherwise the empty string is
+returned without consuming any input."
+  (lambda (stream)
+    (let-values (((result remaining) (parser stream)))
+      (if result
+          (values result remaining)
+          (values "" stream)))))
+
 (define (parse-many parser)
   "Create a parser that uses PARSER as many times as possible until it
 fails and return the results of each successful parse in a list.  This
-- 
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