From 9c9d2802fd191ce4d6684e4b1242b6f5b3f511c2 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Sun, 18 Oct 2015 20:53:19 -0400
Subject: parsers: Add parse-filter.

* syntax-highlight/parsers.scm (parse-filter): New procedure.
---
 syntax-highlight/parsers.scm | 10 ++++++++++
 1 file changed, 10 insertions(+)

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

diff --git a/syntax-highlight/parsers.scm b/syntax-highlight/parsers.scm
index 5349296..bb967db 100644
--- a/syntax-highlight/parsers.scm
+++ b/syntax-highlight/parsers.scm
@@ -33,6 +33,7 @@
             parse-lift
             parse-never
             parse-map
+            parse-filter
             parse-either
             parse-both
             parse-any
@@ -77,6 +78,15 @@
   "Return a new parser that applies PROC to result of PARSER."
   (parse-bind (parse-lift proc) parser))
 
+(define (parse-filter predicate parser)
+  "Create a new parser that succeeds when PARSER is successful and
+PREDICATE is satisfied with the result."
+  (lambda (stream)
+    (let-values (((result remaining) (parser stream)))
+      (if (and result (predicate result))
+          (values result remaining)
+          (parse-fail stream)))))
+
 (define (parse-either first second)
   "Create a parser that tries to parse with FIRST or, if that fails,
 parses SECOND."
-- 
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