From ec36347f8f91d365d3563513e1731ee368ca34fb Mon Sep 17 00:00:00 2001
From: Boutade <thegoofist@protonmail.com>
Date: Wed, 22 Apr 2020 21:34:50 -0500
Subject: <<sep-by* and <<filter

---
 README.org    |  2 +-
 package.lisp  |  2 ++
 parzival.lisp | 13 +++++++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/README.org b/README.org
index 19df989..63f94ad 100644
--- a/README.org
+++ b/README.org
@@ -8,7 +8,7 @@
   In =parzival=, a *parser is a function* that accepts a *stream* and returns
   three values:
   
-  1. A *parse result* (or =nil=), which is any value.
+  1. A *parse result*, which is any value.
   2. A *success indicator*, which is =t= or =nil=.
   3. The possibly modified *stream* that was initially passed in.
 
diff --git a/package.lisp b/package.lisp
index dc26c34..e940a4a 100644
--- a/package.lisp
+++ b/package.lisp
@@ -16,6 +16,7 @@
    #:<<until
    #:<<ignore-until
    #:<<or
+   #:<<filter
    #:<<~
    #:<<?
    #:<<any-char
@@ -68,6 +69,7 @@
    #:<<min-times
    #:<<max-times
    #:<<sep-by
+   #:<<sep-by*
    #:<<brackets
    #:<<char-brackets
    #:<<string
diff --git a/parzival.lisp b/parzival.lisp
index 4c47c32..7eaa611 100644
--- a/parzival.lisp
+++ b/parzival.lisp
@@ -152,6 +152,14 @@ in then. If the parse fails the combinator else is run instead."
                           (<<and <item< #'rec))))))
           #'rec))
 
+(defun <<filter (parser pred)
+  "Condition a successful parse on a predicate"
+  (<<bind parser
+          (lambda (parsed)
+            (if (funcall pred parsed)
+                (<<result parsed)
+                <fail<))))
+
 (defun <<or (&rest parsers)
   "Tries each parser one after the other, rewinding the input stream after each
    failure, and resulting in the first successful parse."
@@ -425,6 +433,10 @@ the character C."
   (<<cons value-parser (<<* (<<and separator-parser value-parser))))
 
 
+(defun <<sep-by* (value-parser separator-parser)
+  "Just like <<SEP-BY, but matches an empty sequence"
+  (<<? (<<sep-by value-parser separator-parser)))
+
 (defun <<brackets (left center right)
   (<<and left (<<bind center
                       (lambda (bracketed-value)
@@ -525,3 +537,4 @@ the character C."
 (defun returning (x)
   "Returns a lambda that returns x no matter what it gets as an argument"
   (lambda (ignore) x))
+
-- 
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