From: Mark Wooding <mdw@distorted.org.uk>
Date: Thu, 26 May 2016 08:26:09 +0000 (+0100)
Subject: src/parser/parser-proto.lisp: Add functions returning standard parsers.
X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~mdw/git/sod/commitdiff_plain/da901cf29aae60b244bd9fcee72a5a2f59d359f2?ds=inline

src/parser/parser-proto.lisp: Add functions returning standard parsers.

Return always-successful and always-failing parsers, with tunable
output.
---

diff --git a/doc/SYMBOLS b/doc/SYMBOLS
index 9df1bd9..86ef28d 100644
--- a/doc/SYMBOLS
+++ b/doc/SYMBOLS
@@ -1484,6 +1484,8 @@ parser-proto.lisp
   cl:not                                        function parser
   cl:or                                         macro parser
   parse
+  parse-empty                                   function
+  parse-fail                                    function
   parser                                        macro
   parser-at-eof-p                               generic
   parser-capture-place                          generic
diff --git a/doc/parsing.tex b/doc/parsing.tex
index ec1667f..c04ab56 100644
--- a/doc/parsing.tex
+++ b/doc/parsing.tex
@@ -43,6 +43,13 @@ consumed any input items.
 \begin{describe}{fun}{combine-parser-failures @<failures> @> @<list>}
 \end{describe}
 
+\begin{describe}{fun}{parse-empty \&optional @<value> @> @<function>}
+\end{describe}
+
+\begin{describe}{fun}
+    {parse-fail @<indicator> \&optional @<consumedp> @> @<function>}
+\end{describe}
+
 %%%--------------------------------------------------------------------------
 \section{File locations} \label{sec:parsing.floc}
 
diff --git a/src/parser/parser-proto.lisp b/src/parser/parser-proto.lisp
index 189e503..7aafe45 100644
--- a/src/parser/parser-proto.lisp
+++ b/src/parser/parser-proto.lisp
@@ -87,6 +87,21 @@ (defun combine-parser-failures (failures)
 	  failures
 	  :initial-value nil))
 
+(export 'parse-empty)
+(defun parse-empty (&optional value)
+  "Return a parser which parses nothing, successfully.
+
+   The parser returns VALUE and consumes nothing."
+  (lambda () (values value t nil)))
+
+(export 'parse-fail)
+(defun parse-fail (indicator &optional consumedp)
+  "Return a parser which fails.
+
+   The parser reports the INDICATOR and (falsely) claims to have consumed
+   input if CONSUMEDP is true."
+  (lambda () (values indicator nil consumedp)))
+
 ;;;--------------------------------------------------------------------------
 ;;; Basic protocol.