X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~mdw/git/sod/blobdiff_plain/97d10f8b1f631cd32ea5ac9118b253cb4142d391..ae7a3c8fb42e457933efbbb9127d5a745096825a:/src/lexer-proto.lisp

diff --git a/src/lexer-proto.lisp b/src/lexer-proto.lisp
index 122da75..b045be7 100644
--- a/src/lexer-proto.lisp
+++ b/src/lexer-proto.lisp
@@ -141,6 +141,14 @@ (defparse error (:context (context token-scanner-context)
 			:ignore-unconsumed ,ignore-unconsumed
 			:force-progress ,force-progress))
 
+(export 'must)
+(defparse must (:context (context token-scanner-context)
+		sub &optional default)
+  "Try to parse SUB; if it fails, report an error, and return DEFAULT.
+
+   This parser can't actually fail."
+  `(parse (error () ,sub (t ,default))))
+
 ;;;--------------------------------------------------------------------------
 ;;; Lexical analysis utilities.