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(in-package :parser-combinators)
;; This is a hack, really.
(defvar *curtail* nil)
(defmacro curtail? (name &body body)
"Parser modifier: add recursion curtailing to PARSER, naming the curtailed parser NAME. Left
recursive parser parser will only be nested once per remaining length of the input string. Note:
this is only necessary for a limited class of left recursive parsers. Non-left recursive parsers
should be implemented using just `named?`, and most left-recursive parsers using that in combination
with `chainl1?`. Also see `expression?`."
(with-unique-names (inp curtail)
`(named? ,name
#'(lambda (,inp)
(let ((,curtail (if *curtail* *curtail* 0)))
(if (>= (+ ,curtail (position-of ,inp))
(length-of ,inp))
(funcall (zero) ,inp)
(let ((*curtail* (1+ ,curtail)))
(funcall ,@body ,inp))))))))