[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: peg.el --- Parsing Expression Grammars in Emacs Lisp
From: |
weber |
Subject: |
Re: peg.el --- Parsing Expression Grammars in Emacs Lisp |
Date: |
Thu, 27 Nov 2008 02:17:18 -0800 (PST) |
User-agent: |
G2/1.0 |
This is nice, thanks. I'll try to start using it in my scripts :)
A quick note: the function characterp doesn't exist here (i'm using
22.3)
Aliasing it to numberp seems to work for now..
Cheers,
Hugo
Helmut Eller wrote:
> New in this version:
>
> * fix a few bugs, most notably *-expressions didn't backtrack properly.
>
> * a more convenient syntax for character sets, e.g. [A-Z "-]"] now
> means all charecters from A to Z, minus, and right brackets.
>
> * more realistic examples
>
> * minimal test suite
>
> Helmut.
>
> ;;; peg.el --- Parsing Expression Grammars in Emacs Lisp
> ;;
> ;; Copyright 2008 Helmut Eller <address@hidden>.
> ;;
> ;; This file is licensed under the terms of the GNU General Public
> ;; License as distributed with Emacs (press C-h C-c for details).
> ;;
> ;;; Commentary:
> ;;
> ;; Parsing Expression Grammars (PEG) are a formalism in the spirit of
> ;; Context Free Grammars (CFG) with some simplifications which makes
> ;; the implementation of PEGs as top-down parser particularly simple
> ;; and easy to understand [**].
> ;;
> ;; This file implements a macro `peg-parse' which parses the current
> ;; buffer according to a PEG. E.g. we can match integers with a PEG
> ;; like this:
> ;;
> ;; (peg-parse (number sign digit (* digit))
> ;; (sign (or "+" "-" ""))
> ;; (digit '"09")))
> ;;
> ;; In contrast to regexps, PEGs allow us to define recursive rules. A
> ;; PEG is a list of rules. A rule is written as (NAME . PE).
> ;; E.g. (sign (or "+" "-" "")) is a rule with the name "sign". For
> ;; convenience, PE is implicitly wrapped in a and. The syntax for
> ;; Parsing Expression (PE) is a follows:
> ;;
> ;; Description Lisp Haskell as in [*]
> ;; Sequence (and e1 e2) e1 e2
> ;; Prioritized Choice (or e1 e2) e1 / e2
> ;; Not-predicate (not e) !e
> ;; And-predicate (if e) &e
> ;; Any character (any) .
> ;; Literal string "abc" "abc"
> ;; Character C (char c) 'c'
> ;; Zero-or-more (* e) e*
> ;; One-or-more (+ e) e+
> ;; Optional (opt e) e?
> ;; Character range (range a b) [a-b]
> ;; Character set [a-b "+*" ?x] [a-b+*x]
> ;;
> ;; `peg-parse' also supports parsing actions, i.e. Lisp snippets which
> ;; are executed when a PE matches. This can be used to construct
> ;; syntax trees or for similar tasks. Actions are written as
> ;;
> ;; (action FORM) ; evaluate FORM
> ;; `(VAR... -- FORM...) ; stack action
> ;;
> ;; Actions don't consume input, but are executed at the point of
> ;; match. A "stack action" takes VARs from the "value stack" and
> ;; pushes the result of evaluating FORMs to that stack. See
> ;; `peg-ex-parse-int' for an example.
> ;;
> ;; References:
> ;;
> ;; [*] Bryan Ford. Parsing Expression Grammars: a Recognition-Based
> ;; Syntactic Foundation. In POPL'04: Proceedings of the 31st ACM
> ;; SIGPLAN-SIGACT symposium on Principles of Programming Languages,
> ;; pages 111-122, New York, NY, USA, 2004. ACM Press.
> ;;
> ;; [**] Baker, Henry G. "Pragmatic Parsing in Common Lisp". ACM Lisp
> ;; Pointers 4(2), April--June 1991, pp. 3--15.
> ;;
>
> ;;; Code:
>
> (defmacro peg-parse (&rest rules)
> "Match RULES at point.
> Return (T STACK) if the match succeed and nil on failure."
> (peg-translate-rules rules))
>
> (defmacro peg-parse-exp (exp)
> "Match the parsing expression EXP at point.
> Note: a PE can't \"call\" rules by name."
> `(let ((peg-thunks nil))
> (when ,(peg-translate-exp (peg-normalize exp))
> (peg-postprocess peg-thunks))))
>
> ;; A table of the PEG rules. Used during compilation to resolve
> ;; references to named rules.
> (defvar peg-rules)
>
> ;; used at runtime for backtracking. It's a list ((POS . THUNK)...).
> ;; Each THUNK is executed at the corresponding POS. Thunks are
> ;; executed in a postprocessing step, not during parsing.
> (defvar peg-thunks)
>
> ;; The basic idea is to translate each rule to a lisp function.
> ;; The result looks like
> ;; (let ((rule1 (lambda () code-for-rule1))
> ;; ...
> ;; (ruleN (lambda () code-for-ruleN)))
> ;; (funcall rule1))
> ;;
> ;; code-for-ruleX returns t if the rule matches and nil otherwise.
> ;;
> (defun peg-translate-rules (rules)
> "Translate the PEG RULES, to a top-down parser."
> (let ((peg-rules (make-hash-table :size 20)))
> (dolist (rule rules)
> (puthash (car rule) 'defer peg-rules))
> (dolist (rule rules)
> (puthash (car rule) (peg-normalize `(and . ,(cdr rule))) peg-rules))
> (peg-check-cycles peg-rules)
> `(let ((peg-thunks '()))
> (let ,(mapcar (lambda (rule)
> (let ((name (car rule)))
> `(,name
> (lambda ()
> ,(peg-translate-exp (gethash name peg-rules))))))
> rules)
> (when (funcall ,(car (car rules)))
> (peg-postprocess peg-thunks))))))
>
> (defun peg-method-table-name (method-name)
> (intern (format "peg-%s-methods" method-name)))
>
> (defmacro peg-define-method-table (name)
> (let ((tab (peg-method-table-name name)))
> `(progn
> (defvar ,tab)
> (setq ,tab (make-hash-table :size 20)))))
>
> (defmacro peg-add-method (method type args &rest body)
> (declare (indent 3))
> `(puthash ',type (lambda ,args . ,body) ,(peg-method-table-name method)))
>
> (peg-define-method-table normalize)
>
> ;; Internally we use a regularized syntax, e.g. we only have binary OR
> ;; nodes. Regularized nodes are lists of the form (OP ARGS...).
> (defun peg-normalize (exp)
> "Return a \"normalized\" form of EXP."
> (cond ((and (consp exp)
> (let ((fun (gethash (car exp) peg-normalize-methods)))
> (and fun
> (apply fun (cdr exp))))))
> ((stringp exp)
> (let ((len (length exp)))
> (cond ((zerop len) '(null))
> ((= len 1) `(char ,(aref exp 0)))
> (t `(str ,exp)))))
> ((and (symbolp exp) exp)
> (when (not (gethash exp peg-rules))
> (error "Reference to undefined PEG rule: %S" exp))
> `(call ,exp))
> ((vectorp exp)
> (peg-normalize `(set . ,(append exp '()))))
> (t
> (error "Invalid parsing expression: %S" exp))))
>
> (defvar peg-leaf-types '(null fail any call action char range str eob
> set))
>
> (dolist (type peg-leaf-types)
> (puthash type `(lambda (&rest args) (cons ',type args))
> peg-normalize-methods))
>
> (peg-add-method normalize or (&rest args)
> (cond ((null args) '(fail))
> ((null (cdr args)) (peg-normalize (car args)))
> (t `(or ,(peg-normalize (car args))
> ,(peg-normalize `(or . ,(cdr args)))))))
>
> (peg-add-method normalize and (&rest args)
> (cond ((null args) '(null))
> ((null (cdr args)) (peg-normalize (car args)))
> (t `(and ,(peg-normalize (car args))
> ,(peg-normalize `(and . ,(cdr args)))))))
>
> (peg-add-method normalize * (&rest args)
> `(* ,(peg-normalize `(and . ,args))))
>
> (peg-add-method normalize + (&rest args)
> (let ((e (peg-normalize `(and . ,args))))
> `(and ,e (* ,e))))
>
> (peg-add-method normalize opt (&rest args)
> (let ((e (peg-normalize `(and . ,args))))
> `(or ,e (null))))
>
> (peg-add-method normalize if (&rest args)
> `(if ,(peg-normalize `(and . ,args))))
>
> (peg-add-method normalize not (&rest args)
> `(not ,(peg-normalize `(and . ,args))))
>
> (peg-add-method normalize \` (form)
> (unless (member '-- form)
> (error "Malformed stack action: %S" form))
> (let ((args (cdr (member '-- (reverse form))))
> (values (cdr (member '-- form))))
> (let ((form `(let ,(mapcar (lambda (var) `(,var (pop peg-stack)))
> args)
> (setq peg-stack
> (append (list . ,values) peg-stack)))))
> `(action ,form))))
>
> (peg-add-method normalize set (&rest specs)
> (cond ((null specs) '(fail))
> ((and (null (cdr specs))
> (let ((range (peg-range-designator (car specs))))
> (and range `(range . ,range)))))
> (t
> (let ((args '()))
> (while specs
> (let* ((spec (pop specs))
> (range (peg-range-designator spec)))
> (cond (range
> (push (cons (car range) (cadr range)) args))
> ((or (and (symbolp spec) (eq spec nil))
> (characterp spec))
> (push spec args))
> ((stringp spec)
> (setq args (append spec args)))
> (t (error "Invalid set specifier: %S" spec)))))
> `(set ,(rx-to-string `(any . ,args) t))))))
>
> (peg-add-method normalize *list (&rest args)
> (peg-normalize
> `(and `(-- ())
> (* ,@args `(l e -- (cons e l)))
> `(l -- (nreverse l)))))
>
> (peg-add-method normalize substring (&rest args)
> (peg-normalize
> `(and `(-- (point))
> ,@args
> `(start --
> (buffer-substring-no-properties start (point))))))
>
> (defun peg-range-designator (x)
> (and (symbolp x)
> (let ((str (symbol-name x)))
> (and (= (length str) 3)
> (eq (aref str 1) ?-)
> (list (aref str 0) (aref str 2))))))
>
> (peg-add-method normalize quote (form)
> (error "quote is reverved for future use"))
>
> (peg-define-method-table translate)
>
> ;; This is the main translation function.
> (defun peg-translate-exp (exp)
> "Return the ELisp code to match the PE EXP."
> (let ((translator (or (gethash (car exp) peg-translate-methods)
> (error "No translator for: %S" (car exp)))))
> (apply translator (cdr exp))))
>
> (peg-add-method translate and (e1 e2)
> `(and ,(peg-translate-exp e1)
> ,(peg-translate-exp e2)))
>
> (peg-add-method translate or (e1 e2)
> (let ((cp (peg-make-choicepoint)))
> `(,@(peg-save-choicepoint cp)
> (or ,(peg-translate-exp e1)
> (,@(peg-restore-choicepoint cp)
> ,(peg-translate-exp e2))))))
>
> ;; Choicepoints are used for backtracking. At a choicepoint we save
> ;; enough state, so that we can continue from there if needed.
> (defun peg-make-choicepoint ()
> (cons (make-symbol "point") (make-symbol "thunks")))
>
> (defun peg-save-choicepoint (choicepoint)
> `(let ((,(car choicepoint) (point))
> (,(cdr choicepoint) peg-thunks))))
>
> (defun peg-restore-choicepoint (choicepoint)
> `(progn
> (goto-char ,(car choicepoint))
> (setq peg-thunks ,(cdr choicepoint))))
>
> ;; match empty strings
> (peg-add-method translate null ()
> `t)
>
> ;; match nothing
> (peg-add-method translate fail ()
> `nil)
>
> (peg-add-method translate eob ()
> '(eobp))
>
> (peg-add-method translate * (e)
> (let ((cp (peg-make-choicepoint)))
> `(progn (while (,@(peg-save-choicepoint cp)
> (cond (,(peg-translate-exp e))
> (t ,(peg-restore-choicepoint cp)
> nil))))
> t)))
>
> (peg-add-method translate if (e)
> (let ((cp (peg-make-choicepoint)))
> `(,@(peg-save-choicepoint cp)
> (when ,(peg-translate-exp e)
> ,(peg-restore-choicepoint cp)
> t))))
>
> (peg-add-method translate not (e)
> (let ((cp (peg-make-choicepoint)))
> `(,@(peg-save-choicepoint cp)
> (when (not ,(peg-translate-exp e))
> ,(peg-restore-choicepoint cp)
> t))))
>
> (peg-add-method translate any ()
> '(when (not (eobp))
> (forward-char)
> t))
>
> (peg-add-method translate char (c)
> `(when (eq (char-after) ',c)
> (forward-char)
> t))
>
> (peg-add-method translate set (charset-regexp)
> `(when (looking-at ',charset-regexp)
> (forward-char)
> t))
>
> (peg-add-method translate range (from to)
> `(when (and (<= ',from (char-after))
> (<= (char-after) ',to))
> (forward-char)
> t))
>
> (peg-add-method translate str (str)
> `(when (looking-at ',(regexp-quote str))
> (goto-char (match-end 0))
> t))
>
> (peg-add-method translate call (name)
> (or (gethash name peg-rules)
> (error "Reference to unknown rule: %S" name))
> `(funcall ,name))
>
> (peg-add-method translate action (form)
> `(progn
> (push (cons (point) (lambda () ,form)) peg-thunks)
> t))
>
> (defvar peg-stack)
> (defun peg-postprocess (thunks)
> "Execute \"actions\"."
> (let ((peg-stack '()))
> (dolist (thunk (reverse thunks))
> (goto-char (car thunk))
> (funcall (cdr thunk)))
> (list t peg-stack)))
>
> ;; Left recursion is presumably a common mistate when using PEGs.
> ;; Here we try to detect such mistakes. Essentailly we traverse the
> ;; graph as long as we can without consuming input. When we find a
> ;; recursive call we signal an error.
>
> (defun peg-check-cycles (peg-rules)
> (maphash (lambda (name exp)
> (peg-detect-cycles exp (list name))
> (dolist (node (peg-find-star-nodes exp))
> (peg-detect-cycles node '())))
> peg-rules))
>
> (defun peg-find-star-nodes (exp)
> (let ((type (car exp)))
> (cond ((memq type peg-leaf-types) '())
> (t (let ((kids (apply #'append
> (mapcar #'peg-find-star-nodes (cdr exp)))))
> (if (eq type '*)
> (cons exp kids)
> kids))))))
>
>
> (peg-define-method-table detect-cycles)
>
> (defun peg-detect-cycles (exp path)
> "Signal an error on a cycle.
> Otherwise traverse EXP recursively and return T if EXP can match
> without consuming input. Return nil if EXP definetly consumes
> input. PATH is the list of rules that we have visited so far."
> (apply (or (gethash (car exp) peg-detect-cycles-methods)
> (error "No detect-cycle method for: %S" exp))
> path (cdr exp)))
>
> (peg-add-method detect-cycles call (path name)
> (cond ((member name path)
> (error "Possible left recursion: %s"
> (mapconcat (lambda (x) (format "%s" x))
> (reverse (cons name path)) " -> ")))
> (t
> (peg-detect-cycles (gethash name peg-rules) (cons name path)))))
>
> (peg-add-method detect-cycles and (path e1 e2)
> (and (peg-detect-cycles e1 path)
> (peg-detect-cycles e2 path)))
>
> (peg-add-method detect-cycles or (path e1 e2)
> (or (peg-detect-cycles e1 path)
> (peg-detect-cycles e2 path)))
>
> (peg-add-method detect-cycles * (path e)
> (when (peg-detect-cycles e path)
> (error "Infinite *-loop: %S matches empty string" e))
> t)
>
> (peg-add-method detect-cycles if (path e) (peg-unary-nullable e path))
> (peg-add-method detect-cycles not (path e) (peg-unary-nullable e path))
>
> (defun peg-unary-nullable (exp path)
> (peg-detect-cycles exp path)
> t)
>
> (peg-add-method detect-cycles any (path) nil)
> (peg-add-method detect-cycles char (path c) nil)
> (peg-add-method detect-cycles set (path s) nil)
> (peg-add-method detect-cycles range (path c1 c2) nil)
> (peg-add-method detect-cycles str (path s) (equal s ""))
> (peg-add-method detect-cycles null (path) t)
> (peg-add-method detect-cycles fail (path) nil)
> (peg-add-method detect-cycles eob (path) t)
> (peg-add-method detect-cycles action (path form) t)
>
> ;;; Tests:
>
> (defmacro peg-parse-string (rules string)
> `(with-temp-buffer
> (insert ,string)
> (goto-char (point-min))
> (peg-parse . ,rules)))
>
> (defun peg-test ()
> (interactive)
> (assert (peg-parse-string ((s "a")) "a"))
> (assert (not (peg-parse-string ((s "a")) "b")))
> (assert (peg-parse-string ((s (not "a"))) "b"))
> (assert (not (peg-parse-string ((s (not "a"))) "a")))
> (assert (peg-parse-string ((s (if "a"))) "a"))
> (assert (not (peg-parse-string ((s (if "a"))) "b")))
> (assert (peg-parse-string ((s "ab")) "ab"))
> (assert (not (peg-parse-string ((s "ab")) "ba")))
> (assert (not (peg-parse-string ((s "ab")) "a")))
> (assert (peg-parse-string ((s (range ?0 ?9))) "0"))
> (assert (not (peg-parse-string ((s (range ?0 ?9))) "a")))
> (assert (peg-parse-string ((s [0-9])) "0"))
> (assert (peg-parse-string ((s (any))) "0"))
> (assert (not (peg-parse-string ((s (any))) "")))
> (assert (peg-parse-string ((s (eob))) ""))
> (assert (peg-parse-string ((s (not (eob)))) "a"))
> (assert (peg-parse-string ((s (or "a" "b"))) "a"))
> (assert (peg-parse-string ((s (or "a" "b"))) "b"))
> (assert (not (peg-parse-string ((s (or "a" "b"))) "c")))
> (assert (peg-parse-string ((s (and "a" "b"))) "ab"))
> (assert (peg-parse-string ((s (and "a" "b"))) "abc"))
> (assert (not (peg-parse-string ((s (and "a" "b"))) "ba")))
> (assert (peg-parse-string ((s (and "a" "b" "c"))) "abc"))
> (assert (peg-parse-string ((s (* "a") "b" (eob))) "ab"))
> (assert (not (peg-parse-string ((s (* "a") "b" (eob))) "abc")))
> (assert (peg-parse-string ((s "")) "abc"))
> (assert (peg-parse-string ((s "" (eob))) ""))
> (assert (peg-parse-string ((s (opt "a") "b")) "abc"))
> (assert (peg-parse-string ((s (opt "a") "b")) "bc"))
> (assert (not (peg-parse-string ((s (or))) "ab")))
> (assert (peg-parse-string ((s (and))) "ab")))
>
> (when (featurep 'cl)
> (peg-test))
>
> ;;; Examples:
>
> ;; peg-ex-recognize-int recognizes integers. An integer begins with a
> ;; optional sign, then follows one or more digits. Digits are all
> ;; characters from 0 to 9.
> ;;
> ;; Notes:
> ;; 1) "" matches the empty sequence, i.e. matches without
> ;; consuming input.
> ;; 2) [0-9] is the character range from 0 to 9. This can also be
> ;; written as (range ?0 ?9). [Note that 0-9 is a symbol.]
> (defun peg-ex-recognize-int ()
> (peg-parse (number sign digit (* digit))
> (sign (or "+" "-" ""))
> (digit [0-9])))
>
> ;; peg-ex-parse-int recognizes integers and computes the corresponding
> ;; value. The grammer is the same as for `peg-ex-recognize-int' added
> ;; with parsing actions. Unfortunaletly, the actions add quite a bit
> ;; of clutter.
> ;;
> ;; The action for the sign rule pushes t on the stack for a minus sign
> ;; and nil for plus or no sign.
> ;;
> ;; The action for the digit rule pushes the value for a single digit.
> ;;
> ;; The action `(a b -- (+ (* a 10) b)), takes two items from the stack
> ;; and pushes the first digit times 10 added to second digit.
> ;;
> ;; The action `(minus val -- (if minus (- val) val)), negates the
> ;; value if the minus flag is true.
> (defun peg-ex-parse-int ()
> (peg-parse (number sign
> digit
> (* digit `(a b -- (+ (* a 10) b)))
> `(minus val -- (if minus (- val) val)))
> (sign (or (and "+" `(-- nil))
> (and "-" `(-- t))
> (and "" `(-- nil))))
> (digit [0-9] `(-- (- (char-before) ?0)))))
>
> ;; Put point after the ) and press C-x C-e
> ;; (peg-ex-parse-int)-234234
>
> ;; Parse arithmetic expressions and compute the result as side effect.
> (defun peg-ex-arith ()
> (peg-parse
> (expr (or (and ws sum eol)
> (and (* (not eol) (any)) eol error)))
> (sum product (* (or (and plus product `(a b -- (+ a b)))
> (and minus product `(a b -- (- a b))))))
> (product value (* (or (and times value `(a b -- (* a b)))
> (and divide value `(a b -- (/ a b))))))
> (value (or (and (substring number) `(string -- (string-to-number string)))
> (and open sum close)))
> (number (+ [0-9]) ws)
> (plus "+" ws)
> (minus "-" ws)
> (times "*" ws)
> (divide "/" ws)
> (open "(" ws)
> (close ")" ws)
> (ws (* (or " " "\t")))
> (eol (or "\n" "\r\n" "\r"))
> (error (action (error "Parse error at: %s" (point))))))
>
> ;; (peg-ex-arith) 1 + 2 * 3 * (4 + 5)
>
> ;; Parse URI according to RFC 2396.
> (defun peg-ex-uri ()
> (peg-parse
> (URI-reference (or absoluteURI relativeURI)
> (or (and "#" (substring fragment))
> `(-- nil)))
> (absoluteURI (substring scheme) ":" (or hier-part opaque-part)
> `(scheme user host port path query -- (list :scheme scheme
> :user user
> :host host
> :port port
> :path path
> :query query)))
> (hier-part (or net-path abs-path)
> (or (and "?" (substring query))
> `(-- nil)))
> (net-path "//" authority (or abs-path `(-- nil)))
> (abs-path "/" path-segments)
> (path-segments segment (*list "/" segment) `(s l -- (cons s l)))
> (segment (substring (* pchar) (* ";" param)))
> (param (* pchar))
> (pchar (or unreserved escaped [":@&=+$,"]))
> (query (* uric))
> (fragment (* uric))
> (relativeURI (or net-path abs-path rel-path) (opt "?" query))
> (rel-path rel-segment (opt abs-path))
> (rel-segment (+ unreserved escaped [";@&=+$,"]))
> (authority (or server reg-name))
> (server (or (and (or (and (substring userinfo) "@")
> `(-- nil))
> hostport)
> `(-- nil nil nil)))
> (userinfo (* (or unreserved escaped [";:&=+$,"])))
> (hostport (substring host) (or (and ":" (substring port))
> `(-- nil)))
> (host (or hostname ipv4address))
> (hostname (* domainlabel ".") toplabel (opt "."))
> (domainlabel alphanum
> (opt (* (or alphanum "-") (if alphanum))
> alphanum))
> (toplabel alpha
> (* (or alphanum "-") (if alphanum))
> alphanum)
> (ipv4address (+ digit) "." (+ digit) "." (+ digit) "." (+ digit))
> (port (* digit))
> (scheme alpha (* (or alpha digit ["+-."])))
> (reg-name (or unreserved escaped ["$,;:@&=+"]))
> (opaque-part uric-no-slash (* uric))
> (uric (or reserved unreserved escaped))
> (uric-no-slash (or unreserved escaped [";?:@&=+$,"]))
> (reserved (set ";/?:@&=+$,"))
> (unreserved (or alphanum mark))
> (escaped "%" hex hex)
> (hex (or digit [A-F] [a-f]))
> (mark (set "-_.!~*'()"))
> (alphanum (or alpha digit))
> (alpha (or lowalpha upalpha))
> (lowalpha [a-z])
> (upalpha [A-Z])
> (digit [0-9])))
>
> ;; (peg-ex-uri)file:/bar/baz.html?foo=df
> ;; (peg-ex-uri)http://address@hidden:8080/bar/baz.html?x=1#foo
>
> ;; Parse a lisp style Sexp.
> ;; [To keep the example short, ' and . are handled as ordinary symbol.]
> (defun peg-ex-lisp ()
> (peg-parse
> (sexp (* (or blank comment)) (or string list number symbol))
> (ws (* blank))
> (blank [" \n\t"])
> (comment ";" (* (not (or "\n" (eob))) (any)))
> (string "\"" (substring (* (not "\"") (any))) "\"")
> (number (substring (opt (set "+-")) (+ digit))
> (if terminating)
> `(string -- (string-to-number string)))
> (symbol (substring (and symchar (* (not terminating) symchar)))
> `(s -- (intern s)))
> (symchar [a-z A-Z 0-9 "-;!#%&'*+,./:;<=>address@hidden|}~"])
> (list "(" `(-- (cons nil nil)) `(hd -- hd hd)
> (* sexp `(tl e -- (setcdr tl (list e)))
> ) ws ")" `(hd tl -- (cdr hd)))
> (digit [0-9])
> (terminating (or (set " \n\t();\"'") (eob)))))
>
> ;; (peg-ex-lisp)
>
> ;; We try to detect left recursion and report it as error.
> (defun peg-ex-left-recursion ()
> (eval '(peg-parse (exp (or term
> (and exp "+" exp)))
> (term (or digit
> (and term "*" term)))
> (digit [0-9]))))
>
> (defun peg-ex-infinite-loop ()
> (eval '(peg-parse (exp (* (or "x"
> "y"
> (action (foo))))))))
>
> (provide 'peg)
>
> ;;; peg.el ends here