From: handa <handa>
Date: Thu, 24 Mar 2011 06:52:40 +0000 (+0000)
Subject: *** empty log message ***
X-Git-Url: http://git.chise.org/gitweb/?a=commitdiff_plain;p=m17n%2Fm17n-lib-cs.git

*** empty log message ***
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diff --git a/peg.el b/peg.el
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+;;; peg.el --- Parsing Expression Grammars in Emacs Lisp
+;;
+;; Copyright 2008  Helmut Eller <eller.hel...@gmail.com>.
+;;
+;; Version: 0.4 (2008-Dec-17)
+;;
+;; 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 [Ford, Baker].
+;;
+;; 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    [0-9]))
+;;
+;; 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 Ford's paper
+;; 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:
+;;
+;; [Ford] 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] Baker, Henry G. "Pragmatic Parsing in Common Lisp".  ACM Lisp
+;; Pointers 4(2), April--June 1991, pp. 3--15.
+;;
+
+(unless (>= emacs-major-version 22)
+  (error "peg.el requires Emacs version 22 or newer"))
+
+;;; 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 '()) . ,(mapcar #'car rules))
+       ,@(mapcar (lambda (rule)
+                   (let ((name (car rule)))
+                     `(setq ,name
+                            (lambda ()
+                              ,(peg-translate-exp (gethash name peg-rules))))))
+                 rules)
+       (when (funcall ,(car (car rules)))
+         (peg-postprocess peg-thunks)))))
+
+(eval-and-compile 
+  (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))))
+
+;; FIXME: this duplicates code; could use some loop to avoid that
+(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)
+  (peg-normalize `(stack-action ,form)))
+
+(peg-add-method normalize stack-action (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)
+                   ,@(mapcar (lambda (val) `(push ,val peg-stack)) values))))
+      `(action ,form))))
+
+(defvar peg-char-classes 
+  '(ascii alnum alpha blank cntrl digit graph lower multibyte nonascii print
+          punct space unibyte upper word xdigit))
+
+(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 ,(car range) ,(cdr range))))))
+        (t 
+         (let ((chars '()) (ranges '()) (classes '()))
+           (while specs
+             (let* ((spec (pop specs))
+                    (range (peg-range-designator spec)))
+               (cond (range
+                      (push range ranges))
+                     ((peg-characterp spec)
+                      (push spec chars))
+                     ((stringp spec)
+                      (setq chars (append (reverse (append spec ())) chars)))
+                     ((memq spec peg-char-classes)
+                      (push spec classes))
+                     (t (error "Invalid set specifier: %S" spec)))))
+           (setq ranges (reverse ranges))
+           (setq chars (delete-dups (reverse chars)))
+           (setq classes (reverse classes))
+           (cond ((and (null ranges) 
+                       (null classes)
+                       (cond ((null chars) '(fail))
+                             ((null (cdr chars)) `(char ,(car chars))))))
+                 (t `(set ,ranges ,chars ,classes)))))))
+
+(defun peg-range-designator (x)
+  (and (symbolp x)
+       (let ((str (symbol-name x)))
+         (and (= (length str) 3)
+              (eq (aref str 1) ?-)
+              (< (aref str 0) (aref str 2))
+              (cons (aref str 0) (aref str 2))))))
+
+;; characterp is new in Emacs 23.
+(defun peg-characterp (x)
+  (if (fboundp 'characterp) 
+      (characterp x)
+    (integerp x)))
+
+(peg-add-method normalize list (&rest args)
+  (peg-normalize 
+   (let ((marker (make-symbol "magic-marker")))
+     `(and (stack-action (-- ',marker))
+           ,@args
+           (stack-action (-- 
+                          (let ((l '()))
+                            (while 
+                                (let ((e (pop peg-stack)))
+                                  (cond ((eq e ',marker) nil)
+                                        ((null peg-stack)
+                                         (error "Marker not longer stack"))
+                                        (t (push e l) t))))
+                            l)))))))
+
+(peg-add-method normalize *list (&rest args)
+  (peg-normalize `(list (* . ,args))))
+
+(peg-add-method normalize substring (&rest args)
+  (peg-normalize 
+   `(and `(-- (point))
+         ,@args
+         `(start -- (buffer-substring-no-properties start (point))))))
+
+(peg-add-method normalize region (&rest args)
+  (peg-normalize 
+   `(and `(-- (point))
+         ,@args
+         `(-- (point)))))
+
+(peg-add-method normalize replace (pe replacement)
+  (peg-normalize
+   `(and (stack-action (-- (point)))
+         ,pe
+         (stack-action (start -- (progn
+                                   (delete-region start (point))
+                                   (insert-before-markers ,replacement))))
+         (stack-action (x --)))))
+
+(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 (ranges chars classes)
+  `(when (looking-at ',(peg-make-charset-regexp ranges chars classes))
+     (forward-char)
+     t))
+
+(defun peg-make-charset-regexp (ranges chars classes)
+  (when (and (not ranges) (not classes) (<= (length chars) 1))
+    (error "Bug"))
+  (let ((rbracket (member ?\] chars))
+        (minus (member ?- chars))
+        (hat (member ?^ chars)))
+    (dolist (c '(?\] ?- ?^))
+      (setq chars (delete c chars)))
+    (format "[%s%s%s%s%s%s]"
+            (if rbracket "]" "")
+            (if minus "-" "")
+            (mapconcat (lambda (x) (format "%c-%c" (car x) (cdr x)))
+                       ranges "")
+            (mapconcat (lambda (c) (format "[:%s:]" c)) classes "")
+            (mapconcat (lambda (c) (format "%c" c)) chars "")
+            (if hat "^" ""))))
+
+(peg-add-method translate range (from to)
+  `(when (and (char-after)
+              (<= ',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 (mapcar (lambda (x)
+                             (goto-char (car x))
+                             (cons (point-marker) (cdr x)))
+                           (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 r c k) 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)))
+
+;; We can't expand the macro at compile time, because it needs helper
+;; functions which aren't available yet.  Delay the expansion to
+;; load-time (or later).
+(eval '(progn " 
+(" ;<-- this stops Emacs from indenting the next form
+
+(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 (not (peg-parse-string ((s [0-9])) "a")))
+  (assert (not (peg-parse-string ((s [0-9])) "")))
+  (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))) "b"))
+  (assert (peg-parse-string ((s (* "a") "b" (eob))) "ab"))
+  (assert (peg-parse-string ((s (* "a") "b" (eob))) "aaab"))
+  (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"))
+  (assert (peg-parse-string ((s (and))) ""))
+  (assert (peg-parse-string ((s ["^"])) "^"))
+  (assert (peg-parse-string ((s ["^a"])) "a"))
+  (assert (peg-parse-string ((s ["-"])) "-"))
+  (assert (peg-parse-string ((s ["]-"])) "]"))
+  (assert (peg-parse-string ((s ["^]"])) "^"))
+  (assert (peg-parse-string ((s [alpha])) "z"))
+  (assert (not (peg-parse-string ((s [alpha])) "0")))
+  (assert (not (peg-parse-string ((s [alpha])) "")))
+  (assert (not (peg-parse-string ((s ["][:alpha:]"])) "z")))
+  (assert (equal (peg-parse-string ((s `(-- 1 2))) "") '(t (2 1))))
+  (assert (equal (peg-parse-string ((s `(-- 1 2) `(a b -- a b))) "")
+                 '(t (2 1))))
+  (assert (equal (peg-parse-string ((s (list x y))
+                                    (x `(-- 1))
+                                    (y `(-- 2)))
+                                   "")
+                 '(t ((1 2)))))
+  (assert (equal (peg-parse-string ((s (list (* x)))
+                                    (x "x" `(-- 'x)))
+                                   "xxx")
+                 '(t ((x x x)))))
+  (assert (equal (peg-parse-string ((s (region (* x)))
+                                    (x "x" `(-- 'x)))
+                                   "xxx")
+                 '(t (4 x x x 1))))
+  (assert (equal (peg-parse-string ((s (region (list (* x))))
+                                    (x "x" `(-- 'x 'y)))
+                                   "xxx")
+                 '(t (4 (x y x y x y) 1))))
+  (assert (equal (with-temp-buffer 
+                   (save-excursion (insert "abcdef"))
+                   (list
+                    (peg-parse (x "a" 
+                                  (replace "bc" "x")
+                                  (replace "de" "y")
+                                  "f"))
+                    (buffer-string)))
+                 '((t nil) "axyf")))
+  )
+
+(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))
+                  `(scheme user host port path query fragment -- 
+                           (list :scheme scheme :user user
+                                 :host host :port port
+                                 :path path :query query 
+                                 :fragment fragment)))
+   (absoluteURI (substring scheme) ":" (or hier-part opaque-part))
+   (hier-part ;(-- user host port path query)
+    (or net-path 
+        (and `(-- nil nil nil)
+             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#x
+;; (peg-ex-uri)http://lu...@www.foo.com: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 string list number symbol))
+   (_ (* (or [" \n\t"] comment)))
+   (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 "-;!#%&'*+,./:;<=>?...@[]^_`{|}~"])
+   (list "("            `(-- (cons nil nil)) `(hd -- hd hd)
+         (* sexp        `(tl e -- (setcdr tl (list e)))
+            ) _ ")"     `(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))))))))
+
+)) ; end of eval-when-load
+
+(provide 'peg)
+
+;;; peg.el ends here