;; This module implements a parser form like the parser-tools's
;; `parser', except that it works on an arbitrary CFG (returning
;; the first sucecssful parse).

;; I think this is probably a packrat parser, except that
;; left-recursion is not transformed away. Instead, alternative
;; solutions are computed in parallel, and multiple attempts to
;; compute the same result block until the first one completes.
;; If you get into deadlock, such as when trying to match
;;    <foo> := <foo>
;; then it means that there's no successful parse, so everything
;; that's blocked should fail.

;; The packrat-ness is caching the series of results for a particular
;; non-terminal at a particular starting location. Otherwise, it's
;; backtracking search. Backtracking is implemented through explicit
;; success and failure continuations. Multiple results for a
;; particular nonterminal and location are kept only when they have
;; different lengths. (Otherwise, in the spirit of finding one
;; successful parse, only the first result is kept.)

;; The parser-tools's `parse' is used to transform tokens in the
;; grammar to tokens specific to this parser. In other words, this
;; parser uses `parser' so that it doesn't have to know anything about
;; tokens.

(module cfg-parser mzscheme
  (require (lib "yacc.ss" "parser-tools")
           (lib "list.ss")
           (lib "etc.ss"))
  (require-for-syntax (lib "boundmap.ss" "syntax")
                      (lib "list.ss"))

  (provide cfg-parser)
  
  ;; A raw token, wrapped so that we can recognize it:
  (define-struct tok (name orig-name val))

  ;; Represents the thread scheduler:
  (define-struct tasks (active active-back waits multi-waits cache progress?))

  ;; Used to calculate information on the grammar, such as whether
  ;; a particular non-terminal is "simple" instead of recursively defined.
  (define-for-syntax (nt-fixpoint nts proc nt-ids patss)
    (define (ormap-all val f as bs)
      (cond
       [(null? as) val]
       [else (ormap-all (or (f (car as) (car bs)) val)
			f
			(cdr as) (cdr bs))]))
    (let loop ()
      (when (ormap-all #f
		       (lambda (nt pats)
			 (let ([old (bound-identifier-mapping-get nts nt)])
			   (let ([new (proc nt pats old)])
			     (if (equal? old new)
				 #f
				 (begin
				   (bound-identifier-mapping-put! nts nt new)
				   #t)))))
		       nt-ids patss)
        (loop))))
  
  ;; Tries parse-a followed by parse-b. If parse-a is not simple,
  ;; then after parse-a succeeds once, we parallelize parse-b
  ;; and trying a second result for parse-a.
  (define (parse-and simple-a? parse-a parse-b
                     stream depth end success-k fail-k 
                     max-depth tasks)
    (letrec ([mk-got-k
              (lambda (success-k fail-k)
                (lambda (val stream depth max-depth tasks next1-k)
                  (if simple-a?
                      (parse-b val stream depth end
                               (mk-got2-k success-k fail-k next1-k)
                               (mk-fail2-k success-k fail-k next1-k)
                               max-depth tasks)
                      (parallel-or
                       (lambda (success-k fail-k max-depth tasks)
                         (parse-b val stream depth end
                                  success-k fail-k
                                  max-depth tasks))
                       (lambda (success-k fail-k max-depth tasks)
                         (next1-k (mk-got-k success-k fail-k)
                                  fail-k max-depth tasks))
                       success-k fail-k max-depth tasks))))]
             [mk-got2-k
              (lambda (success-k fail-k next1-k)
                (lambda (val stream depth max-depth tasks next-k)
                  (success-k val stream depth max-depth tasks
                             (lambda (success-k fail-k max-depth tasks)
                               (next-k (mk-got2-k success-k fail-k next1-k)
                                       (mk-fail2-k success-k fail-k next1-k)
                                       max-depth tasks)))))]
             [mk-fail2-k
              (lambda (success-k fail-k next1-k)
                (lambda (max-depth tasks)
                  (next1-k (mk-got-k success-k fail-k)
                           fail-k
                           max-depth
                           tasks)))])
      (parse-a stream depth end
               (mk-got-k success-k fail-k)
               fail-k
               max-depth tasks)))

  ;; Parallel or for non-terminal alternatives
  (define (parse-parallel-or parse-a parse-b stream depth end success-k fail-k max-depth tasks)
    (parallel-or (lambda (success-k fail-k max-depth tasks)
                   (parse-a stream depth end success-k fail-k max-depth tasks))
                 (lambda (success-k fail-k max-depth tasks)
                   (parse-b stream depth end success-k fail-k max-depth tasks))
                 success-k fail-k max-depth tasks))
  
  ;; Generic parallel-or
  (define (parallel-or parse-a parse-b success-k fail-k max-depth tasks)
    (define answer-key (gensym))
    (letrec ([gota-k
              (lambda (val stream depth max-depth tasks next-k)
                (report-answer answer-key
                               max-depth
                               tasks
                               (list val stream depth next-k)))]
             [faila-k
              (lambda (max-depth tasks)
                (report-answer answer-key
                               max-depth
                               tasks
                               null))])
      (let* ([tasks (queue-task
                     tasks
                     (lambda (max-depth tasks)
                       (parse-a gota-k
                                faila-k
                                max-depth tasks)))]
             [tasks (queue-task
                     tasks
                     (lambda (max-depth tasks)
                       (parse-b gota-k
                                faila-k
                                max-depth tasks)))]
             [queue-next (lambda (next-k tasks)
                           (queue-task tasks
                                       (lambda (max-depth tasks)
                                         (next-k gota-k
                                                 faila-k
                                                 max-depth tasks))))])
        (letrec ([mk-got-one
                  (lambda (immediate-next? get-nth success-k)
                    (lambda (val stream depth max-depth tasks next-k)
                      (let ([tasks (if immediate-next?
                                       (queue-next next-k tasks)
                                       tasks)])
                        (success-k val stream depth max-depth 
                                   tasks
                                   (lambda (success-k fail-k max-depth tasks)                                     
                                     (let ([tasks (if immediate-next?
                                                      tasks
                                                      (queue-next next-k tasks))])
                                       (get-nth max-depth tasks success-k fail-k)))))))]
                 [get-first
                  (lambda (max-depth tasks success-k fail-k)
                    (wait-for-answer #f max-depth tasks answer-key
                                     (mk-got-one #t get-first success-k)
                                     (lambda (max-depth tasks)
                                       (get-second max-depth tasks success-k fail-k))
                                     #f))]
                 [get-second
                  (lambda (max-depth tasks success-k fail-k)
                    (wait-for-answer #f max-depth tasks answer-key
                                     (mk-got-one #f get-second success-k)
                                     fail-k #f))])
          (get-first max-depth tasks success-k fail-k)))))

  ;; Non-terminal alternatives where the first is "simple" can be done
  ;; sequentially, which is simpler
  (define (parse-or parse-a parse-b
                        stream depth end success-k fail-k max-depth tasks)
    (letrec ([mk-got-k
              (lambda (success-k fail-k)
                (lambda (val stream depth max-depth tasks next-k)
                  (success-k val stream depth
                             max-depth tasks
                             (lambda (success-k fail-k max-depth tasks)
                               (next-k (mk-got-k success-k fail-k)
                                       (mk-fail-k success-k fail-k)
                                       max-depth tasks)))))]
             [mk-fail-k
              (lambda (success-k fail-k)
                (lambda (max-depth tasks)
                  (parse-b stream depth end success-k fail-k max-depth tasks)))])
      (parse-a stream depth end
               (mk-got-k success-k fail-k)
               (mk-fail-k success-k fail-k)
               max-depth tasks)))
  
  ;; Starts a thread
  (define queue-task
    (opt-lambda (tasks t [progress? #t])
      (make-tasks (tasks-active tasks)
                  (cons t (tasks-active-back tasks))
                  (tasks-waits tasks)
                  (tasks-multi-waits tasks)
                  (tasks-cache tasks)
                  (or progress? (tasks-progress? tasks)))))

  ;; Reports an answer to a waiting thread:
  (define (report-answer answer-key max-depth tasks val)
    (let ([v (hash-table-get (tasks-waits tasks) answer-key (lambda () #f))])
      (if v
          (let ([tasks (make-tasks (cons (v val)
                                         (tasks-active tasks))
                                   (tasks-active-back tasks)
                                   (tasks-waits tasks)
                                   (tasks-multi-waits tasks)
                                   (tasks-cache tasks)
                                   #t)])
            (hash-table-remove! (tasks-waits tasks) answer-key)
            (swap-task max-depth tasks))
          ;; We have an answer ready too fast; wait
          (swap-task max-depth
                     (queue-task tasks
                                 (lambda (max-depth tasks)
                                   (report-answer answer-key max-depth tasks val))
                                 #f)))))

  ;; Reports an answer to multiple waiting threads:
  (define (report-answer-all answer-key max-depth tasks val k)
    (let ([v (hash-table-get (tasks-multi-waits tasks) answer-key (lambda () null))])
      (hash-table-remove! (tasks-multi-waits tasks) answer-key)
      (let ([tasks (make-tasks (append (map (lambda (a) (a val)) v)
                                       (tasks-active tasks))
                               (tasks-active-back tasks)
                               (tasks-waits tasks)
                               (tasks-multi-waits tasks)
                               (tasks-cache tasks)
                               #t)])
        (k max-depth tasks))))

  ;; Waits for an answer; if `multi?' is #f, this is sole waiter, otherwise
  ;;  there might be many. Use wither #t or #f (and `report-answer' or 
  ;;  `report-answer-all', resptively) consistently for a particular answer key.
  (define (wait-for-answer multi? max-depth tasks answer-key success-k fail-k deadlock-k)
    (let ([wait (lambda (val)
                  (lambda (max-depth tasks)
                    (if val
                        (if (null? val)
                            (fail-k max-depth tasks)
                            (let-values ([(val stream depth next-k) (apply values val)])
                              (success-k val stream depth max-depth tasks next-k)))
                        (deadlock-k max-depth tasks))))])
      (if multi?
          (hash-table-put! (tasks-multi-waits tasks) answer-key
                           (cons wait (hash-table-get (tasks-multi-waits tasks) answer-key
                                                      (lambda () null))))
          (hash-table-put! (tasks-waits tasks) answer-key wait))
      (let ([tasks (make-tasks (tasks-active tasks)
                               (tasks-active-back tasks)
                               (tasks-waits tasks)
                               (tasks-multi-waits tasks)
                               (tasks-cache tasks)
                               #t)])
        (swap-task max-depth tasks))))
  
  ;; Swap thread
  (define (swap-task max-depth tasks)
    ;; Swap in first active:
    (if (null? (tasks-active tasks))
        (if (tasks-progress? tasks)
            (swap-task max-depth
                       (make-tasks (reverse (tasks-active-back tasks))
                                   null
                                   (tasks-waits tasks)
                                   (tasks-multi-waits tasks)
                                   (tasks-cache tasks)
                                   #f))
            ;; No progress, so issue failure for all multi-waits
            (if (zero? (hash-table-count (tasks-multi-waits tasks)))
                (error 'swap-task "Deadlock")
                (swap-task max-depth
                           (make-tasks (apply
                                        append
                                        (hash-table-map (tasks-multi-waits tasks)
                                                        (lambda (k l)
                                                          (map (lambda (v) (v #f)) l))))
                                       (tasks-active-back tasks)
                                       (tasks-waits tasks)
                                       (make-hash-table)
                                       (tasks-cache tasks)
                                       #t))))
        (let ([t (car (tasks-active tasks))]
              [tasks (make-tasks (cdr (tasks-active tasks))
                                 (tasks-active-back tasks)
                                 (tasks-waits tasks)
                                 (tasks-multi-waits tasks)
                                 (tasks-cache tasks)
                                 (tasks-progress? tasks))])
          (t max-depth tasks))))

  ;; Finds the symbolic representative of a token class
  (define-for-syntax (map-token toks tok)
    (bound-identifier-mapping-get toks tok
                                  (lambda ()
                                    (let ([id (gensym (syntax-e tok))])
                                      (bound-identifier-mapping-put! toks tok id)
                                      id))))
    
  ;; Builds a matcher for a particular alternative
  (define-for-syntax (build-match nts toks pat handle)
    (let loop ([pat pat]
               [pos 1])
      (if (null? pat)
          #`(success-k #,handle stream depth max-depth tasks 
                       (lambda (success-k fail-k max-depth tasks)
                         (fail-k max-depth tasks)))
          (let ([id (datum->syntax-object (car pat)
                                          (string->symbol (format "$~a" pos)))])
            (cond
              [(bound-identifier-mapping-get nts (car pat) (lambda () #f))
               ;; Match non-termimal
               #`(parse-and
                  ;; First part is simple? (If so, we don't have to parallelize the `and'.)
                  #,(let ([l (bound-identifier-mapping-get nts (car pat) (lambda () #f))])
                      (or (not l)
                          (andmap values (caddr l))))
                  #,(car pat)
                  (lambda (#,id stream depth end success-k fail-k max-depth tasks)
                    #,(loop (cdr pat) (add1 pos)))
                  stream depth 
                  #,(let ([cnt (apply +
                                      (map (lambda (item)
                                             (cond
                                               [(bound-identifier-mapping-get nts item (lambda () #f))
                                                => (lambda (l) (car l))]
                                               [else 1]))
                                           (cdr pat)))])
                      #`(- end #,cnt))
                  success-k fail-k max-depth tasks)]
              [else
               ;; Match token
               (let ([tok-id (map-token toks (car pat))])
                 #`(if (and (pair? stream)
                            (eq? '#,tok-id (tok-name (car stream))))
                       (let ([#,id (tok-val (car stream))]
                             [stream (cdr stream)]
                             [depth (add1 depth)])
                         (let ([max-depth (max max-depth depth)])
                           #,(loop (cdr pat) (add1 pos))))
                       (fail-k max-depth tasks)))])))))
  
  ;; Starts parsing to match a non-terminal. There's a minor
  ;; optimization that checks for known starting tokens. Otherwise,
  ;; use the cache, block if someone else is already trying the match,
  ;; and cache the result if it's computed.
  ;; The cache maps nontermial+startingpos+iteration to a result, where
  ;; the iteration is 0 for the first match attempt, 1 for the second,
  ;; etc.
  (define (parse-nt/share key min-cnt init-tokens stream depth end max-depth tasks success-k fail-k k)
    (if (and (positive? min-cnt)
             (pair? stream)
             (not (memq (tok-name (car stream)) init-tokens)))
        ;; No such leading token; give up
        (fail-k max-depth tasks)
        ;; Run pattern
	(let loop ([n 0]
                   [success-k success-k]
                   [fail-k fail-k]
                   [max-depth max-depth]
                   [tasks tasks]
                   [k k])
          (let ([answer-key (gensym)]
                [table-key (vector key depth n)]
                [old-depth depth]
                [old-stream stream])
            #;(printf "Loop ~a\n" table-key)
            (cond
              [(hash-table-get (tasks-cache tasks) table-key (lambda () #f))
               => (lambda (result)
                    #;(printf "Reuse ~a\n" table-key)
                    (result success-k fail-k max-depth tasks))]
              [else
	       #;(printf "Try ~a ~a\n" table-key (map tok-name stream))
                 (hash-table-put! (tasks-cache tasks) table-key
                                  (lambda (success-k fail-k max-depth tasks)
                                    #;(printf "Wait ~a ~a\n" table-key answer-key)
                                    (wait-for-answer #t max-depth tasks answer-key success-k fail-k
                                                     (lambda (max-depth tasks)
                                                       #;(printf "Deadlock ~a ~a\n" table-key answer-key)
                                                       (fail-k max-depth tasks)))))
                 (let result-loop ([max-depth max-depth][tasks tasks][k k])
                   (letrec ([orig-stream stream]
                            [new-got-k
                             (lambda (val stream depth max-depth tasks next-k)
                               ;; Check whether we already have a result that consumed the same amount:
                               (let ([result-key (vector #f key old-depth depth)])
                                 (cond
                                   [(hash-table-get (tasks-cache tasks) result-key (lambda () #f))
                                    ;; Go for the next-result
                                    (result-loop max-depth
                                                 tasks
                                                 (lambda (end max-depth tasks success-k fail-k)
                                                   (next-k success-k fail-k max-depth tasks)))]
                                   [else
                                    #;(printf "Success ~a ~a\n" table-key 
                                              (map tok-name (let loop ([d old-depth][s old-stream])
                                                              (if (= d depth)
                                                                  null
                                                                  (cons (car s) (loop (add1 d) (cdr s)))))))
                                    (let ([next-k (lambda (success-k fail-k max-depth tasks)
                                                    (loop (add1 n)
                                                          success-k
                                                          fail-k
                                                          max-depth
                                                          tasks
                                                          (lambda (end max-depth tasks success-k fail-k)
                                                            (next-k success-k fail-k max-depth tasks))))])
                                      (hash-table-put! (tasks-cache tasks) result-key #t)
                                      (hash-table-put! (tasks-cache tasks) table-key
                                                       (lambda (success-k fail-k max-depth tasks)
                                                         (success-k val stream depth max-depth tasks next-k)))
                                      (report-answer-all answer-key
                                                         max-depth
                                                         tasks
                                                         (list val stream depth next-k)
                                                         (lambda (max-depth tasks)
                                                           (success-k val stream depth max-depth tasks next-k))))])))]
                            [new-fail-k
                             (lambda (max-depth tasks)
                               #;(printf "Failure ~a\n" table-key)
                               (hash-table-put! (tasks-cache tasks) table-key
                                                (lambda (success-k fail-k max-depth tasks)
                                                  (fail-k max-depth tasks)))
                               (report-answer-all answer-key
                                                  max-depth
                                                  tasks
                                                  null
                                                  (lambda (max-depth tasks)
                                                    (fail-k max-depth tasks))))])
                     (k end max-depth tasks new-got-k new-fail-k)))])))))
  
  (define-syntax (cfg-parser stx)
    (syntax-case stx ()
      [(_ clause ...)
       (let ([clauses (syntax->list #'(clause ...))])
         (let-values ([(start grammar cfg-error parser-clauses)
                       (let loop ([clauses clauses]
                                  [cfg-start #f]
                                  [cfg-grammar #f]
                                  [cfg-error #f]
                                  [parser-clauses null])
                         (if (null? clauses)
                             (values cfg-start
                                     cfg-grammar
                                     cfg-error
                                     (reverse parser-clauses))
                             (syntax-case (car clauses) (start error grammar)
                               [(start tok)
                                (loop (cdr clauses) #'tok cfg-grammar cfg-error parser-clauses)]
                               [(error expr)
                                (loop (cdr clauses) cfg-start cfg-grammar #'expr parser-clauses)]
                               [(grammar [nt [pat handle0 handle ...] ...] ...)
                                (let ([nts (make-bound-identifier-mapping)]
                                      [toks (make-bound-identifier-mapping)]
                                      [nt-ids (syntax->list #'(nt ...))]
                                      [patss (map (lambda (stx)
                                                    (map syntax->list (syntax->list stx)))
                                                  (syntax->list #'((pat ...) ...)))])
                                  (for-each (lambda (nt)
                                              (bound-identifier-mapping-put! nts nt (list 0)))
                                            nt-ids)
                                  ;; Compute min max size for each non-term:
                                  (nt-fixpoint
                                   nts
                                   (lambda (nt pats old-list)
                                     (let ([new-cnt
                                            (apply
                                             min
                                             (map (lambda (pat)
                                                    (apply
                                                     +
                                                     (map (lambda (elem)
                                                            (car
                                                             (bound-identifier-mapping-get nts
                                                                                           elem
                                                                                           (lambda () (list 1)))))
                                                          pat)))
                                                  pats))])
                                       (if (new-cnt . > . (car old-list))
                                           (cons new-cnt (cdr old-list))
                                           old-list)))
                                   nt-ids patss)
                                  ;; Compute set of toks that must appear at the beginning
                                  ;;  for a non-terminal
                                  (nt-fixpoint
                                   nts
                                   (lambda (nt pats old-list)
                                     (let ([new-list
                                            (apply
                                             append
                                             (map (lambda (pat)
                                                    (let loop ([pat pat])
                                                      (if (pair? pat)
                                                          (let ([l (bound-identifier-mapping-get 
                                                                    nts
                                                                    (car pat)
                                                                    (lambda ()
                                                                      (list 1 (map-token toks (car pat)))))])
                                                            ;; If the non-terminal can match 0 things,
                                                            ;;  then it might match something from the
                                                            ;;  next pattern element. Otherwise, it must
                                                            ;;  match the first element:
                                                            (if (zero? (car l))
                                                                (append (cdr l) (loop (cdr pat)))
                                                                (cdr l)))
                                                          null)))
                                                  pats))])
                                       (let ([new (filter (lambda (id)
                                                            (andmap (lambda (id2)
                                                                      (not (eq? id id2)))
                                                                    (cdr old-list)))
                                                          new-list)])
                                         (if (pair? new)
                                             ;; Drop dups in new list:
                                             (let ([new (let loop ([new new])
                                                          (if (null? (cdr new))
                                                              new
                                                              (if (ormap (lambda (id)
                                                                           (eq? (car new) id))
                                                                         (cdr new))
                                                                  (loop (cdr new))
                                                                  (cons (car new) (loop (cdr new))))))])
                                               (cons (car old-list) (append new (cdr old-list))))
                                             old-list))))
                                   nt-ids patss)
                                  ;; Determine left-recursive clauses:
                                  (for-each (lambda (nt pats)
                                              (let ([l (bound-identifier-mapping-get nts nt)])
                                                (bound-identifier-mapping-put! nts nt (list (car l)
                                                                                            (cdr l)
                                                                                            (map (lambda (x) #f) pats)))))
                                            nt-ids patss)
                                  (nt-fixpoint
                                   nts
                                   (lambda (nt pats old-list)
                                     (list (car old-list)
                                           (cadr old-list)
                                           (map (lambda (pat simple?)
                                                  (or simple?
                                                      (let ([l (map (lambda (elem)
                                                                      (bound-identifier-mapping-get 
                                                                       nts
                                                                       elem
                                                                       (lambda () #f)))
                                                                    pat)])
                                                        (andmap (lambda (i)
                                                                  (or (not i)
                                                                      (andmap values (caddr i))))
                                                                l))))
                                                pats (caddr old-list))))
                                   nt-ids patss)
                                  ;; Build a definiton for each non-term:
                                  (loop (cdr clauses)
                                        cfg-start
                                        (map (lambda (nt pats handles)
                                               (define info (bound-identifier-mapping-get nts nt))
                                               (list nt
                                                     #`(let ([key (gensym '#,nt)])
                                                         (lambda (stream depth end success-k fail-k max-depth tasks)
                                                           (parse-nt/share
                                                            key #,(car info) '#,(cadr info) stream depth end
                                                            max-depth tasks
                                                            success-k fail-k
                                                            (lambda (end max-depth tasks success-k fail-k)
                                                              #,(let loop ([pats pats]
                                                                           [handles (syntax->list handles)]
                                                                           [simple?s (caddr info)])
                                                                  (if (null? pats)
                                                                      #'(fail-k max-depth tasks)
                                                                      #`(#,(if (or (null? (cdr pats))
                                                                                   (car simple?s))
                                                                               #'parse-or
                                                                               #'parse-parallel-or)
                                                                         (lambda (stream depth end success-k fail-k max-depth tasks)
                                                                           #,(build-match nts
                                                                                          toks 
                                                                                          (car pats)
                                                                                          (car handles)))
                                                                         (lambda (stream depth end success-k fail-k max-depth tasks)
                                                                           #,(loop (cdr pats)
                                                                                   (cdr handles)
                                                                                   (cdr simple?s)))
                                                                         stream depth end success-k fail-k max-depth tasks)))))))))
                                             nt-ids
                                             patss
                                             (syntax->list #'(((begin handle0 handle ...) ...) ...)))
                                        cfg-error
                                        (list*
                                         (with-syntax ([((tok . tok-id) ...)
                                                        (bound-identifier-mapping-map toks cons)])
                                           (with-syntax ([($tok ...)
                                                          (map (lambda (id)
                                                                 (datum->syntax-object id '$1))
                                                               (syntax->list #'(tok ...)))])
                                             #`(grammar (start [() null]
                                                               [(atok start) (cons $1 $2)])
                                                        (atok [(tok) (make-tok 'tok-id 'tok $tok)] ...))))
                                         #`(start start)
                                         parser-clauses)))]
                               [(grammar . _)
                                (raise-syntax-error
                                 #f
                                 "bad grammar clause"
                                 stx
                                 (car #f))]
                               [_else
                                (loop (cdr clauses)
                                      cfg-start
                                      cfg-grammar
                                      cfg-error
                                      (cons (car clauses) parser-clauses))])))])
           #`(let ([orig-parse (parser 
                                [error (lambda (a b c)
                                         (error 'cfg-parser "unexpected ~a token: ~a" b c))]
                                . #,parser-clauses)]
                   [error-proc #,cfg-error])
               (letrec #,grammar 
                 (lambda (get-tok)
                   (let ([tok-list (orig-parse get-tok)])
                     (letrec ([success-k
                               (lambda (val stream depth max-depth tasks next) 
                                 (if (null? stream)
                                     val
                                     (next success-k fail-k max-depth tasks)))]
                              [fail-k (lambda (max-depth tasks)
                                        (let ([bad-tok (list-ref tok-list 
                                                                 (min (sub1 (length tok-list))
                                                                      max-depth))])
                                          (if error-proc
                                              (error-proc #t
                                                          (tok-orig-name bad-tok)
                                                          (tok-val bad-tok))
                                              (error
                                               'cfg-parse
                                               "failed at ~a" 
                                               (tok-val bad-tok)))))])
                       (#,start tok-list 0 
                          (length tok-list)
                          success-k
                          fail-k
                          0 (make-tasks null null 
                                        (make-hash-table) (make-hash-table)
                                        (make-hash-table 'equal) #t)))))))))]))

#|
  ;; Tests used during development

  (require (lib "lex.ss" "parser-tools"))
  
  (define-tokens non-terminals (PLUS MINUS STAR BAR COLON EOF))
  
  (define lex
    (lexer
     ["+" (token-PLUS '+)]
     ["-" (token-MINUS '-)]
     ["*" (token-STAR '*)]
     ["|" (token-BAR '||)]
     [":" (token-COLON '|:|)]
     [(eof) (token-EOF 'eof)]))
  
  (define parse
    (cfg-parser
     (tokens non-terminals)
     (start <program>)
     (end EOF)
     (error (lambda (a b stx) 
              (error 'parse "failed at ~s" stx)))
     (grammar [<program> [(PLUS) "plus"]
                         [(<minus-program> BAR <minus-program>) (list $1 $2 $3)]
                         [(<program> COLON) (list $1)]]
              [<minus-program> [(MINUS) "minus"]
                               [(<program> STAR) (cons $1 $2)]]
              [<simple> [(<alts> <alts> <alts> MINUS) "yes"]]
              [<alts> [(PLUS) 'plus]
                      [(MINUS) 'minus]]
              [<random> [() '0]
                        [(<random> PLUS) (add1 $1)]
                        [(<random> PLUS) (add1 $1)]])))
  
  (define (result)
    (let ([p (open-input-string #;"+*|-|-*|+**" #;"-|+*|+**" 
	      #;"+*|+**|-" #;"-|-*|-|-*"
	      #;"-|-*|-|-**|-|-*|-|-**"
	      "-|-*|-|-**|-|-*|-|-***|-|-*|-|-**|-|-*|-|-****|-|-*|-|-**|-|-*|-|-***\
               |-|-*|-|-**|-|-*|-|-*****|-|-*|-|-**|-|-*|-|-***|-|-*|-|-**|-|-*|-|-****|\
               -|-*|-|-**|-|-*|-|-***|-|-*|-|-**|-|-*|-|-*****"
	      ;; This one fails:
	      #;"+*")])
      (time (parse (lambda () (lex p))))))

|#

  )