brag/parser-tools/cfg-parser.rkt

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

;; I'm pretty sure that this is an implementation of Earley's
;; algorithm.

;; To a first approximation, it's a backtracking parser. Alternative
;; for a non-terminal 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 fails.

;; A cache holds the series of results for a particular non-terminal
;; at a particular starting location. (A series is used, instead of a
;; sinlge result, for backtracking.) Otherwise, the parser uses
;; 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 yaragg-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.
;;



(require yaragg/parser-tools/yacc
         yaragg/parser-tools/lex)

(require (for-syntax racket/base
                     syntax/boundmap
                     yaragg/parser-tools/private-lex/token-syntax))

(provide cfg-parser)

;; A raw token, wrapped so that we can recognize it:
(struct tok (name orig-name val start end))

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

(define-for-syntax make-token-identifier-mapping make-hasheq)
(define-for-syntax (token-identifier-mapping-get t tok [fail #f])
  (if fail
      (hash-ref t (syntax-e tok) fail)
      (hash-ref t (syntax-e tok))))

(define-for-syntax (token-identifier-mapping-put! t tok v)
  (hash-set! t (syntax-e tok) v))

(define-for-syntax (token-identifier-mapping-map t f)
  (hash-map t f))

;; 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
                     (λ (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 last-consumed-token depth end success-k fail-k 
                   max-depth tasks)
  (define ((mk-got-k success-k fail-k) val stream last-consumed-token depth max-depth tasks next1-k)
    (if simple-a?
        (parse-b val stream last-consumed-token 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
         (λ (success-k fail-k max-depth tasks)
           (parse-b val stream last-consumed-token depth end
                    success-k fail-k
                    max-depth tasks))
         (λ (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)))
  
  (define ((mk-got2-k success-k fail-k next1-k) val stream last-consumed-token depth max-depth tasks next-k)
    (success-k val stream last-consumed-token depth max-depth tasks
               (λ (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))))
  
  (define ((mk-fail2-k success-k fail-k next1-k) max-depth tasks)
    (next1-k (mk-got-k success-k fail-k) fail-k max-depth tasks))
  
  (parse-a stream last-consumed-token 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 last-consumed-token depth end success-k fail-k max-depth tasks)
  (parallel-or (λ (success-k fail-k max-depth tasks)
                 (parse-a stream last-consumed-token depth end success-k fail-k max-depth tasks))
               (λ (success-k fail-k max-depth tasks)
                 (parse-b stream last-consumed-token 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))
  (define (gota-k val stream last-consumed-token depth max-depth tasks next-k)
    (report-answer answer-key
                   max-depth
                   tasks
                   (list val stream last-consumed-token depth next-k)))
  (define (faila-k max-depth tasks) 
    (report-answer answer-key
                   max-depth
                   tasks
                   null))
  (let* ([tasks (queue-task tasks (λ (max-depth tasks)
                                    (parse-a gota-k faila-k max-depth tasks)))]
         [tasks (queue-task tasks (λ (max-depth tasks)
                                    (parse-b gota-k faila-k max-depth tasks)))]
         [queue-next (λ (next-k tasks)
                       (queue-task tasks (λ (max-depth tasks)
                                           (next-k gota-k faila-k max-depth tasks))))])
    (define ((mk-got-one immediate-next? get-nth success-k) val stream last-consumed-token depth max-depth tasks next-k)
      (let ([tasks (if immediate-next?
                       (queue-next next-k tasks)
                       tasks)])
        (success-k val stream last-consumed-token depth max-depth 
                   tasks
                   (λ (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))))))
    (define (get-first max-depth tasks success-k fail-k)
      (wait-for-answer #f max-depth tasks answer-key
                       (mk-got-one #t get-first success-k)
                       (λ (max-depth tasks)
                         (get-second max-depth tasks success-k fail-k))
                       #f))
    (define (get-second 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 last-consumed-token depth end success-k fail-k max-depth tasks)
  (define ((mk-got-k success-k fail-k) val stream last-consumed-token depth max-depth tasks next-k)
    (success-k val stream last-consumed-token depth
               max-depth tasks
               (λ (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))))
  (define ((mk-fail-k success-k fail-k) max-depth tasks)
    (parse-b stream last-consumed-token depth end success-k fail-k max-depth tasks))
  (parse-a stream last-consumed-token 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 ts t [progress? #t])
  (tasks (tasks-active ts)
          (cons t (tasks-active-back ts))
              (tasks-waits ts)
              (tasks-multi-waits ts)
              (tasks-cache ts)
              (or progress? (tasks-progress? ts))))

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

;; 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 ts answer-key success-k fail-k deadlock-k)
  (let ([wait (λ (val)
                (λ (max-depth tasks)
                  (if val
                      (if (null? val)
                          (fail-k max-depth tasks)
                          (let-values ([(val stream last-consumed-token depth next-k) (apply values val)])
                            (success-k val stream last-consumed-token depth max-depth tasks next-k)))
                      (deadlock-k max-depth tasks))))])
    (if multi?
        (hash-set! (tasks-multi-waits ts) answer-key
                   (cons wait (hash-ref (tasks-multi-waits ts) answer-key
                                        (λ () null))))
        (hash-set! (tasks-waits ts) answer-key wait))
    (let ([ts (tasks (tasks-active ts)
                             (tasks-active-back ts)
                             (tasks-waits ts)
                             (tasks-multi-waits ts)
                             (tasks-cache ts)
                             #t)])
      (swap-task max-depth ts))))

;; Swap thread
(define (swap-task max-depth ts)
  ;; Swap in first active:
  (if (null? (tasks-active ts))
      (if (tasks-progress? ts)
          (swap-task max-depth
                     (tasks (reverse (tasks-active-back ts))
                                 null
                                 (tasks-waits ts)
                                 (tasks-multi-waits ts)
                                 (tasks-cache ts)
                                 #f))
          ;; No progress, so issue failure for all multi-waits
          (if (zero? (hash-count (tasks-multi-waits ts)))
              (error 'swap-task "Deadlock")
              (swap-task max-depth
                         (tasks (apply
                                      append
                                      (hash-map (tasks-multi-waits ts)
                                                (λ (k l)
                                                  (map (λ (v) (v #f)) l))))
                                     (tasks-active-back ts)
                                     (tasks-waits ts)
                                     (make-hasheq)
                                     (tasks-cache ts)
                                     #t))))
      (let ([t (car (tasks-active ts))]
            [ts (tasks (cdr (tasks-active ts))
                               (tasks-active-back ts)
                               (tasks-waits ts)
                               (tasks-multi-waits ts)
                               (tasks-cache ts)
                               (tasks-progress? ts))])
        (t max-depth ts))))

;; Finds the symbolic representative of a token class
(define-for-syntax (map-token toks tok)
  (car (token-identifier-mapping-get toks tok)))

(define no-pos-val (position #f #f #f))
(define-for-syntax no-pos 
  (let ([npv ((syntax-local-certifier) #'no-pos-val)])
    (λ (stx) npv)))
(define-for-syntax ((at-tok-pos sel expr) stx)
  #`(let ([v #,expr]) (if v (#,sel v) no-pos-val)))

;; Builds a matcher for a particular alternative
(define-for-syntax (build-match nts toks pat handle $ctx)
  (let loop ([pat pat]
             [pos 1])
    (if (null? pat)
        #`(success-k #,handle stream last-consumed-token depth max-depth tasks 
                     (λ (success-k fail-k max-depth tasks)
                       (fail-k max-depth tasks)))
        (let ([id (datum->syntax (car pat) (string->symbol (format "$~a" pos)))]
              [id-start-pos (datum->syntax (car pat) (string->symbol (format "$~a-start-pos" pos)))]
              [id-end-pos (datum->syntax (car pat) (string->symbol (format "$~a-end-pos" pos)))]
              [n-end-pos (and (null? (cdr pat)) (datum->syntax (car pat) '$n-end-pos))])
          (cond
            [(bound-identifier-mapping-get nts (car pat) (λ () #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) (λ () #f))])
                    (or (not l)
                        (andmap values (caddr l))))
                #,(car pat)
                (let ([original-stream stream])
                  (λ (#,id stream last-consumed-token depth end success-k fail-k max-depth tasks)
                    (let-syntax ([#,id-start-pos (at-tok-pos #'(if (eq? original-stream stream)
                                                                   tok-end
                                                                   tok-start)
                                                             #'(if (eq? original-stream stream)
                                                                   last-consumed-token
                                                                   (and (pair? original-stream) 
                                                                        (car original-stream))))]
                                 [#,id-end-pos (at-tok-pos #'tok-end #'last-consumed-token)]
                                 #,@(if n-end-pos
                                        #`([#,n-end-pos (at-tok-pos #'tok-end #'last-consumed-token)])
                                        null))
                      #,(loop (cdr pat) (add1 pos)))))
                stream last-consumed-token depth 
                #,(let ([cnt (apply +
                                    (map (λ (item)
                                           (cond
                                             [(bound-identifier-mapping-get nts item (λ () #f))
                                              => (λ (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* ([stream-a (car stream)]
                            [#,id (tok-val stream-a)]
                            [last-consumed-token (car stream)]
                            [stream (cdr stream)]
                            [depth (add1 depth)])
                       (let ([max-depth (max max-depth depth)])
                         (let-syntax ([#,id-start-pos (at-tok-pos #'tok-start #'stream-a)]
                                      [#,id-end-pos (at-tok-pos #'tok-end #'stream-a)]
                                      #,@(if n-end-pos
                                             #`([#,n-end-pos (at-tok-pos #'tok-end #'stream-a)])
                                             null))
                           #,(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 last-consumed-token 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])
        (define answer-key (gensym))
        (define table-key (vector key depth n))
        (define old-depth depth)
        (define old-stream stream)
        #;(printf "Loop ~a\n" table-key)
        (cond
          [(hash-ref (tasks-cache tasks) table-key (λ () #f))
           => (λ (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-set! (tasks-cache tasks) table-key
                      (λ (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
                                         (λ (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])
             (define orig-stream stream)
             (define (new-got-k val stream last-consumed-token depth max-depth tasks next-k)
               ;; Check whether we already have a result that consumed the same amount:
               (define result-key (vector #f key old-depth depth))
               (cond
                 [(hash-ref (tasks-cache tasks) result-key (λ () #f))
                  ;; Go for the next-result
                  (result-loop max-depth
                               tasks
                               (λ (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 (λ (success-k fail-k max-depth tasks)
                                  (loop (add1 n)
                                        success-k
                                        fail-k
                                        max-depth
                                        tasks
                                        (λ (end max-depth tasks success-k fail-k)
                                          (next-k success-k fail-k max-depth tasks))))])
                    (hash-set! (tasks-cache tasks) result-key #t)
                    (hash-set! (tasks-cache tasks) table-key
                               (λ (success-k fail-k max-depth tasks)
                                 (success-k val stream last-consumed-token depth max-depth tasks next-k)))
                    (report-answer-all answer-key
                                       max-depth
                                       tasks
                                       (list val stream last-consumed-token depth next-k)
                                       (λ (max-depth tasks)
                                         (success-k val stream last-consumed-token depth max-depth tasks next-k))))]))
             (define (new-fail-k max-depth tasks)
               #;(printf "Failure ~a\n" table-key)
               (hash-set! (tasks-cache tasks) table-key
                          (λ (success-k fail-k max-depth tasks)
                            (fail-k max-depth tasks)))
               (report-answer-all answer-key
                                  max-depth
                                  tasks
                                  null
                                  (λ (max-depth tasks)
                                    (fail-k max-depth tasks))))
             (k end max-depth tasks new-got-k new-fail-k))]))))

;; These temp identifiers can't be `gensym` or `generate-temporary`
;; because they have to be consistent between module loads
;; (IIUC, the parser is multi-threaded, and this approach is not thread-safe)
;; so I see no alternative to the old standby of making them ludicrously unlikely
(define-for-syntax start-id-temp 'start_jihqolbbafscgxvsufnepvmxqipnxgmlpxukmdoqxqzmzgaogaftbkbyqjttwwfimifowdxfyekjiixdmtprfkcvfciraehoeuaz)
(define-for-syntax atok-id-temp 'atok_wrutdjgecmybyfipiwsgjlvsveryodlgassuzcargiuznzgdghrykfqfbwcjgzdhdoeqxcucmtjkuyucskzethozhqkasphdwbht)
(define-syntax (cfg-parser stx)
  (syntax-case stx ()
    [(_ CLAUSE ...)
     (let ([clauses (syntax->list #'(CLAUSE ...))])
       (let-values ([(start grammar cfg-error parser-clauses src-pos?)
                     (let ([all-toks (apply
                                      append
                                      (for/list ([clause (in-list clauses)])
                                                (syntax-case clause (tokens)
                                                  [(tokens T ...)
                                                   (apply
                                                    append
                                                    (for/list ([t (in-list (syntax->list #'(T ...)))])
                                                              (define v (syntax-local-value t (λ () #f)))
                                                              (cond
                                                                [(terminals-def? v)
                                                                 (for/list ([v (in-list (syntax->list (terminals-def-t v)))])
                                                                           (cons v #f))]
                                                                [(e-terminals-def? v)
                                                                 (for/list ([v (in-list (syntax->list (e-terminals-def-t v)))])
                                                                           (cons v #t))]
                                                                [else null])))]
                                                  [_else null])))]
                           [all-end-toks (apply
                                          append
                                          (for/list ([clause (in-list clauses)])
                                                    (syntax-case clause (end)
                                                      [(end T ...)
                                                       (syntax->list #'(T ...))]
                                                      [_else null])))])
                       (let loop ([clauses clauses]
                                  [cfg-start #f]
                                  [cfg-grammar #f]
                                  [cfg-error #f]
                                  [src-pos? #f]
                                  [parser-clauses null])
                         (if (null? clauses)
                             (values cfg-start
                                     cfg-grammar
                                     cfg-error
                                     (reverse parser-clauses)
                                     src-pos?)
                             (syntax-case (car clauses) (start error grammar src-pos)
                               [(start TOK)
                                (loop (cdr clauses) #'TOK cfg-grammar cfg-error src-pos? parser-clauses)]
                               [(error EXPR)
                                (loop (cdr clauses) cfg-start cfg-grammar #'EXPR src-pos? parser-clauses)]
                               [(grammar [NT [PAT HANDLE0 HANDLE ...] ...] ...)
                                (let ([nts (make-bound-identifier-mapping)]
                                      [toks (make-token-identifier-mapping)]
                                      [end-toks (make-token-identifier-mapping)]
                                      [nt-ids (syntax->list #'(NT ...))]
                                      [patss (map (λ (stx)
                                                    (map syntax->list (syntax->list stx)))
                                                  (syntax->list #'((PAT ...) ...)))])
                                  (for ([nt (in-list nt-ids)])
                                       (bound-identifier-mapping-put! nts nt (list 0)))
                                  (for ([t (in-list all-end-toks)])
                                       (token-identifier-mapping-put! end-toks t #t))
                                  (for ([t (in-list all-toks)]
                                        #:unless (token-identifier-mapping-get end-toks (car t) (λ () #f)))
                                       (define id (gensym (syntax-e (car t))))
                                       (token-identifier-mapping-put! toks (car t) (cons id (cdr t))))
                                  ;; Compute min max size for each non-term:
                                  (nt-fixpoint
                                   nts
                                   (λ (nt pats old-list)
                                     (let ([new-cnt
                                            (apply min (for/list ([pat (in-list pats)])
                                                                 (for/sum ([elem (in-list pat)])
                                                                          (car (bound-identifier-mapping-get
                                                                                nts elem  (λ () (list 1)))))))])
                                       (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
                                   (λ (nt pats old-list)
                                     (let ([new-list
                                            (apply
                                             append
                                             (for/list ([pat (in-list pats)])
                                                       (let loop ([pat pat])
                                                         (if (pair? pat)
                                                             (let ([l (bound-identifier-mapping-get 
                                                                       nts
                                                                       (car pat)
                                                                       (λ ()
                                                                         (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))))])
                                       (let ([new (filter (λ (id)
                                                            (andmap (λ (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 (λ (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 (λ (nt pats)
                                              (let ([l (bound-identifier-mapping-get nts nt)])
                                                (bound-identifier-mapping-put! nts nt (list (car l)
                                                                                            (cdr l)
                                                                                            (map (λ (x) #f) pats)))))
                                            nt-ids patss)
                                  (nt-fixpoint
                                   nts
                                   (λ (nt pats old-list)
                                     (list (car old-list)
                                           (cadr old-list)
                                           (map (λ (pat simple?)
                                                  (or simple?
                                                      (let ([l (map (λ (elem)
                                                                      (bound-identifier-mapping-get 
                                                                       nts
                                                                       elem
                                                                       (λ () #f)))
                                                                    pat)])
                                                        (andmap (λ (i)
                                                                  (or (not i)
                                                                      (andmap values (caddr i))))
                                                                l))))
                                                pats (caddr old-list))))
                                   nt-ids patss)
                                  ;; Build a definition for each non-term:
                                  (loop (cdr clauses)
                                        cfg-start
                                        (map (λ (nt pats handles $ctxs)
                                               (define info (bound-identifier-mapping-get nts nt))
                                               (list nt
                                                     #`(let ([key (gensym '#,nt)])
                                                         (λ (stream last-consumed-token depth end success-k fail-k max-depth tasks)
                                                           (parse-nt/share
                                                            key #,(car info) '#,(cadr info) stream last-consumed-token depth end
                                                            max-depth tasks
                                                            success-k fail-k
                                                            (λ (end max-depth tasks success-k fail-k)
                                                              #,(let loop ([pats pats]
                                                                           [handles (syntax->list handles)]
                                                                           [$ctxs (syntax->list $ctxs)]
                                                                           [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)
                                                                         (λ (stream last-consumed-token depth end success-k fail-k max-depth tasks)
                                                                           #,(build-match nts
                                                                                          toks 
                                                                                          (car pats)
                                                                                          (car handles)
                                                                                          (car $ctxs)))
                                                                         (λ (stream last-consumed-token depth end success-k fail-k max-depth tasks)
                                                                           #,(loop (cdr pats)
                                                                                   (cdr handles)
                                                                                   (cdr $ctxs)
                                                                                   (cdr simple?s)))
                                                                         stream last-consumed-token depth end success-k fail-k max-depth tasks)))))))))
                                             nt-ids
                                             patss
                                             (syntax->list #'(((begin HANDLE0 HANDLE ...) ...) ...))
                                             (syntax->list #'((HANDLE0 ...) ...)))
                                        cfg-error
                                        src-pos?
                                        (list*
                                         (with-syntax ([((tk tok-id . $e) ...)
                                                        (token-identifier-mapping-map toks
                                                                                      (λ (k v)
                                                                                        (list* k
                                                                                               (car v)
                                                                                               (if (cdr v)
                                                                                                   #f
                                                                                                   '$1))))]
                                                       [(pos ...) 
                                                        (if src-pos?
                                                            #'($1-start-pos $1-end-pos)
                                                            #'(#f #f))]
                                                       ;; rename `start` and `atok` to temp ids
                                                       ;; so that "start" and "atok" can be used as literal string tokens in a grammar.
                                                       ;; not sure why this works, but it passes all tests.
                                                       [%start start-id-temp]
                                                       [%atok atok-id-temp])
                                           #`(grammar (%start [() null]
                                                              [(%atok %start) (cons $1 $2)])
                                                      (%atok [(tk) (tok 'tok-id 'tk $e pos ...)] ...)))
                                         (with-syntax ([%start start-id-temp])
                                           #`(start %start))
                                         parser-clauses)))]
                               [(grammar . _)
                                (raise-syntax-error
                                 #f
                                 "bad grammar clause"
                                 stx
                                 (car clauses))]
                               [(src-pos)
                                (loop (cdr clauses)
                                      cfg-start
                                      cfg-grammar
                                      cfg-error
                                      #t
                                      (cons (car clauses) parser-clauses))]
                               [_else
                                (loop (cdr clauses)
                                      cfg-start
                                      cfg-grammar
                                      cfg-error
                                      src-pos?
                                      (cons (car clauses) parser-clauses))]))))])
         #`(let ([orig-parse (parser 
                              [error (λ (a b c)
                                       (error 'cfg-parser "unexpected ~a token: ~a" b c))]
                              . #,parser-clauses)]
                 [error-proc #,cfg-error])
             (letrec #,grammar 
               (λ (get-tok)
                 (let ([tok-list (orig-parse get-tok)])
                   (letrec ([success-k
                             (λ (val stream last-consumed-token depth max-depth tasks next) 
                               (if (null? stream)
                                   val
                                   (next success-k fail-k max-depth tasks)))]
                            [fail-k (λ (max-depth tasks)
                                      (cond
                                        [(null? tok-list)
                                         (if error-proc
                                             (error-proc #t
                                                         'no-tokens
                                                         #f
                                                         (position #f #f #f)
                                                         (position #f #f #f))
                                             (error
                                              'cfg-parse
                                              "no tokens"))]
                                        [else
                                         (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)
                                                           (tok-start bad-tok)
                                                           (tok-end bad-tok))
                                               (error
                                                'cfg-parse
                                                "failed at ~a" 
                                                (tok-val bad-tok))))]))])
                     (#,start tok-list
                              ;; we simulate a token at the very beginning with zero width
                              ;; for use with the position-generating code (*-start-pos, *-end-pos).
                              (if (null? tok-list)
                                  (tok #f #f #f
                                       (position 1
                                                 #,(if src-pos? #'1 #'#f) 
                                                 #,(if src-pos? #'0 #'#f))
                                       (position 1 
                                                 #,(if src-pos? #'1 #'#f)
                                                 #,(if src-pos? #'0 #'#f)))
                                  (tok (tok-name (car tok-list))
                                       (tok-orig-name (car tok-list))
                                       (tok-val (car tok-list))
                                       (tok-start (car tok-list))
                                       (tok-start (car tok-list))))
                              0 
                              (length tok-list)
                              success-k
                              fail-k
                              0 
                              (tasks null null 
                                          (make-hasheq) (make-hasheq)
                                          (make-hash) #t)))))))))]))


(module* test racket/base
  (require (submod "..")
           yaragg/parser-tools/lex
           racket/block
           rackunit)

  ;; Test: parsing regular expressions.
  ;; Here is a test case on locations:
  (block
   (define-tokens regexp-tokens (ANCHOR STAR OR LIT LPAREN RPAREN EOF))
   (define lex (lexer-src-pos ["|" (token-OR lexeme)]
                              ["^" (token-ANCHOR lexeme)]
                              ["*" (token-STAR lexeme)]
                              [(repetition 1 +inf.0 alphabetic) (token-LIT lexeme)]
                              ["(" (token-LPAREN lexeme)]
                              [")" (token-RPAREN lexeme)]
                              [whitespace (return-without-pos (lex input-port))]
                              [(eof) (token-EOF 'eof)]))
   (define -parse (cfg-parser
                   (tokens regexp-tokens)
                   (start top)
                   (end EOF)
                   (src-pos)
                   (grammar [top [(maybe-anchor regexp)
                                  (cond [$1 
                                         `(anchored ,$2 ,(pos->sexp $1-start-pos) ,(pos->sexp $2-end-pos))]
                                        [else
                                         `(unanchored ,$2 ,(pos->sexp $1-start-pos) ,(pos->sexp $2-end-pos))])]]
                            [maybe-anchor [(ANCHOR) #t]
                                          [() #f]]
                            [regexp [(regexp STAR) `(star ,$1 ,(pos->sexp $1-start-pos) ,(pos->sexp $2-end-pos))]
                                    [(regexp OR regexp) `(or ,$1 ,$3 ,(pos->sexp $1-start-pos) ,(pos->sexp $3-end-pos))]
                                    [(LPAREN regexp RPAREN) `(group ,$2 ,(pos->sexp $1-start-pos) ,(pos->sexp $3-end-pos))]
                                    [(LIT) `(lit ,$1 ,(pos->sexp $1-start-pos) ,(pos->sexp $1-end-pos))]])))
   (define (pos->sexp pos)
     (position-offset pos))
   
   (define (parse s)
     (define ip (open-input-string s))
     (port-count-lines! ip)
     (-parse (λ () (lex ip))))
   
   (check-equal? (parse "abc")
                 '(unanchored (lit "abc" 1 4) 1 4))
   (check-equal? (parse "a | (b*) | c")
                 '(unanchored (or (or (lit "a" 1 2)
                                      (group (star (lit "b" 6 7) 6 8) 5 9)
                                      1 9)
                                  (lit "c" 12 13)
                                  1 13)
                              1 13)))
  
  
  
  
  
  ;; Tests used during development
  (define-tokens non-terminals (PLUS MINUS STAR BAR COLON EOF))
  
  (define lex
    (lexer
     ["+" (token-PLUS '+)]
     ["-" (token-MINUS '-)]
     ["*" (token-STAR '*)]
     ["|" (token-BAR '||)]
     [":" (token-COLON '|:|)]
     [whitespace (lex input-port)]
     [(eof) (token-EOF 'eof)]))
  
  (define parse
    (cfg-parser
     (tokens non-terminals)
     (start <program>)
     (end EOF)
     (error (λ (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)]])))
  
  (let ([p (open-input-string #;"+*|-|-*|+**" #;"-|+*|+**" 
                              #;"+*|+**|-" #;"-|-*|-|-*"
                              #;"-|-*|-|-**|-|-*|-|-**"
                              "-|-*|-|-**|-|-*|-|-***|-|-*|-|-**|-|-*|-|-****|-|-*|-|-**|-|-*|-|-***
               |-|-*|-|-**|-|-*|-|-*****|-|-*|-|-**|-|-*|-|-***|-|-*|-|-**|-|-*|-|-****|
               -|-*|-|-**|-|-*|-|-***|-|-*|-|-**|-|-*|-|-*****"
                              ;; This one fails:
                              #;"+*")])
    (check-equal? (parse (λ () (lex p)))
                  '((((((((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *)
                        ||
                        (((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *))
                       .
                       *)
                      ||
                      (((((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *)
                        ||
                        (((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *))
                       .
                       *))
                     .
                     *)
                    ||
                    (((((((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *)
                        ||
                        (((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *))
                       .
                       *)
                      ||
                      (((((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *)
                        ||
                        (((((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *) || (((("minus" || "minus") . *) || (("minus" || "minus") . *)) . *)) . *))
                       .
                       *))
                     .
                     *)))))