(define default-dto
  (let ()

    ;; implementation of "default" dto, used as a filler for undefined
    ;; functions in other dtos

    ;; primitives
    (define (not-implemented name)
      (lambda (dto . args)
        (raise (dictionary-error (string-append name " not implemented") dto))))
    (define default-dictionary? (not-implemented "dictionary?"))
    (define default-dict-pure? (not-implemented "dict-pure?"))
    (define default-dict-size (not-implemented "dict-size"))
    (define default-dict-find-update (not-implemented "dict-find-update"))

    (define (dict-find-update* dto dict key fail success)
      (if (dict-pure? dto dict)
          (dict-find-update dto dict key fail success)
          (dict-find-update! dto dict key fail success)))
    
    (define (dict-find-update*/dict dto dict key fail success)
      (if (dict-pure? dto dict)
          (dict-find-update dto dict key fail success)
          (begin
            (dict-find-update! dto dict key fail success)
            dict)))

    (define (dict-delete-all* dto dict keys)
      (if (dict-pure? dto dict)
          (dict-delete-all dto dict keys)
          (begin
            (dict-delete-all! dto dict keys)
            dict)))

    (define (dict-update* dto dict key updater fail success)
      (if (dict-pure? dto dict)
          (dict-update dto dict key updater fail success)
          (begin
            (dict-update! dto dict key updater fail success)
            dict)))

    (define (dict-filter* dto pred dictionary)
      (if (dict-pure? dto dictionary)
          (dict-filter dto pred dictionary)
          (begin
            (dict-filter! dto pred dictionary)
            dictionary)))

    (define (dict-replace* dto dict key val)
      (if (dict-pure? dto dict)
          (dict-replace dto dict key val)
          (begin
            (dict-replace! dto dict key val)
            dict)))

    (define (default-dict-empty? dto dictionary)
      (= 0 (dict-size dto dictionary)))

    (define (default-dict=? dto = dict1 dict2)
      (define (check-entries* keys)
        (cond
          ((null? keys) #t)
          (else (let* ((key (car keys))
                       (d1-value (dict-ref dto dict1 key)))
                  (dict-ref dto dict2 key
                            (lambda () #f)
                            (lambda (d2-value)
                              (if (= d1-value d2-value)
                                  (check-entries* (cdr keys))
                                  #f)))))))
      (and (= (dict-size dto dict1)
              (dict-size dto dict2))
           (check-entries* (dict-keys dto dict1))))

    (define (default-dict-contains? dto dictionary key)
      (dict-ref dto dictionary key
                (lambda () #f)
                (lambda (x) #t)))

    (define (default-dict-ref dto dictionary key failure success)
      (dict-find-update* dto dictionary key
                     (lambda (insert ignore)
                       (failure))
                     (lambda (key value update remove)
                       (success value))))

    (define (default-dict-ref/default dto dictionary key default)
      (dict-ref dto dictionary key
                (lambda () default)
                (lambda (x) x)))

    ;; private
    (define (default-dict-set* dto dictionary use-old? objs)
      (let loop ((objs objs)
                 (dictionary dictionary))
        (cond
         ((null? objs)
          dictionary)
         ((null? (cdr objs))
          (error "mismatch of key / values argument list" objs))
         (else (let* ((key (car objs))
                      (value (cadr objs))
                      (new-d (dict-find-update*/dict dto dictionary key
                                         (lambda (insert ignore)
                                           (insert value))
                                         (lambda (key old-value update delete)
                                           (update key (if use-old? old-value value))))))
                 (loop (cddr objs)
                       new-d))))))

    (define (default-dict-set dto dictionary . objs)
      (default-dict-set* dto dictionary #f objs))

    (define (default-dict-adjoin dto dictionary . objs)
      (default-dict-set* dto dictionary #t objs))

    (define (default-dict-delete dto dictionary . keys)
      (dict-delete-all* dto dictionary keys))

    (define (default-dict-delete-all dto dictionary keylist)
      (let loop ((keylist keylist)
                 (d dictionary))
        (cond
          ((null? keylist) d)
          (else (let* ((key (car keylist))
                       (new-d (dict-find-update*/dict dto d key
                                          (lambda (_ ignore)
                                            (ignore))
                                          (lambda (key old-value _ delete)
                                            (delete)))))
                  (loop (cdr keylist)
                        new-d))))))

    (define (default-dict-replace dto dictionary key value)
      (dict-find-update* dto dictionary key
                  (lambda (_ ignore)
                    (ignore))
                  (lambda (key old-value update _)
                    (update key value))))

    (define (default-dict-intern dto dictionary key failure)
      (define pure (dict-pure? dto dictionary))
      (dict-find-update* dto dictionary key
                  (lambda (insert _)
                    (let ((value (failure)))
                     (if pure
                         (values (insert value) value)   
                         (begin
                           (insert value)
                           value))))
                  (lambda (key value update _)
                    (if pure
                        (values dictionary value)
                        value))))

    (define (default-dict-update dto dictionary key updater failure success)
      (dict-find-update* dto dictionary key
                  (lambda (insert ignore)
                    (insert (updater (failure))))
                  (lambda (key value update _)
                    (update key (updater (success value))))))

    (define (default-dict-update/default dto dictionary key updater default)
      (dict-update* dto dictionary key updater
                    (lambda () default)
                    (lambda (x) x)))

    (define (default-dict-pop dto dictionary)
      (define (do-pop)
        (call/cc
         (lambda (cont)
           (dict-for-each dto
                          (lambda (key value)
                            (define new-dict
                              (dict-delete-all* dto dictionary (list key)))
                            (if (dict-pure? dto dictionary)
                                (cont new-dict key value)   
                                (cont key value)))
                          dictionary))))
      (define empty? (dict-empty? dto dictionary))
      (if empty?
          (error "popped empty dictionary")
          (do-pop)))

    (define default-dict-map (not-implemented "dict-map"))

    (define (default-dict-filter dto pred dictionary)
      (define keys (dict-keys dto dictionary))
      (define keys-to-delete
        (filter
         (lambda (key)
           (not (pred key (dict-ref dto dictionary key))))
         keys))
      (dict-delete-all* dto dictionary keys-to-delete))

    (define (default-dict-remove dto pred dictionary)
      (dict-filter* dto (lambda (key value)
                          (not (pred key value)))
                    dictionary))

    (define (default-dict-count dto pred dictionary)
      (dict-fold dto
                 (lambda (key value acc)
                   (if (pred key value)
                       (+ 1 acc)
                       acc))
                 0
                 dictionary))

    (define (default-dict-any dto pred dictionary)
      (call/cc
       (lambda (cont)
         (dict-for-each dto
                        (lambda (key value)
                          (define ret (pred key value))
                          (when ret
                            (cont ret)))
                        dictionary)
         #f)))

    (define (default-dict-every dto pred dictionary)
      (define last #t)
      (call/cc
       (lambda (cont)
         (dict-for-each dto
                        (lambda (key value)
                          (define ret (pred key value))
                          (when (not ret)
                            (cont #f))
                          (set! last ret))
                        dictionary)
         last)))

    (define (default-dict-keys dto dictionary)
      (reverse
       (dict-fold dto
                  (lambda (key value acc)
                    (cons key acc))
                  '()
                  dictionary)))

    (define (default-dict-values dto dictionary)
      (reverse
       (dict-fold dto
                  (lambda (key value acc)
                    (cons value acc))
                  '()
                  dictionary)))

    (define (default-dict-entries dto dictionary)
      (define pair
        (dict-fold dto
                   (lambda (key value acc)
                     (cons (cons key (car acc))
                           (cons value (cdr acc))))
                   (cons '() '())
                   dictionary))
      (values (reverse (car pair))
              (reverse (cdr pair))))

    (define (default-dict-fold dto proc knil dictionary)
      (define acc knil)
      (dict-for-each dto
                     (lambda (key value)
                       (set! acc (proc key value acc)))
                     dictionary)
      acc)

    (define (default-dict-map->list dto proc dictionary)
      (define reverse-lst
        (dict-fold dto
                   (lambda (key value lst)
                     (cons (proc key value) lst))
                   '()
                   dictionary))
      (reverse reverse-lst))

    (define (default-dict->alist dto dictionary)
      (dict-map->list dto
                      cons
                      dictionary))

    (define default-dict-comparator (not-implemented "dict-comparator"))

    (define (default-dict-for-each dto proc dict start end)
      (define (any . _) #t)
      (define cmp (dict-comparator dto dict))
      (define lower
        (if start
            (lambda (el) (>=? cmp el start))
            any))
      (define upper
        (if end
            (lambda (el) (<=? cmp el end))
            any))
      (define (accept el)
        (and (upper el) (lower el)))
      (define map-proc
        (if (dict-pure? dto dict)
            dict-map
            dict-map!))
      
      (map-proc 
        dto
        (lambda (key value)
          (when (accept key)
            (proc key value))
          value)
        dict))

    (define (default-dict->generator dto dict start end)
      
      (define (any . _) #t)
      (define cmp (dict-comparator dto dict))
      (define lower
        (if start
            (lambda (el) (>=? cmp el start))
            any))
      (define upper
        (if end
            (lambda (el) (<=? cmp el end))
            any))
      (define (accept el)
        (and (upper el) (lower el)))

      ;; proc that takes yield value and yield continuation when yield is called
      ;; shouldn't return
      (define yield-handler #f)
      
      (define (yield value)
        (when (or (eof-object? value)
                  (accept (car value)))
          (call/cc (lambda (yield-cont)
                     (yield-handler value yield-cont))) ))
      
      (define (generate)
        (dict-for-each dto
                       (lambda (key value)
                         (yield (cons key value)))
                       dict)
        (yield (eof-object)))
      
      ;; continuation at the point of last yield
      (define yield-cont #f)
      
      ;; check if eof return was seen; if yes, keep returning eof
      ;; for further invocations
      (define eof #f)
      
      (define (get-next-value exit)
        (set! yield-handler
          (lambda (value new-yield-cont)
            (set! yield-cont new-yield-cont)
            (when (eof-object? value)
              (set! eof #t)
              ;; unset continuation reference to allow
              ;; gc clean everything up
              (set! yield-cont #f))
            (exit value)))
        
        (cond
          ;; eof seen -- keep returning eof
          (eof (eof-object))
          ;; no yield called yet -- start the generator
          ((not yield-cont) (generate))
          ;; continue from last yield position
          (else (yield-cont #t))))
      
      (lambda ()
        (call/cc get-next-value)))

    (define (default-dict-accumulator dto dict acc-proc)
      (define pure (dict-pure? dto dict))
      (lambda (arg)
        (if (eof-object? arg)
            dict
            (if pure
                (set! dict (acc-proc dto dict (car arg) (cdr arg)))   
                (acc-proc dto dict (car arg) (cdr arg))))))

    (define (default-dict-set-accumulator dto dict)
      (if (dict-pure? dto dict)
          (default-dict-accumulator dto dict dict-set)
          (default-dict-accumulator dto dict dict-set!)))

    (define (default-dict-adjoin-accumulator dto dict)
      (if (dict-pure? dto dict)
          (default-dict-accumulator dto dict dict-adjoin)
          (default-dict-accumulator dto dict dict-adjoin!)))

    (let ()
      (define null-dto (make-dto-private (make-vector dict-procedures-count #f)))
      (define default-dto
        (make-modified-dto
         null-dto
         dictionary?-id default-dictionary?
         dict-empty?-id default-dict-empty?
         dict-contains?-id default-dict-contains?
         dict=?-id default-dict=?
         dict-pure?-id default-dict-pure?
         dict-ref-id default-dict-ref
         dict-ref/default-id default-dict-ref/default
         dict-set-id default-dict-set
         dict-adjoin-id default-dict-adjoin
         dict-delete-id default-dict-delete
         dict-delete-all-id default-dict-delete-all
         dict-replace-id default-dict-replace
         dict-intern-id default-dict-intern
         dict-update-id default-dict-update
         dict-update/default-id default-dict-update/default
         dict-pop-id default-dict-pop
         dict-map-id default-dict-map
         dict-filter-id default-dict-filter
         dict-remove-id default-dict-remove
         dict-find-update-id default-dict-find-update
         dict-size-id default-dict-size
         dict-count-id default-dict-count
         dict-any-id default-dict-any
         dict-every-id default-dict-every
         dict-keys-id default-dict-keys
         dict-values-id default-dict-values
         dict-entries-id default-dict-entries
         dict-fold-id default-dict-fold
         dict-map->list-id default-dict-map->list
         dict->alist-id default-dict->alist
         dict-comparator-id default-dict-comparator

         dict-for-each-id default-dict-for-each

         ;; generator procedures
         dict->generator-id default-dict->generator
         dict-set-accumulator-id default-dict-set-accumulator
         dict-adjoin-accumulator-id default-dict-adjoin-accumulator))

      ;; sanity check
      (vector-for-each
       (lambda (proc index)
         (unless (and proc (procedure? proc))
           (error "Missing or wrong default procedure definition" proc index)))
       (procvec default-dto)
       (list->vector (iota dict-procedures-count)))

      default-dto)))