about summary refs log tree commit diff stats
path: root/archive/2.vm/065duplex_list.mu
blob: 3a7de8f65aaf3731ab0443559334f357a4838222 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
# A doubly linked list permits bidirectional traversal.

container duplex-list:_elem [
  value:_elem
  next:&:duplex-list:_elem
  prev:&:duplex-list:_elem
]

def push x:_elem, in:&:duplex-list:_elem/contained-in:result -> result:&:duplex-list:_elem [
  local-scope
  load-inputs
  result:&:duplex-list:_elem <- new {(duplex-list _elem): type}
  *result <- merge x, in, null
  return-unless in
  put *in, prev:offset, result
]

def first in:&:duplex-list:_elem -> result:_elem [
  local-scope
  load-inputs
  {
    break-if in
    zero:&:_elem <- new _elem:type
    zero-result:_elem <- copy *zero
    abandon zero
    return zero-result
  }
  result <- get *in, value:offset
]

def next in:&:duplex-list:_elem -> result:&:duplex-list:_elem/contained-in:in [
  local-scope
  load-inputs
  return-unless in, null
  result <- get *in, next:offset
]

def prev in:&:duplex-list:_elem -> result:&:duplex-list:_elem/contained-in:in [
  local-scope
  load-inputs
  return-unless in, null
  result <- get *in, prev:offset
  return result
]

scenario duplex-list-handling [
  run [
    local-scope
    # reserve locations 0-9 to check for missing null check
    10:num/raw <- copy 34
    11:num/raw <- copy 35
    list:&:duplex-list:num <- push 3, null
    list <- push 4, list
    list <- push 5, list
    list2:&:duplex-list:num <- copy list
    20:num/raw <- first list2
    list2 <- next list2
    21:num/raw <- first list2
    list2 <- next list2
    22:num/raw <- first list2
    30:&:duplex-list:num/raw <- next list2
    31:num/raw <- first 30:&:duplex-list:num/raw
    32:&:duplex-list:num/raw <- next 30:&:duplex-list:num/raw
    33:&:duplex-list:num/raw <- prev 30:&:duplex-list:num/raw
    list2 <- prev list2
    40:num/raw <- first list2
    list2 <- prev list2
    41:num/raw <- first list2
    50:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    0 <- 0  # no modifications to null pointers
    10 <- 34
    11 <- 35
    20 <- 5  # scanning next
    21 <- 4
    22 <- 3
    30 <- 0  # null
    31 <- 0  # first of null
    32 <- 0  # next of null
    33 <- 0  # prev of null
    40 <- 4  # then start scanning prev
    41 <- 5
    50 <- 1  # list back at start
  ]
]

def length l:&:duplex-list:_elem -> result:num [
  local-scope
  load-inputs
  result <- copy 0
  {
    break-unless l
    result <- add result, 1
    l <- next l
    loop
  }
]

# insert 'x' after 'in'
def insert x:_elem, in:&:duplex-list:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  new-node:&:duplex-list:_elem <- new {(duplex-list _elem): type}
  *new-node <- put *new-node, value:offset, x
  # save old next before changing it
  next-node:&:duplex-list:_elem <- get *in, next:offset
  *in <- put *in, next:offset, new-node
  *new-node <- put *new-node, prev:offset, in
  *new-node <- put *new-node, next:offset, next-node
  return-unless next-node
  *next-node <- put *next-node, prev:offset, new-node
]

scenario inserting-into-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # inside list
    list2 <- insert 6, list2
    # check structure like before
    list2 <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    list2 <- next list2
    13:num/raw <- first list2
    list2 <- prev list2
    20:num/raw <- first list2
    list2 <- prev list2
    21:num/raw <- first list2
    list2 <- prev list2
    22:num/raw <- first list2
    30:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 5  # scanning next
    11 <- 4
    12 <- 6  # inserted element
    13 <- 3
    20 <- 6  # then prev
    21 <- 4
    22 <- 5
    30 <- 1  # list back at start
  ]
]

scenario inserting-at-end-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # inside list
    list2 <- next list2  # now at end of list
    list2 <- insert 6, list2
    # check structure like before
    list2 <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    list2 <- next list2
    13:num/raw <- first list2
    list2 <- prev list2
    20:num/raw <- first list2
    list2 <- prev list2
    21:num/raw <- first list2
    list2 <- prev list2
    22:num/raw <- first list2
    30:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 5  # scanning next
    11 <- 4
    12 <- 3
    13 <- 6  # inserted element
    20 <- 3  # then prev
    21 <- 4
    22 <- 5
    30 <- 1  # list back at start
  ]
]

scenario inserting-after-start-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list <- insert 6, list
    # check structure like before
    list2:&:duplex-list:num <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    list2 <- next list2
    13:num/raw <- first list2
    list2 <- prev list2
    20:num/raw <- first list2
    list2 <- prev list2
    21:num/raw <- first list2
    list2 <- prev list2
    22:num/raw <- first list2
    30:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 5  # scanning next
    11 <- 6  # inserted element
    12 <- 4
    13 <- 3
    20 <- 4  # then prev
    21 <- 6
    22 <- 5
    30 <- 1  # list back at start
  ]
]

# remove 'x' from its surrounding list 'in'
#
# Returns null if and only if list is empty. Beware: in that case any other
# pointers to the head are now invalid.
def remove x:&:duplex-list:_elem/contained-in:in, in:&:duplex-list:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  # if 'x' is null, return
  return-unless x
  next-node:&:duplex-list:_elem <- get *x, next:offset
  prev-node:&:duplex-list:_elem <- get *x, prev:offset
  # null x's pointers
  *x <- put *x, next:offset, null
  *x <- put *x, prev:offset, null
  # if next-node is not null, set its prev pointer
  {
    break-unless next-node
    *next-node <- put *next-node, prev:offset, prev-node
  }
  # if prev-node is not null, set its next pointer and return
  {
    break-unless prev-node
    *prev-node <- put *prev-node, next:offset, next-node
    return
  }
  # if prev-node is null, then we removed the head node at 'in'
  # return the new head rather than the old 'in'
  return next-node
]

scenario removing-from-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # second element
    list <- remove list2, list
    10:bool/raw <- equal list2, null
    # check structure like before
    list2 <- copy list
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    20:&:duplex-list:num/raw <- next list2
    list2 <- prev list2
    30:num/raw <- first list2
    40:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 0  # remove returned non-null
    11 <- 5  # scanning next, skipping deleted element
    12 <- 3
    20 <- 0  # no more elements
    30 <- 5  # prev of final element
    40 <- 1  # list back at start
  ]
]

scenario removing-from-start-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list <- remove list, list
    # check structure like before
    list2:&:duplex-list:num <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    20:&:duplex-list:num/raw <- next list2
    list2 <- prev list2
    30:num/raw <- first list2
    40:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 4  # scanning next, skipping deleted element
    11 <- 3
    20 <- 0  # no more elements
    30 <- 4  # prev of final element
    40 <- 1  # list back at start
  ]
]

scenario removing-from-end-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    # delete last element
    list2:&:duplex-list:num <- next list
    list2 <- next list2
    list <- remove list2, list
    10:bool/raw <- equal list2, null
    # check structure like before
    list2 <- copy list
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    20:&:duplex-list:num/raw <- next list2
    list2 <- prev list2
    30:num/raw <- first list2
    40:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 0  # remove returned non-null
    11 <- 5  # scanning next, skipping deleted element
    12 <- 4
    20 <- 0  # no more elements
    30 <- 5  # prev of final element
    40 <- 1  # list back at start
  ]
]

scenario removing-from-singleton-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  run [
    list <- remove list, list
    1:num/raw <- deaddress list
  ]
  memory-should-contain [
    1 <- 0  # back to an empty list
  ]
]

def remove x:&:duplex-list:_elem/contained-in:in, n:num, in:&:duplex-list:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  i:num <- copy 0
  curr:&:duplex-list:_elem <- copy x
  {
    done?:bool <- greater-or-equal i, n
    break-if done?
    break-unless curr
    next:&:duplex-list:_elem <- next curr
    in <- remove curr, in
    curr <- copy next
    i <- add i, 1
    loop
  }
]

scenario removing-multiple-from-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # second element
    list <- remove list2, 2, list
    stash list
  ]
  trace-should-contain [
    app: 5
  ]
]

# remove values between 'start' and 'end' (both exclusive).
# also clear pointers back out from start/end for hygiene.
# set end to 0 to delete everything past start.
# can't set start to 0 to delete everything before end, because there's no
# clean way to return the new head pointer.
def remove-between start:&:duplex-list:_elem, end:&:duplex-list:_elem/contained-in:start -> start:&:duplex-list:_elem [
  local-scope
  load-inputs
  next:&:duplex-list:_elem <- get *start, next:offset
  nothing-to-delete?:bool <- equal next, end
  return-if nothing-to-delete?
  assert next, [malformed duplex list]
  # start->next->prev = 0
  # start->next = end
  *next <- put *next, prev:offset, null
  *start <- put *start, next:offset, end
  {
    break-if end
    stash [spliced:] next
    return
  }
  # end->prev->next = 0
  # end->prev = start
  prev:&:duplex-list:_elem <- get *end, prev:offset
  assert prev, [malformed duplex list - 2]
  *prev <- put *prev, next:offset, null
  stash [spliced:] next
  *end <- put *end, prev:offset, start
]

scenario remove-range [
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  local-scope
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # delete 16 onwards
    # first pointer: to the third element
    list2:&:duplex-list:num <- next list
    list2 <- next list2
    list2 <- remove-between list2, null
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 15
    20 <- 0
  ]
  trace-should-contain [
    app: spliced: 16 <-> 17 <-> 18
  ]
]

scenario remove-range-to-final [
  local-scope
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # delete 15, 16 and 17
    # start pointer: to the second element
    list2:&:duplex-list:num <- next list
    # end pointer: to the last (sixth) element
    end:&:duplex-list:num <- next list2
    end <- next end
    end <- next end
    end <- next end
    remove-between list2, end
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 18
    20 <- 0  # no more elements
  ]
  trace-should-contain [
    app: spliced: 15 <-> 16 <-> 17
  ]
]

scenario remove-range-to-penultimate [
  local-scope
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # delete 15 and 16
    # start pointer: to the second element
    list2:&:duplex-list:num <- next list
    # end pointer: to the last (sixth) element
    end:&:duplex-list:num <- next list2
    end <- next end
    end <- next end
    remove-between list2, end
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    list <- next list
    13:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 17
    13 <- 18
    20 <- 0  # no more elements
  ]
  trace-should-contain [
    app: spliced: 15 <-> 16
  ]
]

scenario remove-range-empty [
  local-scope
  # construct a duplex list with three elements [13, 14, 15]
  list:&:duplex-list:num <- push 15, null
  list <- push 14, list
  list <- push 13, list
  run [
    # delete between first and second element (i.e. nothing)
    list2:&:duplex-list:num <- next list
    remove-between list, list2
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  # no change
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 15
    20 <- 0
  ]
]

scenario remove-range-to-end [
  local-scope
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # remove the third element and beyond
    list2:&:duplex-list:num <- next list
    remove-between list2, null
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    20 <- 0
  ]
]

# insert list beginning at 'start' after 'in'
def splice in:&:duplex-list:_elem, start:&:duplex-list:_elem/contained-in:in -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  return-unless in
  return-unless start
  end:&:duplex-list:_elem <- last start
  next:&:duplex-list:_elem <- next in
  {
    break-unless next
    *end <- put *end, next:offset, next
    *next <- put *next, prev:offset, end
  }
  *in <- put *in, next:offset, start
  *start <- put *start, prev:offset, in
]

# insert contents of 'new' after 'in'
def insert in:&:duplex-list:_elem, new:&:@:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  return-unless in
  return-unless new
  len:num <- length *new
  return-unless len
  curr:&:duplex-list:_elem <- copy in
  idx:num <- copy 0
  {
    done?:bool <- greater-or-equal idx, len
    break-if done?
    c:_elem <- index *new, idx
    insert c, curr
    # next iter
    curr <- next curr
    idx <- add idx, 1
    loop
  }
]

def append in:&:duplex-list:_elem, new:&:duplex-list:_elem/contained-in:in -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  last:&:duplex-list:_elem <- last in
  *last <- put *last, next:offset, new
  return-unless new
  *new <- put *new, prev:offset, last
]

def last in:&:duplex-list:_elem -> result:&:duplex-list:_elem [
  local-scope
  load-inputs
  result <- copy in
  {
    next:&:duplex-list:_elem <- next result
    break-unless next
    result <- copy next
    loop
  }
]

# does a duplex list start with a certain sequence of elements?
def match x:&:duplex-list:_elem, y:&:@:_elem -> result:bool [
  local-scope
  load-inputs
  i:num <- copy 0
  max:num <- length *y
  {
    done?:bool <- greater-or-equal i, max
    break-if done?
    expected:_elem <- index *y, i
    return-unless x, false/no-match
    curr:_elem <- first x
    curr-matches?:bool <- equal curr, expected
    return-unless curr-matches?, false/no-match
    x <- next x
    i <- add i, 1
    loop
  }
  return true/successful-match
]

scenario duplex-list-match [
  local-scope
  list:&:duplex-list:char <- push 97/a, null
  list <- push 98/b, list
  list <- push 99/c, list
  list <- push 100/d, list
  run [
    10:bool/raw <- match list, []
    11:bool/raw <- match list, [d]
    12:bool/raw <- match list, [dc]
    13:bool/raw <- match list, [dcba]
    14:bool/raw <- match list, [dd]
    15:bool/raw <- match list, [dcbax]
  ]
  memory-should-contain [
    10 <- 1  # matches []
    11 <- 1  # matches [d]
    12 <- 1  # matches [dc]
    13 <- 1  # matches [dcba]
    14 <- 0  # does not match [dd]
    15 <- 0  # does not match [dcbax]
  ]
]

# helper for debugging
def dump-from x:&:duplex-list:_elem [
  local-scope
  load-inputs
  $print x, [: ]
  {
    break-unless x
    c:_elem <- get *x, value:offset
    $print c, [ ]
    x <- next x
    {
      is-newline?:bool <- equal c, 10/newline
      break-unless is-newline?
      $print 10/newline
      $print x, [: ]
    }
    loop
  }
  $print 10/newline, [---], 10/newline
]

scenario stash-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 1, null
  list <- push 2, list
  list <- push 3, list
  run [
    stash [list:], list
  ]
  trace-should-contain [
    app: list: 3 <-> 2 <-> 1
  ]
]

def to-text in:&:duplex-list:_elem -> result:text [
  local-scope
  load-inputs
  buf:&:buffer:char <- new-buffer 80
  buf <- to-buffer in, buf
  result <- buffer-to-array buf
]

# variant of 'to-text' which stops printing after a few elements (and so is robust to cycles)
def to-text-line in:&:duplex-list:_elem -> result:text [
  local-scope
  load-inputs
  buf:&:buffer:char <- new-buffer 80
  buf <- to-buffer in, buf, 6  # max elements to display
  result <- buffer-to-array buf
]

def to-buffer in:&:duplex-list:_elem, buf:&:buffer:char -> buf:&:buffer:char [
  local-scope
  load-inputs
  {
    break-if in
    buf <- append buf, [[]]
    return
  }
  # append in.value to buf
  val:_elem <- get *in, value:offset
  buf <- append buf, val
  # now prepare next
  next:&:duplex-list:_elem <- next in
  nextn:num <- deaddress next
  return-unless next
  buf <- append buf, [ <-> ]
  # and recurse
  remaining:num, optional-input-found?:bool <- next-input
  {
    break-if optional-input-found?
    # unlimited recursion
    buf <- to-buffer next, buf
    return
  }
  {
    break-unless remaining
    # limited recursion
    remaining <- subtract remaining, 1
    buf <- to-buffer next, buf, remaining
    return
  }
  # past recursion depth; insert ellipses and stop
  append buf, [...]
]

scenario stash-empty-duplex-list [
  local-scope
  x:&:duplex-list:num <- copy null
  run [
    stash x
  ]
  trace-should-contain [
    app: []
  ]
]