summaryrefslogtreecommitdiff
path: root/macros/latex/contrib/l3kernel/l3quark.dtx
blob: 8106e1746e50865adcc687764a006964da78bd1e (plain)
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
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
% \iffalse meta-comment
%
%% File: l3quark.dtx
%
% Copyright (C) 1990-2021 The LaTeX Project
%
% It may be distributed and/or modified under the conditions of the
% LaTeX Project Public License (LPPL), either version 1.3c of this
% license or (at your option) any later version.  The latest version
% of this license is in the file
%
%    https://www.latex-project.org/lppl.txt
%
% This file is part of the "l3kernel bundle" (The Work in LPPL)
% and all files in that bundle must be distributed together.
%
% -----------------------------------------------------------------------
%
% The development version of the bundle can be found at
%
%    https://github.com/latex3/latex3
%
% for those people who are interested.
%
%<*driver>
\documentclass[full,kernel]{l3doc}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \title{^^A
%   The \pkg{l3quark} package\\ Quarks^^A
% }
%
% \author{^^A
%  The \LaTeX{} Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released 2021-05-07}
%
% \maketitle
%
% \begin{documentation}
%
% Two special types of constants in \LaTeX3 are \enquote{quarks} and
% \enquote{scan marks}. By convention all constants of type quark
% start out with |\q_|, and scan marks start with |\s_|.
%
% \section{Quarks}
%
% Quarks are control sequences that expand to themselves and should
% therefore \emph{never} be executed directly in the code.
% This would result in an endless loop!
%
% They are meant to be used as delimiter in weird functions, the most
% common use case being the `stop token' (\emph{i.e.}~\cs{q_stop}).
% For example, when writing a macro to parse a user-defined date
% \begin{verbatim}
%   \date_parse:n {19/June/1981}
% \end{verbatim}
% one might write a command such as
% \begin{verbatim}
%   \cs_new:Npn \date_parse:n #1 { \date_parse_aux:w #1 \q_stop }
%   \cs_new:Npn \date_parse_aux:w #1 / #2 / #3 \q_stop
%     { <do something with the date> }
% \end{verbatim}
%
% Quarks are sometimes also used as error return values for functions
% that receive erroneous input.
% For example, in the function \cs{prop_get:NnN} to retrieve a value stored
% in some key of a property list, if the key does not exist then the return value
% is the quark \cs{q_no_value}.
% As mentioned above, such quarks are extremely fragile and it is imperative
% when using such functions that code is carefully written to check for
% pathological cases to avoid leakage of a quark into an uncontrolled
% environment.
%
% Quarks also permit the
% following ingenious trick when parsing tokens:
% when you pick up a token in a temporary variable
% and you want to know whether you have picked up a particular quark,
% all you have to do is compare the temporary variable to the quark using
% \cs{tl_if_eq:NNTF}. A set of special quark testing functions is set up
% below. All the quark testing functions are expandable although the
% ones testing only single tokens are much faster.
% An example of the quark testing functions and their use in recursion
% can be seen in the implementation of \cs{clist_map_function:NN}.
%
% \section{Defining quarks}
%
% \begin{function}{\quark_new:N}
%   \begin{syntax}
%     \cs{quark_new:N} \meta{quark}
%   \end{syntax}
%   Creates a new \meta{quark} which expands only to \meta{quark}.
%   The \meta{quark} is defined globally, and an error message
%   is raised if the name was already taken.
% \end{function}
%
% \begin{variable}{\q_stop}
%   Used as a marker for delimited arguments, such as
%   \begin{verbatim}
%     \cs_set:Npn \tmp:w #1#2 \q_stop {#1}
%   \end{verbatim}
% \end{variable}
%
% \begin{variable}{\q_mark}
%   Used as a marker for delimited arguments when \cs{q_stop} is
%   already in use.
% \end{variable}
%
% \begin{variable}{\q_nil}
%   Quark to mark a null value in structured variables or functions. Used
%   as an end delimiter when this may itself need to be tested
%   (in contrast to \cs{q_stop}, which is only ever used as a delimiter).
% \end{variable}
%
% \begin{variable}{\q_no_value}
%   A canonical value for a missing value, when one is requested from
%   a data structure. This is therefore used as a \enquote{return} value
%   by functions such as \cs{prop_get:NnN} if there is no data to
%   return.
% \end{variable}
%
% \section{Quark tests}
%
% The method used to define quarks means that the single token (\texttt{N})
% tests are faster than the multi-token (\texttt{n}) tests. The latter
% should therefore only be used when the argument can definitely take
% more than a single token.
%
% \begin{function}[EXP,pTF]{\quark_if_nil:N}
%   \begin{syntax}
%     \cs{quark_if_nil_p:N} \meta{token}
%     \cs{quark_if_nil:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is equal to \cs{q_nil}.
% \end{function}
%
% \begin{function}[EXP,pTF]{\quark_if_nil:n, \quark_if_nil:o, \quark_if_nil:V}
%   \begin{syntax}
%     \cs{quark_if_nil_p:n} \Arg{token list} \\
%     \cs{quark_if_nil:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token list} contains only \cs{q_nil} (distinct
%   from \meta{token list} being empty or containing \cs{q_nil} plus one
%   or more other tokens).
% \end{function}
%
% \begin{function}[EXP,pTF]{\quark_if_no_value:N, \quark_if_no_value:c}
%   \begin{syntax}
%     \cs{quark_if_no_value_p:N} \meta{token} \\
%     \cs{quark_if_no_value:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is equal to \cs{q_no_value}.
% \end{function}
%
% \begin{function}[EXP,pTF]{\quark_if_no_value:n}
%   \begin{syntax}
%      \cs{quark_if_no_value_p:n} \Arg{token list} \\
%     \cs{quark_if_no_value:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token list} contains only \cs{q_no_value}
%   (distinct from \meta{token list} being empty or containing
%   \cs{q_no_value} plus one or more other tokens).
% \end{function}
%
% \section{Recursion}
%
% This module provides a uniform interface to intercepting and
% terminating loops as when one is doing tail recursion. The building
% blocks follow below and an example is shown in
% Section~\ref{sec:l3quark:quark-example}.
%
% \begin{variable}{\q_recursion_tail}
%   This quark is appended to the data structure in question and
%   appears as a real element there. This means it gets any list
%   separators around it.
% \end{variable}
%
% \begin{variable}{\q_recursion_stop}
%   This quark is added \emph{after} the data structure. Its purpose
%   is to make it possible to terminate the recursion at any point
%   easily.
% \end{variable}
%
% \begin{function}[EXP]{\quark_if_recursion_tail_stop:N}
%   \begin{syntax}
%     \cs{quark_if_recursion_tail_stop:N} \meta{token}
%   \end{syntax}
%   Tests if \meta{token} contains only the marker
%   \cs{q_recursion_tail}, and if so uses
%   \cs{use_none_delimit_by_q_recursion_stop:w} to terminate the
%   recursion that this belongs to. The
%   recursion input must include the marker tokens \cs{q_recursion_tail}
%   and \cs{q_recursion_stop} as the last two items.
% \end{function}
%
% \begin{function}[EXP,updated = 2011-09-06]
%   {\quark_if_recursion_tail_stop:n, \quark_if_recursion_tail_stop:o}
%   \begin{syntax}
%     \cs{quark_if_recursion_tail_stop:n} \Arg{token list}
%   \end{syntax}
%   Tests if the \meta{token list} contains only
%   \cs{q_recursion_tail}, and if so uses
%   \cs{use_none_delimit_by_q_recursion_stop:w} to terminate the
%   recursion that this belongs to. The
%   recursion input must include the marker tokens \cs{q_recursion_tail}
%   and \cs{q_recursion_stop} as the last two items.
% \end{function}
%
% \begin{function}[EXP]{\quark_if_recursion_tail_stop_do:Nn}
%   \begin{syntax}
%     \cs{quark_if_recursion_tail_stop_do:Nn} \meta{token} \Arg{insertion}
%   \end{syntax}
%   Tests if \meta{token} contains only the marker
%   \cs{q_recursion_tail}, and if so uses
%   \cs{use_i_delimit_by_q_recursion_stop:w} to terminate the
%   recursion that this belongs to. The
%   recursion input must include the marker tokens \cs{q_recursion_tail}
%   and \cs{q_recursion_stop} as the last two items. The \meta{insertion}
%   code is then added to the input stream after the recursion has
%   ended.
% \end{function}
%
% \begin{function}[EXP,updated = 2011-09-06]
%   {\quark_if_recursion_tail_stop_do:nn, \quark_if_recursion_tail_stop_do:on}
%   \begin{syntax}
%     \cs{quark_if_recursion_tail_stop_do:nn} \Arg{token list} \Arg{insertion}
%   \end{syntax}
%   Tests if the \meta{token list} contains only
%   \cs{q_recursion_tail}, and if so uses
%   \cs{use_i_delimit_by_q_recursion_stop:w} to terminate the
%   recursion that this belongs to. The
%   recursion input must include the marker tokens \cs{q_recursion_tail}
%   and \cs{q_recursion_stop} as the last two items. The \meta{insertion}
%   code is then added to the input stream after the recursion has
%   ended.
% \end{function}
%
%
% \begin{function}[EXP,added = 2018-04-10]
%   {
%     \quark_if_recursion_tail_break:NN,
%     \quark_if_recursion_tail_break:nN
%   }
%   \begin{syntax}
%     \cs{quark_if_recursion_tail_break:nN} \Arg{token list} \cs[no-index]{\meta{type}_map_break:}
%   \end{syntax}
%   Tests if \meta{token list} contains only \cs{q_recursion_tail}, and
%   if so terminates the recursion using \cs[no-index]{\meta{type}_map_break:}.
%   The recursion end should be marked by \cs{prg_break_point:Nn}
%   \cs[no-index]{\meta{type}_map_break:}.
% \end{function}
%
% \subsection{An example of recursion with quarks}
% \label{sec:l3quark:quark-example}
%
% Quarks are mainly used internally in the \pkg{expl3} code to define
% recursion functions such as \cs{tl_map_inline:nn} and so on.
% Here is a small example to demonstrate how to use quarks in this fashion.
% We shall define a command called |\my_map_dbl:nn| which takes a token list
% and applies an operation to every \emph{pair} of tokens.
% For example, |\my_map_dbl:nn {abcd} {[--#1--#2--]~}| would produce
% \enquote{\ttfamily [--a--b--]~[--c--d--]~}.
% Using quarks to define such functions simplifies their logic and ensures
% robustness in many cases.
%
%
% Here's the definition of |\my_map_dbl:nn|.
% First of all, define the function that does the processing based on the
% inline function argument |#2|.
% Then initiate the recursion using an internal function.
% The token list |#1| is terminated using \cs{q_recursion_tail}, with
% delimiters according to the type of recursion (here a pair of
% \cs{q_recursion_tail}), concluding with \cs{q_recursion_stop}.
% These quarks are used to mark the end of the token list being operated upon.
%\begin{verbatim}
% \cs_new:Npn \my_map_dbl:nn #1#2
%  {
%    \cs_set:Npn \__my_map_dbl_fn:nn ##1 ##2 {#2}
%    \__my_map_dbl:nn #1 \q_recursion_tail \q_recursion_tail
%    \q_recursion_stop
%  }
%\end{verbatim}
%
% The definition of the internal recursion function follows.
% First check if either of the input tokens are the termination quarks.
% Then, if not, apply the inline function to the two arguments.
%\begin{verbatim}
% \cs_new:Nn \__my_map_dbl:nn
%  {
%    \quark_if_recursion_tail_stop:n {#1}
%    \quark_if_recursion_tail_stop:n {#2}
%    \__my_map_dbl_fn:nn {#1} {#2}
%\end{verbatim}
% Finally, recurse:
%\begin{verbatim}
%    \__my_map_dbl:nn
%  }
%\end{verbatim}
% Note that contrarily to \LaTeX3 built-in mapping functions, this
% mapping function cannot be nested, since the second map would overwrite
% the definition of |\__my_map_dbl_fn:nn|.
%
% \section{Scan marks}
%
% Scan marks are control sequences set equal to \cs{scan_stop:},
% hence never expand in an expansion context and are (largely)
% invisible if they are encountered in a typesetting context.
%
% Like quarks, they can be used as delimiters in weird functions
% and are often safer to use for this purpose.
% Since they are harmless when executed by \TeX{} in non-expandable
% contexts, they can be used to mark the end of a set of instructions.
% This allows to skip to that point if the end of the instructions
% should not be performed (see \pkg{l3regex}).
%
% \begin{function}[added = 2018-04-01]{\scan_new:N}
%   \begin{syntax}
%     \cs{scan_new:N} \meta{scan mark}
%   \end{syntax}
%   Creates a new \meta{scan mark} which is set equal to \cs{scan_stop:}.
%   The \meta{scan mark} is defined globally, and an error message
%   is raised if the name was already taken by another scan mark.
% \end{function}
%
% \begin{variable}[added = 2018-04-01]{\s_stop}
%   Used at the end of a set of instructions, as a marker
%   that can be jumped to using \cs{use_none_delimit_by_s_stop:w}.
% \end{variable}
%
% \begin{function}[EXP,added = 2018-04-01]{\use_none_delimit_by_s_stop:w}
%   \begin{syntax}
%     \cs{use_none_delimit_by_s_stop:w} \meta{tokens} \cs{s_stop}
%   \end{syntax}
%   Removes the \meta{tokens} and \cs{s_stop} from the input stream.
%   This leads to a low-level \TeX{} error if \cs{s_stop} is absent.
% \end{function}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3quark} implementation}
%
% \TestFiles{m3quark001.lvt}
%
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
%
% \subsection{Quarks}
%
%    \begin{macrocode}
%<@@=quark>
%    \end{macrocode}
%
% \begin{macro}{\quark_new:N}
% \UnitTested
%    Allocate a new quark.
%    \begin{macrocode}
\cs_new_protected:Npn \quark_new:N #1
  {
    \__kernel_chk_if_free_cs:N #1
    \cs_gset_nopar:Npn #1 {#1}
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\q_nil, \q_mark, \q_no_value, \q_stop}
%   Some \enquote{public} quarks. \cs{q_stop} is an \enquote{end of
%   argument} marker, \cs{q_nil} is a empty value and \cs{q_no_value}
%   marks an empty argument.
%    \begin{macrocode}
\quark_new:N \q_nil
\quark_new:N \q_mark
\quark_new:N \q_no_value
\quark_new:N \q_stop
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\q_recursion_tail, \q_recursion_stop}
%   Quarks for ending recursions. Only ever used there!
%   \cs{q_recursion_tail} is appended to whatever list structure we are
%   doing recursion on, meaning it is added as a proper list item with
%   whatever list separator is in use.  \cs{q_recursion_stop} is placed
%   directly after the list.
%    \begin{macrocode}
\quark_new:N \q_recursion_tail
\quark_new:N \q_recursion_stop
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\s_@@}
%   Private scan mark used in \pkg{l3quark}.  We don't have \pkg{l3scan}
%   yet, so we declare the scan mark here and add it to the scan mark pool
%   later.
%    \begin{macrocode}
\cs_new_eq:NN \s_@@ \scan_stop:
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\q_@@_nil}
%   Private quark use for some tests.
%    \begin{macrocode}
\quark_new:N \q_@@_nil
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}[EXP]{\quark_if_recursion_tail_stop:N}
% \UnitTested
% \begin{macro}[EXP]{\quark_if_recursion_tail_stop_do:Nn}
% \UnitTested
%   When doing recursions, it is easy to spend a lot of time testing if the
%   end marker has been found. To avoid this, a dedicated end marker is used
%   each time a recursion is set up. Thus if the marker is found everything
%   can be wrapper up and finished off. The simple case is when the test
%   can guarantee that only a single token is being tested. In this case,
%   there is just a dedicated copy of the standard quark test. Both a gobbling
%   version and one inserting end code are provided.
%    \begin{macrocode}
\cs_new:Npn \quark_if_recursion_tail_stop:N #1
  {
    \if_meaning:w \q_recursion_tail #1
      \exp_after:wN \use_none_delimit_by_q_recursion_stop:w
    \fi:
  }
\cs_new:Npn \quark_if_recursion_tail_stop_do:Nn #1
  {
    \if_meaning:w \q_recursion_tail #1
      \exp_after:wN \use_i_delimit_by_q_recursion_stop:nw
    \else:
      \exp_after:wN \use_none:n
    \fi:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]
%   {\quark_if_recursion_tail_stop:n, \quark_if_recursion_tail_stop:o}
% \UnitTested
% \begin{macro}[EXP]
%   {\quark_if_recursion_tail_stop_do:nn, \quark_if_recursion_tail_stop_do:on}
% \UnitTested
% \begin{macro}[EXP]{\@@_if_recursion_tail:w}
%   See \cs{quark_if_nil:nTF} for the details.  Expanding
%   \cs{@@_if_recursion_tail:w} once in front of the tokens chosen here
%   gives an empty result if and only if |#1|~is exactly
%   \cs{q_recursion_tail}.
%    \begin{macrocode}
\cs_new:Npn \quark_if_recursion_tail_stop:n #1
  {
    \tl_if_empty:oTF
      { \@@_if_recursion_tail:w {} #1 {} ?! \q_recursion_tail ??! }
      { \use_none_delimit_by_q_recursion_stop:w }
      { }
  }
\cs_new:Npn \quark_if_recursion_tail_stop_do:nn #1
  {
    \tl_if_empty:oTF
      { \@@_if_recursion_tail:w {} #1 {} ?! \q_recursion_tail ??! }
      { \use_i_delimit_by_q_recursion_stop:nw }
      { \use_none:n }
  }
\cs_new:Npn \@@_if_recursion_tail:w
    #1 \q_recursion_tail #2 ? #3 ?! { #1 #2 }
\cs_generate_variant:Nn \quark_if_recursion_tail_stop:n { o }
\cs_generate_variant:Nn \quark_if_recursion_tail_stop_do:nn { o }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\quark_if_recursion_tail_break:NN}
% \begin{macro}[EXP]{\quark_if_recursion_tail_break:nN}
%   Analogues of the \cs[index=quark_if_recursion_tail_stop:n]
%   {quark_if_recursion_tail_stop\ldots{}} functions.
%   Break the mapping using |#2|.
%    \begin{macrocode}
\cs_new:Npn \quark_if_recursion_tail_break:NN #1#2
  {
    \if_meaning:w \q_recursion_tail #1
      \exp_after:wN #2
    \fi:
  }
\cs_new:Npn \quark_if_recursion_tail_break:nN #1#2
  {
    \tl_if_empty:oT
      { \@@_if_recursion_tail:w {} #1 {} ?! \q_recursion_tail ??! }
      {#2}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[pTF]{\quark_if_nil:N}
% \UnitTested
% \begin{macro}[pTF]{\quark_if_no_value:N, \quark_if_no_value:c}
% \UnitTested
%   Here we test if we found a special quark as the first argument.
%   We better start with \cs{q_no_value} as the first argument since
%   the whole thing may otherwise loop if |#1| is wrongly given
%   a string like |aabc| instead of a single token.\footnote{It may
%   still loop in special circumstances however!}
%    \begin{macrocode}
\prg_new_conditional:Npnn \quark_if_nil:N #1 { p, T , F , TF }
  {
    \if_meaning:w \q_nil #1
      \prg_return_true:
    \else:
      \prg_return_false:
    \fi:
  }
\prg_new_conditional:Npnn \quark_if_no_value:N #1 { p, T , F , TF }
  {
    \if_meaning:w \q_no_value #1
      \prg_return_true:
    \else:
      \prg_return_false:
    \fi:
  }
\prg_generate_conditional_variant:Nnn \quark_if_no_value:N
  { c } { p , T , F , TF }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[pTF]{\quark_if_nil:n, \quark_if_nil:V, \quark_if_nil:o}
% \UnitTested
% \begin{macro}[pTF]{\quark_if_no_value:n}
% \UnitTested
% \begin{macro}{\@@_if_nil:w, \@@_if_no_value:w}
% \begin{macro}[EXP]{\@@_if_empty_if:o}
%   Let us explain \cs{quark_if_nil:nTF}.  Expanding \cs{@@_if_nil:w}
%   once is safe thanks to the trailing \cs{q_nil} |??!|.  The result of
%   expanding once is empty if and only if both delimited arguments |#1|
%   and~|#2| are empty and |#3|~is delimited by the last tokens~|?!|.
%   Thanks to the leading~|{}|, the argument~|#1| is empty if and only
%   if the argument of \cs{quark_if_nil:n} starts with \cs{q_nil}.  The
%   argument~|#2| is empty if and only if this \cs{q_nil} is followed
%   immediately by~|?| or by~|{}?|, coming either from the trailing
%   tokens in the definition of \cs{quark_if_nil:n}, or from its
%   argument.  In the first case, \cs{@@_if_nil:w} is followed by
%   |{}\q_nil| |{}?| |!\q_nil|~|??!|, hence |#3|~is delimited by the
%   final~|?!|, and the test returns \texttt{true} as wanted.  In the
%   second case, the result is not empty since the first~|?!| in the
%   definition of \cs{quark_if_nil:n} stop~|#3|. The auxiliary here
%   is the same as \cs{__tl_if_empty_if:o}, with the same comments
%   applying.
%    \begin{macrocode}
\prg_new_conditional:Npnn \quark_if_nil:n #1 { p, T , F , TF }
  {
    \@@_if_empty_if:o
      { \@@_if_nil:w {} #1 {} ? ! \q_nil ? ? ! }
      \prg_return_true:
    \else:
      \prg_return_false:
    \fi:
  }
\cs_new:Npn \@@_if_nil:w #1 \q_nil #2 ? #3 ? ! { #1 #2 }
\prg_new_conditional:Npnn \quark_if_no_value:n #1 { p, T , F , TF }
  {
    \@@_if_empty_if:o
      { \@@_if_no_value:w {} #1 {} ? ! \q_no_value ? ? ! }
      \prg_return_true:
    \else:
      \prg_return_false:
    \fi:
  }
\cs_new:Npn \@@_if_no_value:w #1 \q_no_value #2 ? #3 ? ! { #1 #2 }
\prg_generate_conditional_variant:Nnn \quark_if_nil:n
  { V , o } { p , TF , T , F }
\cs_new:Npn \@@_if_empty_if:o #1
  {
    \exp_after:wN \if_meaning:w \exp_after:wN \q_nil
      \__kernel_tl_to_str:w \exp_after:wN {#1} \q_nil
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\__kernel_quark_new_test:N}
%   The function \cs{__kernel_quark_new_test:N} defines |#1| in a
%   similar way as \cs[no-index]{quark_if_recursion_tail_...} functions
%   (as described below), using
%   \cs[no-index]{q__\meta{namespace}_recursion_tail} as the test quark
%   and \cs[no-index]{q__\meta{namespace}_recursion_stop} as the
%   delimiter quark, where the \meta{namespace} is determined as the
%   first |_|-delimited part in~|#1|.
%
%   There are six possible function types which this function can define,
%   and which is defined depends on the signature of the function being
%   defined:
%   \begin{description}
%     \def\makelabel#1{\texttt{:#1}~}
%     \item[n]  gives an analogue of \cs{quark_if_recursion_tail_stop:n}
%     \item[nn] gives an analogue of \cs{quark_if_recursion_tail_stop_do:nn}
%     \item[nN] gives an analogue of \cs{quark_if_recursion_tail_break:nN}
%     \item[N]  gives an analogue of \cs{quark_if_recursion_tail_stop:N}
%     \item[Nn] gives an analogue of \cs{quark_if_recursion_tail_stop_do:Nn}
%     \item[NN] gives an analogue of \cs{quark_if_recursion_tail_break:NN}
%   \end{description}
%   Any other signature causes an error, as does a function without signature.
%
% \begin{macro}{\__kernel_quark_new_conditional:Nn}
%   Similar to \cs{__kernel_quark_new_test:N}, but defines quark
%   branching conditionals like \cs{quark_if_nil:nTF} that test for the
%   quark \cs[no-index]{q__\meta{namespace}_\meta{name}}.
%   The \meta{namespace} and \meta{name} are determined from the
%   conditional~|#1|, which must take the rather rigid form
%   \cs[no-index]{__\meta{namespace}_quark_if_\meta{name}:\meta{arg spec}}.
%   There are only two cases for the \meta{arg spec} here:
%   \begin{description}
%     \def\makelabel#1{\texttt{:#1}~}
%     \item[n] gives an analogue of \cs{quark_if_nil:nTF}
%     \item[N] gives an analogue of \cs{quark_if_nil:NTF}
%   \end{description}
%   Any other signature causes an error, as does a function without signature.
%   We use low-level emptiness tests as \pkg{l3tl} is not available yet when these
%   functions are used; thankfully we only care about whether strings
%   are empty so a simple \cs{if_meaning:w} \cs{q_nil} \meta{string}
%   \cs{q_nil} suffices.
%
% \begin{macro}{\@@_new_test:NNNn, \@@_new_test:Nccn,
%   \@@_new_test_aux:nnNNnnnn}
% \begin{macro}{\@@_new_conditional:Nnnn, \@@_new_conditional:Nxxn}
%    \begin{macrocode}
\cs_new_protected:Npn \__kernel_quark_new_test:N #1
  { \@@_new_test_aux:Nx #1 { \@@_module_name:N #1 } }
\cs_new_protected:Npn \@@_new_test_aux:Nn #1 #2
  {
    \if_meaning:w \q_nil #2 \q_nil
      \__kernel_msg_error:nnx { quark } { invalid-function }
        { \token_to_str:N #1 }
    \else:
      \@@_new_test:Nccn #1
        { q__#2_recursion_tail } { q__#2_recursion_stop } { __#2 }
    \fi:
  }
\cs_generate_variant:Nn \@@_new_test_aux:Nn { Nx }
\cs_new_protected:Npn \@@_new_test:NNNn #1
  {
    \exp_last_unbraced:Nf \@@_new_test_aux:nnNNnnnn
      { \cs_split_function:N #1 }
      #1 { test }
  }
\cs_generate_variant:Nn \@@_new_test:NNNn { Ncc }
\cs_new_protected:Npn \__kernel_quark_new_conditional:Nn #1
  {
    \@@_new_conditional:Nxxn #1
      { \@@_quark_conditional_name:N #1 }
      { \@@_module_name:N #1 }
  }
\cs_new_protected:Npn \@@_new_conditional:Nnnn #1#2#3#4
  {
    \if_meaning:w \q_nil #2 \q_nil
      \__kernel_msg_error:nnx { quark } { invalid-function }
        { \token_to_str:N #1 }
    \else:
      \if_meaning:w \q_nil #3 \q_nil
        \__kernel_msg_error:nnx { quark } { invalid-function }
          { \token_to_str:N #1 }
      \else:
        \exp_last_unbraced:Nf \@@_new_test_aux:nnNNnnnn
          { \cs_split_function:N #1 }
          #1 { conditional }
          {#2} {#3} {#4}
      \fi:
    \fi:
  }
\cs_generate_variant:Nn \@@_new_conditional:Nnnn { Nxx }
\cs_new_protected:Npn \@@_new_test_aux:nnNNnnnn #1 #2 #3 #4 #5
  {
    \cs_if_exist_use:cTF { @@_new_#5_#2:Nnnn } { #4 }
      {
        \__kernel_msg_error:nnxx { quark } { invalid-function }
          { \token_to_str:N #4 } {#2}
        \use_none:nnn
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{
%     \@@_new_test_n:Nnnn, \@@_new_test_nn:Nnnn,
%     \@@_new_test_N:Nnnn, \@@_new_test_Nn:Nnnn,
%     \@@_new_test_NN:Nnnn, \@@_new_test_NN:Nnnn,
%   }
%   These macros implement the six possibilities mentioned above, passing
%   the right arguments to \cs{@@_new_test_aux_do:nNNnnnnNNn},
%   which defines some auxiliaries, and then to
%   \cs{@@_new_test_define_tl:nNnNNn} (|:n(n)| variants) or to
%   \cs{@@_new_test_define_ifx:nNnNNn} (|:N(n)|) which define the
%   main conditionals.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_new_test_n:Nnnn #1 #2 #3 #4
  {
    \@@_new_test_aux_do:nNNnnnnNNn {#4} #2 #3 { none } { } { } { }
      \@@_new_test_define_tl:nNnNNn #1 { }
  }
\cs_new_protected:Npn \@@_new_test_nn:Nnnn #1 #2 #3 #4
  {
    \@@_new_test_aux_do:nNNnnnnNNn {#4} #2 #3 { i } { n } {##1} {##2}
      \@@_new_test_define_tl:nNnNNn #1 { \use_none:n }
  }
\cs_new_protected:Npn \@@_new_test_nN:Nnnn #1 #2 #3 #4
  {
    \@@_new_test_aux_do:nNNnnnnNNn {#4} #2 #3 { i } { n } {##1} {##2}
      \@@_new_test_define_break_tl:nNNNNn #1 { }
  }
\cs_new_protected:Npn \@@_new_test_N:Nnnn #1 #2 #3 #4
  {
    \@@_new_test_aux_do:nNNnnnnNNn {#4} #2 #3 { none } { } { } { }
      \@@_new_test_define_ifx:nNnNNn #1 { }
  }
\cs_new_protected:Npn \@@_new_test_Nn:Nnnn #1 #2 #3 #4
  {
    \@@_new_test_aux_do:nNNnnnnNNn {#4} #2 #3 { i } { n } {##1} {##2}
      \@@_new_test_define_ifx:nNnNNn #1
      { \else: \exp_after:wN \use_none:n }
  }
\cs_new_protected:Npn \@@_new_test_NN:Nnnn #1 #2 #3 #4
  {
    \@@_new_test_aux_do:nNNnnnnNNn {#4} #2 #3 { i } { n } {##1} {##2}
      \@@_new_test_define_break_ifx:nNNNNn #1 { }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{
%     \@@_new_test_aux_do:nNNnnnnNNn,
%     \@@_test_define_aux:NNNNnnNNn
%   }
%   \cs{@@_new_test_aux_do:nNNnnnnNNn} makes the control sequence names
%   which will be used by \cs{@@_test_define_aux:NNNNnnNNn}, and then later
%   by \cs{@@_new_test_define_tl:nNnNNn} or
%   \cs{@@_new_test_define_ifx:nNnNNn}. The control sequences defined
%   here are analogous to \cs{@@_if_recursion_tail:w} and to
%   \cs[no-index]{use_(none|i)_delimit_by_q_recursion_stop:(|n)w}.
%
%   The name is composed by the name-space and the name of the quarks.
%   Suppose \cs{__kernel_quark_new_test:N} was used with:
%   \begin{verbatim}
%     \__kernel_quark_new_test:N \__test_quark_tail:n
%   \end{verbatim}
%   then the first auxiliary will be \cs[no-index]{__test_quark_recursion_tail:w},
%   and the second one will be
%   \cs[no-index]{__test_use_none_delimit_by_q_recursion_stop:w}.
%
%   Note that the actual quarks are \emph{not} defined here. They should
%   be defined separately using \cs{quark_new:N}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_new_test_aux_do:nNNnnnnNNn #1 #2 #3 #4 #5
  {
    \exp_args:Ncc \@@_test_define_aux:NNNNnnNNn
      { #1 _quark_recursion_tail:w }
      { #1 _use_ #4 _delimit_by_q_recursion_stop: #5 w }
      #2 #3
  }
\cs_new_protected:Npn \@@_test_define_aux:NNNNnnNNn #1 #2 #3 #4 #5 #6 #7
  {
    \cs_gset:Npn #1  ##1 #3 ##2 ? ##3 ?! { ##1 ##2 }
    \cs_gset:Npn #2  ##1 #6 #4 {#5}
    #7 {##1} #1 #2 #3
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{
%     \@@_new_test_define_tl:nNnNNn,
%     \@@_new_test_define_ifx:nNnNNn
%   }
% \begin{macro}{
%     \@@_new_test_define_break_tl:nNNNNn,
%     \@@_new_test_define_break_ifx:nNNNNn
%   }
%   Finally, these two macros define the main conditional function using
%   what's been set up before.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_new_test_define_tl:nNnNNn #1 #2 #3 #4 #5 #6
  {
    \cs_new:Npn #5 #1
      {
        \tl_if_empty:oTF
          { #2 {} ##1 {} ?! #4 ??! }
          {#3} {#6}
      }
  }
\cs_new_protected:Npn \@@_new_test_define_ifx:nNnNNn #1 #2 #3 #4 #5 #6
  {
    \cs_new:Npn #5 #1
      {
        \if_meaning:w #4 ##1
          \exp_after:wN #3
          #6
        \fi:
      }
  }
\cs_new_protected:Npn \@@_new_test_define_break_tl:nNNNNn #1 #2 #3
  { \@@_new_test_define_tl:nNnNNn {##1##2} #2 {##2} }
\cs_new_protected:Npn \@@_new_test_define_break_ifx:nNNNNn #1 #2 #3
  { \@@_new_test_define_ifx:nNnNNn {##1##2} #2 {##2} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{
%     \@@_new_conditional_n:Nnnn,
%     \@@_new_conditional_N:Nnnn,
%   }
%   These macros implement the two possibilities for branching quark
%   conditionals, passing
%   the right arguments to \cs{@@_new_conditional_aux_do:NNnnn},
%   which defines some auxiliaries and defines the
%   main conditionals.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_new_conditional_n:Nnnn
  { \@@_new_conditional_aux_do:NNnnn \use_i:nn }
\cs_new_protected:Npn \@@_new_conditional_N:Nnnn
  { \@@_new_conditional_aux_do:NNnnn \use_ii:nn }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{
%     \@@_new_conditional_aux_do:NNnnn,
%     \@@_new_conditional_define:NNNNn,
%   }
%   Similar to the previous macros, but branching conditionals only
%   require one auxiliary, so we take a shortcut.  In
%   \cs{@@_new_conditional_define:NNNNn}, |#4| is \cs{use_i:nn} to
%   define the \texttt{n}-type function (which needs an auxiliary) and
%   is \cs{use_ii:nn} to define the \texttt{N}-type function.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_new_conditional_aux_do:NNnnn #1 #2 #3 #4
  {
    \exp_args:Ncc \@@_new_conditional_define:NNNNn
      { __ #4 _if_quark_ #3 :w } { q__ #4 _ #3 } #2 #1
  }
\cs_new_protected:Npn \@@_new_conditional_define:NNNNn #1 #2 #3 #4 #5
  {
    #4 { \cs_gset:Npn #1  ##1 #2 ##2 ? ##3 ?! { ##1 ##2 } } { }
    \exp_args:Nno \use:n { \prg_new_conditional:Npnn #3 ##1 {#5} }
      {
        #4 { \@@_if_empty_if:o { #1 {} ##1 {} ?! #2 ??! } }
           { \if_meaning:w #2 ##1 }
        \prg_return_true: \else: \prg_return_false: \fi:
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_module_name:N}
% \begin{macro}[EXP]{
%     \@@_module_name:w,
%     \@@_module_name_loop:w,
%     \@@_module_name_end:w
%   }
%   \cs{@@_module_name:N} takes a control sequence and returns its
%   \meta{module} name, determined as the first non-empty
%   non-single-character word, separated by |_| or~|:|.  These rules
%   give the correct result for public functions
%   \cs[no-index]{\meta{module}_\ldots{}}, private functions
%   \cs[no-index]{__\meta{module}_\ldots{}}, and variables such as
%   \cs[no-index]{l_\meta{module}_\ldots{}}.  If no valid module is
%   found the result is an empty string.  The approach is to first cut
%   off everything after the (first) |:| if any is present, then
%   repeatedly grab |_|-delimited words until finding one of length at
%   least~$2$ (we use low-level tests as \pkg{l3tl} is not fully
%   available when \cs{__kernel_quark_new_test:N} is first used.
%   If no \meta{module} is found (such as in \cs{::n}) we
%   get the trailing marker \cs{use_none:n} |{}|, which expands to
%   nothing.
%    \begin{macrocode}
\cs_set:Npn \@@_tmp:w #1#2
  {
    \cs_new:Npn \@@_module_name:N ##1
      {
        \exp_last_unbraced:Nf \@@_module_name:w
          { \cs_to_str:N ##1 } #1 \s_@@
      }
    \cs_new:Npn \@@_module_name:w ##1 #1 ##2 \s_@@
      { \@@_module_name_loop:w ##1 #2 \use_none:n { } #2 \s_@@ }
    \cs_new:Npn \@@_module_name_loop:w ##1 #2
      {
        \use_i_ii:nnn \if_meaning:w \prg_do_nothing:
            ##1 \prg_do_nothing: \prg_do_nothing:
          \exp_after:wN \@@_module_name_loop:w
        \else:
          \@@_module_name_end:w ##1
        \fi:
      }
    \cs_new:Npn \@@_module_name_end:w
      ##1 \fi: ##2 \s_@@ { \fi: ##1 }
  }
\exp_after:wN \@@_tmp:w \tl_to_str:n { : _ }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_quark_conditional_name:N, \@@_quark_conditional_name:w}
%   \cs{@@_quark_conditional_name:N} determines the quark name that the quark
%   conditional function~|##1| queries, as the part of the function name
%   between |_quark_if_| and the trailing~|:|.  Again we define it
%   through \cs{@@_tmp:w}, which receives |:| as |#1| and |_quark_if_|
%   as |#2|.  The auxiliary \cs{@@_quark_conditional_name:w} returns the part
%   between the first |_quark_if_| and the next~|:|, and we apply this
%   auxiliary to the function name followed by |:| (in case the function
%   name is lacking a signature), and |_quark_if_:| so that
%   \cs{@@_quark_conditional_name:N} returns an empty string if |_quark_if_| is
%   not present.
%    \begin{macrocode}
\cs_set:Npn \@@_tmp:w #1 #2 \s_@@
  {
    \cs_new:Npn \@@_quark_conditional_name:N ##1
      {
        \exp_last_unbraced:Nf \@@_quark_conditional_name:w
          { \cs_to_str:N ##1 } #1 #2 #1 \s_@@
      }
    \cs_new:Npn \@@_quark_conditional_name:w
      ##1 #2 ##2 #1 ##3 \s_@@ {##2}
  }
\exp_after:wN \@@_tmp:w \tl_to_str:n { : _quark_if_ } \s_@@
%    \end{macrocode}
% \end{macro}
%
% \subsection{Scan marks}
%
%    \begin{macrocode}
%<@@=scan>
%    \end{macrocode}
%
% \begin{variable}{\g_@@_marks_tl}
% \UnitTested
%   The list of all scan marks currently declared.
%   No \pkg{l3tl} yet, so define this by hand.
%    \begin{macrocode}
\cs_gset:Npn \g_@@_marks_tl { }
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\scan_new:N}
% \UnitTested
%   Check whether the variable is already a scan mark,
%   then declare it to be equal to \cs{scan_stop:} globally.
%    \begin{macrocode}
\cs_new_protected:Npn \scan_new:N #1
  {
    \tl_if_in:NnTF \g_@@_marks_tl { #1 }
      {
        \__kernel_msg_error:nnx { scanmark } { already-defined }
          { \token_to_str:N #1 }
      }
      {
        \tl_gput_right:Nn \g_@@_marks_tl {#1}
        \cs_new_eq:NN #1 \scan_stop:
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\s_stop}
% \UnitTested
%   We only declare one scan mark here, more can be defined
%   by specific modules.
%   Can't use \cs{scan_new:N} yet because \pkg{l3tl} isn't loaded,
%   so define \cs{s_stop} by hand and add it to \cs{g_@@_marks_tl}.
%   We also add \cs{s__quark} (declared earlier) to the pool here.
%   Since it lives in a different namespace, a little \pkg{l3docstrip}
%   cheating is necessary.
%    \begin{macrocode}
\cs_new_eq:NN \s_stop \scan_stop:
\cs_gset_nopar:Npx \g_@@_marks_tl
  {
    \exp_not:o \g_@@_marks_tl
    \s_stop
%<@@=quark>
    \s_@@
%<@@=scan>
  }
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}[EXP]{\use_none_delimit_by_s_stop:w}
% \UnitTested
%   Similar to \cs{use_none_delimit_by_q_stop:w}.
%    \begin{macrocode}
\cs_new:Npn \use_none_delimit_by_s_stop:w #1 \s_stop { }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%</package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex