### Annotation of gforth/gforth.ds, revision 1.2

1.1       anton       1: \input texinfo   @c -*-texinfo-*-
2: @comment The source is gforth.ds, from which gforth.texi is generated
3: @comment %**start of header (This is for running Texinfo on a region.)
4: @setfilename gforth-info
5: @settitle GNU Forth Manual
6: @setchapternewpage odd
7: @comment %**end of header (This is for running Texinfo on a region.)
8:
9: @ifinfo
10: This file documents GNU Forth 0.0
11:
13:
14:      Permission is granted to make and distribute verbatim copies of
15:      this manual provided the copyright notice and this permission notice
16:      are preserved on all copies.
17:
18:      @ignore
19:      Permission is granted to process this file through TeX and print the
20:      results, provided the printed document carries a copying permission
21:      notice identical to this one except for the removal of this paragraph
22:      (this paragraph not being relevant to the printed manual).
23:
24:      @end ignore
25:      Permission is granted to copy and distribute modified versions of this
26:      manual under the conditions for verbatim copying, provided also that the
27:      sections entitled "Distribution" and "General Public License" are
28:      included exactly as in the original, and provided that the entire
29:      resulting derived work is distributed under the terms of a permission
30:      notice identical to this one.
31:
32:      Permission is granted to copy and distribute translations of this manual
33:      into another language, under the above conditions for modified versions,
34:      except that the sections entitled "Distribution" and "General Public
35:      License" may be included in a translation approved by the author instead
36:      of in the original English.
37: @end ifinfo
38:
39: @titlepage
40: @sp 10
41: @center @titlefont{GNU Forth Manual}
42: @sp 2
43: @center for version 0.0
44: @sp 2
45: @center Anton Ertl
46:
47: @comment  The following two commands start the copyright page.
48: @page
49: @vskip 0pt plus 1filll
51:
52: @comment !! Published by ... or You can get a copy of this manual ...
53:
54:      Permission is granted to make and distribute verbatim copies of
55:      this manual provided the copyright notice and this permission notice
56:      are preserved on all copies.
57:
58:      Permission is granted to copy and distribute modified versions of this
59:      manual under the conditions for verbatim copying, provided also that the
60:      sections entitled "Distribution" and "General Public License" are
61:      included exactly as in the original, and provided that the entire
62:      resulting derived work is distributed under the terms of a permission
63:      notice identical to this one.
64:
65:      Permission is granted to copy and distribute translations of this manual
66:      into another language, under the above conditions for modified versions,
67:      except that the sections entitled "Distribution" and "General Public
68:      License" may be included in a translation approved by the author instead
69:      of in the original English.
70: @end titlepage
71:
72:
73: @node Top, License, (dir), (dir)
74: @ifinfo
75: GNU Forth is a free implementation of ANS Forth available on many
76: personal machines. This manual corresponds to version 0.0.
77: @end ifinfo
78:
81: * Goals::               About the GNU Forth Project
82: * Other Books::         Things you might want to read
83: * Invocation::          Starting GNU Forth
84: * Words::               Forth words available in GNU Forth
85: * ANS conformance::     Implementation-defined options etc.
86: * Model::               The abstract machine of GNU Forth
87: * Emacs and GForth::    The GForth Mode
88: * Internals::           Implementation details
89: * Bugs::                How to report them
90: * Pedigree::            Ancestors of GNU Forth
91: * Word Index::          An item for each Forth word
92: * Node Index::          An item for each node
94:
95: @node License, Goals, Top, Top
97: !! Insert GPL here
98:
99: @iftex
100: @unnumbered Preface
101: This manual documents GNU Forth. The reader is expected to know
102: Forth. This manual is primarily a reference manual. @xref{Other Books}
103: for introductory material.
104: @end iftex
105:
106: @node    Goals, Other Books, License, Top
107: @comment node-name,     next,           previous, up
108: @chapter Goals of GNU Forth
109: @cindex Goals
110: The goal of the GNU Forth Project is to develop a standard model for
111: ANSI Forth. This can be split into several subgoals:
112:
113: @itemize @bullet
114: @item
115: GNU Forth should conform to the ANSI Forth standard.
116: @item
117: It should be a model, i.e. it should define all the
118: implementation-dependent things.
119: @item
120: It should become standard, i.e. widely accepted and used. This goal
121: is the most difficult one.
122: @end itemize
123:
124: To achieve these goals GNU Forth should be
125: @itemize @bullet
126: @item
127: Similar to previous models (fig-Forth, F83)
128: @item
129: Powerful. It should provide for all the things that are considered
130: necessary today and even some that are not yet considered necessary.
131: @item
132: Efficient. It should not get the reputation of being exceptionally
133: slow.
134: @item
135: Free.
136: @item
137: Available on many machines/easy to port.
138: @end itemize
139:
140: Have we achieved these goals? GNU Forth conforms to the ANS Forth
141: standard; it may be considered a model, but we have not yet documented
142: which parts of the model are stable and which parts we are likely to
143: change; it certainly has not yet become a de facto standard. It has some
144: similarities and some differences to previous models; It has some
145: powerful features, but not yet everything that we envisioned; on RISCs
146: it is as fast as interpreters programmed in assembly, on
147: register-starved machines it is not so fast, but still faster than any
148: other C-based interpretive implementation; it is free and available on
149: many machines.
150:
151: @node Other Books, Invocation, Goals, Top
152: @chapter Other books on ANS Forth
153:
154: As the standard is relatively new, there are not many books out yet. It
155: is not recommended to learn Forth by using GNU Forth and a book that is
156: not written for ANS Forth, as you will not know your mistakes from the
157: deviations of the book.
158:
159: There is, of course, the standard, the definite reference if you want to
160: write ANS Forth programs. It will be available in printed form from
161: Global Engineering Documents !! somtime in spring or summer 1994. If you
162: are lucky, you can still get dpANS6 (the draft that was approved as
163: standard) by aftp from ftp.uu.net:/vendor/minerva/x3j14.
164:
165: @cite{Forth: The new model} by Jack Woehr (!! Publisher) is an
166: introductory book based on a draft version of the standard. It does not
167: cover the whole standard. It also contains interesting background
168: information (Jack Woehr was in the ANS Forth Technical Committe). It is
169: not appropriate for complete newbies, but programmers experienced in
170: other languages should find it ok.
171:
172: @node Invocation, Words, Other Books, Top
173: @chapter Invocation
174:
175: You will usually just say @code{gforth}. In many other cases the default
176: GNU Forth image will be invoked like this:
177:
178: @example
179: gforth [files] [-e forth-code]
180: @end example
181:
182: executing the contents of the files and the Forth code in the order they
183: are given.
184:
185: In general, the command line looks like this:
186:
187: @example
188: gforth [initialization options] [image-specific options]
189: @end example
190:
191: The initialization options must come before the rest of the command
192: line. They are:
193:
194: @table @code
195: @item --image-file @var{file}
197: @file{gforth.fi}.
198:
199: @item --path @var{path}
200: Uses @var{path} for searching the image file and Forth source code
201: files instead of the default in the environment variable
202: @code{GFORTHPATH} or the path specified at installation time (typically
203: @file{/usr/local/lib/gforth:.}). A path is given as a @code{:}-separated
204: list.
205:
206: @item --dictionary-size @var{size}
207: @item -m @var{size}
208: Allocate @var{size} space for the Forth dictionary space instead of
209: using the default specified in the image (typically 256K). The
210: @var{size} specification consists of an integer and a unit (e.g.,
211: @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
212: size, in this case Cells), @code{k} (kilobytes), and @code{M}
213: (Megabytes). If no unit is specified, @code{e} is used.
214:
215: @item --data-stack-size @var{size}
216: @item -d @var{size}
217: Allocate @var{size} space for the data stack instead of using the
218: default specified in the image (typically 16K).
219:
220: @item --return-stack-size @var{size}
221: @item -r @var{size}
222: Allocate @var{size} space for the return stack instead of using the
223: default specified in the image (typically 16K).
224:
225: @item --fp-stack-size @var{size}
226: @item -f @var{size}
227: Allocate @var{size} space for the floating point stack instead of
228: using the default specified in the image (typically 16K). In this case
229: the unit specifier @code{e} refers to floating point numbers.
230:
231: @item --locals-stack-size @var{size}
232: @item -l @var{size}
233: Allocate @var{size} space for the locals stack instead of using the
234: default specified in the image (typically 16K).
235:
236: @end table
237:
238: As explained above, the image-specific command-line arguments for the
239: default image @file{gforth.fi} consist of a sequence of filenames and
240: @code{-e @var{forth-code}} options that are interpreted in the seqence
241: in which they are given. The @code{-e @var{forth-code}} or
242: @code{--evaluate @var{forth-code}} option evaluates the forth
243: code. This option takes only one argument; if you want to evaluate more
244: Forth words, you have to quote them or use several @code{-e}s. To exit
245: after processing the command line (instead of entering interactive mode)
246: append @code{-e bye} to the command line.
247:
248: Not yet implemented:
249: On startup the system first executes the system initialization file
250: (unless the option @code{--no-init-file} is given; note that the system
251: resulting from using this option may not be ANS Forth conformant). Then
252: the user initialization file @file{.gforth.fs} is executed, unless the
253: option @code{--no-rc} is given; this file is first searched in @file{.},
254: then in @file{~}, then in the normal path (see above).
255:
256: @node Words,  , Invocation, Top
257: @chapter Forth Words
258:
260: * Notation::
261: * Arithmetic::
262: * Stack Manipulation::
263: * Memory access::
264: * Control Structures::
265: * Local Variables::
266: * Defining Words::
267: * Vocabularies::
268: * Files::
269: * Blocks::
270: * Other I/O::
271: * Programming Tools::
273:
274: @node Notation, Arithmetic, Words, Words
275: @section Notation
276:
277: The Forth words are described in this section in the glossary notation
278: that has become a de-facto standard for Forth texts, i.e.
279:
280: @quotation
281: @var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}
282: @var{Description}
283: @end quotation
284:
285: @table @var
286: @item word
287: The name of the word. BTW, GNU Forth is case insensitive, so you can
288: type the words in in lower case.
289:
290: @item Stack effect
291: The stack effect is written in the notation @code{@var{before} --
292: @var{after}}, where @var{before} and @var{after} describe the top of
293: stack entries before and after the execution of the word. The rest of
294: the stack is not touched by the word. The top of stack is rightmost,
295: i.e., a stack sequence is written as it is typed in. Note that GNU Forth
296: uses a separate floating point stack, but a unified stack
297: notation. Also, return stack effects are not shown in @var{stack
298: effect}, but in @var{Description}. The name of a stack item describes
299: the type and/or the function of the item. See below for a discussion of
300: the types.
301:
302: @item pronunciation
303: How the word is pronounced
304:
305: @item wordset
306: The ANS Forth standard is divided into several wordsets. A standard
307: system need not support all of them. So, the fewer wordsets your program
308: uses the more portable it will be in theory. However, we suspect that
309: most ANS Forth systems on personal machines will feature all
310: wordsets. Words that are not defined in the ANS standard have
311: @code{gforth} as wordset.
312:
313: @item Description
314: A description of the behaviour of the word.
315: @end table
316:
317: The name of a stack item corresponds in the following way with its type:
318:
319: @table @code
320: @item name starts with
321: Type
322: @item f
323: Bool, i.e. @code{false} or @code{true}.
324: @item c
325: Char
326: @item w
327: Cell, can contain an integer or an address
328: @item n
329: signed integer
330: @item u
331: unsigned integer
332: @item d
333: double sized signed integer
334: @item ud
335: double sized unsigned integer
336: @item r
337: Float
338: @item a_
340: @item c_
341: Char-aligned address (note that a Char is two bytes in Windows NT)
342: @item f_
344: @item df_
345: Address aligned for IEEE double precision float
346: @item sf_
347: Address aligned for IEEE single precision float
348: @item xt
349: Execution token, same size as Cell
350: @item wid
351: Wordlist ID, same size as Cell
352: @item f83name
353: Pointer to a name structure
354: @end table
355:
356: @node Arithmetic,  , Notation, Words
357: @section Arithmetic
358: Forth arithmetic is not checked, i.e., you will not hear about integer
359: overflow on addition or multiplication, you may hear about division by
360: zero if you are lucky. The operator is written after the operands, but
361: the operands are still in the original order. I.e., the infix @code{2-1}
362: corresponds to @code{2 1 -}. Forth offers a variety of division
363: operators. If you perform division with potentially negative operands,
364: you do not want to use @code{/} or @code{/mod} with its undefined
365: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
366: former).
367:
368: @subsection Single precision
369: doc-+
370: doc--
371: doc-*
372: doc-/
373: doc-mod
374: doc-/mod
375: doc-negate
376: doc-abs
377: doc-min
378: doc-max
379:
380: @subsection Bitwise operations
381: doc-and
382: doc-or
383: doc-xor
384: doc-invert
385: doc-2*
386: doc-2/
387:
388: @subsection Mixed precision
389: doc-m+
390: doc-*/
391: doc-*/mod
392: doc-m*
393: doc-um*
394: doc-m*/
395: doc-um/mod
396: doc-fm/mod
397: doc-sm/rem
398:
399: @subsection Double precision
400: doc-d+
401: doc-d-
402: doc-dnegate
403: doc-dabs
404: doc-dmin
405: doc-dmax
406:
407: @node Stack Manipulation,,,
408: @section Stack Manipulation
409:
410: gforth has a data stack (aka parameter stack) for characters, cells,
411: addresses, and double cells, a floating point stack for floating point
412: numbers, a return stack for storing the return addresses of colon
413: definitions and other data, and a locals stack for storing local
414: variables. Note that while every sane Forth has a separate floating
415: point stack, this is not strictly required; an ANS Forth system could
416: theoretically keep floating point numbers on the data stack. As an
417: additional difficulty, you don't know how many cells a floating point
418: number takes. It is reportedly possible to write words in a way that
419: they work also for a unified stack model, but we do not recommend trying
420: it. Also, a Forth system is allowed to keep the local variables on the
421: return stack. This is reasonable, as local variables usually eliminate
422: the need to use the return stack explicitly. So, if you want to produce
423: a standard complying program and if you are using local variables in a
424: word, forget about return stack manipulations in that word (see the
425: standard document for the exact rules).
426:
427: @subsection Data stack
428: doc-drop
429: doc-nip
430: doc-dup
431: doc-over
432: doc-tuck
433: doc-swap
434: doc-rot
435: doc--rot
436: doc-?dup
437: doc-pick
438: doc-roll
439: doc-2drop
440: doc-2nip
441: doc-2dup
442: doc-2over
443: doc-2tuck
444: doc-2swap
445: doc-2rot
446:
447: @subsection Floating point stack
448: doc-fdrop
449: doc-fnip
450: doc-fdup
451: doc-fover
452: doc-ftuck
453: doc-fswap
454: doc-frot
455:
456: @subsection Return stack
457: doc->r
458: doc-r>
459: doc-r@
460: doc-rdrop
461: doc-2>r
462: doc-2r>
463: doc-2r@
464: doc-2rdrop
465:
466: @subsection Locals stack
467:
468: @subsection Stack pointer manipulation
469: doc-sp@
470: doc-sp!
471: doc-fp@
472: doc-fp!
473: doc-rp@
474: doc-rp!
475: doc-lp@
476: doc-lp!
477:
478: @node Memory access
479: @section Memory access
480:
481: @subsection Stack-Memory transfers
482:
483: doc-@
484: doc-!
485: doc-+!
486: doc-c@
487: doc-c!
488: doc-2@
489: doc-2!
490: doc-f@
491: doc-f!
492: doc-sf@
493: doc-sf!
494: doc-df@
495: doc-df!
496:
498:
499: ANS Forth does not specify the sizes of the data types. Instead, it
500: offers a number of words for computing sizes and doing address
501: arithmetic. Basically, address arithmetic is performed in terms of
502: address units (aus); on most systems the address unit is one byte. Note
503: that a character may have more than one au, so @code{chars} is no noop
504: (on systems where it is a noop, it compiles to nothing).
505:
506: ANS Forth also defines words for aligning addresses for specific
507: addresses. Many computers require that accesses to specific data types
508: must only occur at specific addresses; e.g., that cells may only be
509: accessed at addresses divisible by 4. Even if a machine allows unaligned
510: accesses, it can usually perform aligned accesses faster.
511:
512: For the performance-concious: alignment operations are usually only
513: necessary during the definition of a data structure, not during the
514: (more frequent) accesses to it.
515:
516: ANS Forth defines no words for character-aligning addresses. This is not
517: an oversight, but reflects the fact that addresses that are not
518: char-aligned have no use in the standard and therefore will not be
519: created.
520:
521: The standard guarantees that addresses returned by @code{CREATE}d words
523: are aligned for all purposes.
524:
525: doc-chars
526: doc-char+
527: doc-cells
528: doc-cell+
529: doc-align
530: doc-aligned
531: doc-floats
532: doc-float+
533: doc-falign
534: doc-faligned
535: doc-sfloats
536: doc-sfloat+
537: doc-sfalign
538: doc-sfaligned
539: doc-dfloats
540: doc-dfloat+
541: doc-dfalign
542: doc-dfaligned
544:
545: @subsection Memory block access
546:
547: doc-move
548: doc-erase
549:
550: While the previous words work on address units, the rest works on
551: characters.
552:
553: doc-cmove
554: doc-cmove>
555: doc-fill
556: doc-blank
557:
558: @node Control Structures
559: @section Control Structures
560:
561: Control structures in Forth cannot be used in interpret state, only in
562: compile state, i.e., in a colon definition. We do not like this
563: limitation, but have not seen a satisfying way around it yet, although
564: many schemes have been proposed.
565:
566: @subsection Selection
567:
568: @example
569: @var{flag}
570: IF
571:   @var{code}
572: ENDIF
573: @end example
574: or
575: @example
576: @var{flag}
577: IF
578:   @var{code1}
579: ELSE
580:   @var{code2}
581: ENDIF
582: @end example
583:
584: You can use @code{THEN} instead of {ENDIF}. Indeed, @code{THEN} is
585: standard, and @code{ENDIF} is not, although it is quite popular. We
586: recommend using @code{ENDIF}, because it is less confusing for people
587: who also know other languages (and is not prone to reinforcing negative
588: prejudices against Forth in these people). Adding @code{ENDIF} to a
589: system that only supplies @code{THEN} is simple:
590: @example
591: : endif   POSTPONE then ; immediate
592: @end example
593:
594: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
595: (adv.)}  has the following meanings:
596: @quotation
597: ... 2b: following next after in order ... 3d: as a necessary consequence
598: (if you were there, then you saw them).
599: @end quotation
600: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
601: and many other programming languages has the meaning 3d.]
602:
603: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
604: can avoid using @code{?dup}.
605:
606: @example
607: @var{n}
608: CASE
609:   @var{n1} OF @var{code1} ENDOF
610:   @var{n2} OF @var{code2} ENDOF
611:   @dots
612: ENDCASE
613: @end example
614:
615: Executes the first @var{codei}, where the @var{ni} is equal to
616: @var{n}. A default case can be added by simply writing the code after
617: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
618: but must not consume it.
619:
620: @subsection Simple Loops
621:
622: @example
623: BEGIN
624:   @var{code1}
625:   @var{flag}
626: WHILE
627:   @var{code2}
628: REPEAT
629: @end example
630:
631: @var{code1} is executed and @var{flag} is computed. If it is true,
632: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
633:
634: @example
635: BEGIN
636:   @var{code}
637:   @var{flag}
638: UNTIL
639: @end example
640:
641: @var{code} is executed. The loop is restarted if @code{flag} is false.
642:
643: @example
644: BEGIN
645:   @var{code}
646: AGAIN
647: @end example
648:
649: This is an endless loop.
650:
651: @subsection Counted Loops
652:
653: The basic counted loop is:
654: @example
655: @var{limit} @var{start}
656: ?DO
657:   @var{body}
658: LOOP
659: @end example
660:
661: This performs one iteration for every integer, starting from @var{start}
662: and up to, but excluding @var{limit}. The counter, aka index, can be
663: accessed with @code{i}. E.g., the loop
664: @example
665: 10 0 ?DO
666:   i .
667: LOOP
668: @end example
669: prints
670: @example
671: 0 1 2 3 4 5 6 7 8 9
672: @end example
673: The index of the innermost loop can be accessed with @code{i}, the index
674: of the next loop with @code{j}, and the index of the third loop with
675: @code{k}.
676:
677: The loop control data are kept on the return stack, so there are some
678: restrictions on mixing return stack accesses and counted loop
679: words. E.g., if you put values on the return stack outside the loop, you
680: cannot read them inside the loop. If you put values on the return stack
681: within a loop, you have to remove them before the end of the loop and
682: before accessing the index of the loop.
683:
684: There are several variations on the counted loop:
685:
686: @code{LEAVE} leaves the innermost counted loop immediately.
687:
688: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
689: index by @var{n} instead of by 1. The loop is terminated when the border
690: between @var{limit-1} and @var{limit} is crossed. E.g.:
691:
1.2     ! anton     692: @code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}
1.1       anton     693:
1.2     ! anton     694: @code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}
1.1       anton     695:
696: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
697:
1.2     ! anton     698: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
1.1       anton     699:
1.2     ! anton     700: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
1.1       anton     701:
702: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
703: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
704: case behaves symmetrical to the positive case:
705:
1.2     ! anton     706: @code{-2 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
1.1       anton     707:
1.2     ! anton     708: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0}
1.1       anton     709:
1.2     ! anton     710: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
1.1       anton     711:
1.2     ! anton     712: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
1.1       anton     713: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
714: Forth standard.
715:
716: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
717: when the start and the limit value are equal. We do not recommend using
718: @code{DO}. It will just give you maintenance troubles.
719:
720: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
721: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
722: return stack so @code{EXIT} can get to its return address.
723:
724: Another counted loop is
725: @example
726: @var{n}
727: FOR
728:   @var{body}
729: NEXT
730: @end example
731: This is the preferred loop of native code compiler writers who are too
732: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
733: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
734: and ending with 0. Other Forth systems may behave differently, even if
735: they support @code{FOR} loops.
736:
1.2     ! anton     737: @subsection Arbitrary control structures
!           738:
!           739: ANS Forth permits and supports using control structures in a non-nested
!           740: way. Information about incomplete control structures is stored on the
!           741: control-flow stack. This stack may be implemented on the Forth data
!           742: stack, and this is what we have done in gforth.
!           743:
!           744: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
!           745: entry represents a backward branch target. A few words are the basis for
!           746: building any control structure possible (except control structures that
!           747: need storage, like calls, coroutines, and backtracking).
!           748:
!           749: if
!           751: then
!           752: begin
!           753: until
!           754: again
!           755: cs-pick
!           756: cs-roll
!           757:
!           758: On many systems control-flow stack items take one word, in gforth they
!           759: currently take three (this may change in the future). Therefore it is a
!           760: really good idea to manipulate the control flow stack with
!           761: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
!           762: words.
!           763:
!           764: Some standard control structure words are built from these words:
!           765:
!           766: else
!           767: while
!           768: repeat
!           769:
!           770: Counted loop words constitute a separate group of words:
!           771:
!           772: ?do
!           773: do
!           774: for
!           775: loop
!           776: s+loop
!           777: +loop
!           778: next
!           779: leave
!           780: ?leave
!           781: unloop
!           782: undo
!           783:
!           784: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
!           785: @i{do-sys}. Our system allows it, but it's your job to ensure that for
!           786: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
!           787: through the program (@code{LOOP} etc. compile an @code{UNLOOP}). Also,
!           788: you have to ensure that all @code{LEAVE}s are resolved (by using one of
!           789: the loop-ending words or @code{UNDO}).
!           790:
!           791: Another group of control structure words are
!           792:
!           793: case
!           794: endcase
!           795: of
!           796: endof
!           797:
!           798: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
!           799: @code{cs-roll}.
!           800:
1.1       anton     801: @node Locals
802: @section Locals
803:
1.2     ! anton     804: Local variables can make Forth programming more enjoyable and Forth
!           805: programs easier to read. Unfortunately, the locals of ANS Forth are
!           806: laden with restrictions. Therefore, we provide not only the ANS Forth
!           807: locals wordset, but also our own, more powerful locals wordset (we
!           808: implemented the ANS Forth locals wordset through our locals wordset).
!           809:
!           812:
!           813: @subsection gforth locals
!           814:
!           815: Locals can be defined with
!           816:
!           817: @example
!           818: @{ local1 local2 ... -- comment @}
!           819: @end example
!           820: or
!           821: @example
!           822: @{ local1 local2 ... @}
!           823: @end example
!           824:
!           825: E.g.,
!           826: @example
!           827: : max @{ n1 n2 -- n3 @}
!           828:  n1 n2 > if
!           829:    n1
!           830:  else
!           831:    n2
!           832:  endif ;
!           833: @end example
!           834:
!           835: The similarity of locals definitions with stack comments is intended. A
!           836: locals definition often replaces the stack comment of a word. The order
!           837: of the locals corresponds to the order in a stack comment and everything
!           838: after the @code{--} is really a comment.
!           839:
!           840: This similarity has one disadvantage: It is too easy to confuse locals
!           841: declarations with stack comments, causing bugs and making them hard to
!           842: find. However, this problem can be avoided by appropriate coding
!           843: conventions: Do not use both notations in the same program. If you do,
!           844: they should be distinguished using additional means, e.g. by position.
!           845:
!           846: The name of the local may be preceded by a type specifier, e.g.,
!           847: @code{F:} for a floating point value:
!           848:
!           849: @example
!           850: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
!           851: \ complex multiplication
!           852:  Ar Br f* Ai Bi f* f-
!           853:  Ar Bi f* Ai Br f* f+ ;
!           854: @end example
!           855:
!           856: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
!           857: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
!           858: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
!           859: with @code{W:}, @code{D:} etc.) produces its value and can be changed
!           860: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
!           861: produces its address (which becomes invalid when the variable's scope is
!           862: left). E.g., the standard word @code{emit} can be defined in therms of
!           863: @code{type} like this:
!           864:
!           865: @example
!           866: : emit @{ C^ char* -- @}
!           867:     char* 1 type ;
!           868: @end example
!           869:
!           870: A local without type specifier is a @code{W:} local. Both flavours of
!           871: locals are initialized with values from the data or FP stack.
!           872:
!           873: Currently there is no way to define locals with user-defined data
!           874: structures, but we are working on it.
!           875:
!           876: GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions:
!           877:
!           878: @subsubsection Where are locals visible by name?
!           879:
!           880: Basically, the answer is that locals are visible where you would expect
!           881: it in block-structured languages, and sometimes a little longer. If you
!           882: want to restrict the scope of a local, enclose its definition in
!           883: @code{SCOPE}...@code{ENDSCOPE}.
!           884:
!           885: doc-scope
!           886: doc-endscope
!           887:
!           888: These words behave like control structure words, so you can use them
!           889: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
!           890: arbitrary ways.
!           891:
!           892: If you want a more exact answer to the visibility question, here's the
!           893: basic principle: A local is visible in all places that can only be
!           894: reached through the definition of the local@footnote{In compiler
!           895: construction terminology, all places dominated by the definition of the
!           896: local.}. In other words, it is not visible in places that can be reached
!           897: without going through the definition of the local. E.g., locals defined
!           898: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
!           899: defined in @code{BEGIN}...@code{UNTIL} are visible after the
!           900: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
!           901:
!           902: The reasoning behind this solution is: We want to have the locals
!           903: visible as long as it is meaningful. The user can always make the
!           904: visibility shorter by using explicit scoping. In a place that can
!           905: only be reached through the definition of a local, the meaning of a
!           906: local name is clear. In other places it is not: How is the local
!           907: initialized at the control flow path that does not contain the
!           908: definition? Which local is meant, if the same name is defined twice in
!           909: two independent control flow paths?
!           910:
!           911: This should be enough detail for nearly all users, so you can skip the
!           912: rest of this section. If you relly must know all the gory details and
!           914:
!           915: In order to implement this rule, the compiler has to know which places
!           916: are unreachable. It knows this automatically after @code{AHEAD},
!           917: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
!           918: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
!           919: compiler that the control flow never reaches that place. If
!           920: @code{UNREACHABLE} is not used where it could, the only consequence is
!           921: that the visibility of some locals is more limited than the rule above
!           922: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
!           923: lie to the compiler), buggy code will be produced.
!           924:
!           925: Another problem with this rule is that at @code{BEGIN}, the compiler
!           926: does not know which locals will be visible on the incoming back-edge
!           927: . All problems discussed in the following are due to this ignorance of
!           928: the compiler (we discuss the problems using @code{BEGIN} loops as
!           929: examples; the discussion also applies to @code{?DO} and other
!           930: loops). Perhaps the most insidious example is:
!           931: @example
!           933: BEGIN
!           934:   x
!           935: [ 1 CS-ROLL ] THEN
!           936:   { x }
!           937:   ...
!           938: UNTIL
!           939: @end example
!           940:
!           941: This should be legal according to the visibility rule. The use of
!           942: @code{x} can only be reached through the definition; but that appears
!           943: textually below the use.
!           944:
!           945: From this example it is clear that the visibility rules cannot be fully
!           946: implemented without major headaches. Our implementation treats common
!           947: cases as advertised and the exceptions are treated in a safe way: The
!           948: compiler makes a reasonable guess about the locals visible after a
!           949: @code{BEGIN}; if it is too pessimistic, the
!           950: user will get a spurious error about the local not being defined; if the
!           951: compiler is too optimistic, it will notice this later and issue a
!           952: warning. In the case above the compiler would complain about @code{x}
!           953: being undefined at its use. You can see from the obscure examples in
!           954: this section that it takes quite unusual control structures to get the
!           955: compiler into trouble, and even then it will often do fine.
!           956:
!           957: If the @code{BEGIN} is reachable from above, the most optimistic guess
!           958: is that all locals visible before the @code{BEGIN} will also be
!           959: visible after the @code{BEGIN}. This guess is valid for all loops that
!           960: are entered only through the @code{BEGIN}, in particular, for normal
!           961: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
!           962: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
!           963: compiler. When the branch to the @code{BEGIN} is finally generated by
!           964: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
!           965: warns the user if it was too optimisitic:
!           966: @example
!           967: IF
!           968:   { x }
!           969: BEGIN
!           970:   \ x ?
!           971: [ 1 cs-roll ] THEN
!           972:   ...
!           973: UNTIL
!           974: @end example
!           975:
!           976: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
!           977: optimistically assumes that it lives until the @code{THEN}. It notices
!           978: this difference when it compiles the @code{UNTIL} and issues a
!           979: warning. The user can avoid the warning, and make sure that @code{x}
!           980: is not used in the wrong area by using explicit scoping:
!           981: @example
!           982: IF
!           983:   SCOPE
!           984:   { x }
!           985:   ENDSCOPE
!           986: BEGIN
!           987: [ 1 cs-roll ] THEN
!           988:   ...
!           989: UNTIL
!           990: @end example
!           991:
!           992: Since the guess is optimistic, there will be no spurious error messages
!           994:
!           995: If the @code{BEGIN} is not reachable from above (e.g., after
!           996: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
!           997: optimistic guess, as the locals visible after the @code{BEGIN} may be
!           998: defined later. Therefore, the compiler assumes that no locals are
!           999: visible after the @code{BEGIN}. However, the useer can use
!          1000: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
!          1001: visible at the BEGIN as at the point where the item was created.
!          1002:
!          1003: doc-assume-live
!          1004:
!          1005: E.g.,
!          1006: @example
!          1007: { x }
!          1009: ASSUME-LIVE
!          1010: BEGIN
!          1011:   x
!          1012: [ 1 CS-ROLL ] THEN
!          1013:   ...
!          1014: UNTIL
!          1015: @end example
!          1016:
!          1017: Other cases where the locals are defined before the @code{BEGIN} can be
!          1018: handled by inserting an appropriate @code{CS-ROLL} before the
!          1019: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
!          1020: behind the @code{ASSUME-LIVE}).
!          1021:
!          1022: Cases where locals are defined after the @code{BEGIN} (but should be
!          1023: visible immediately after the @code{BEGIN}) can only be handled by
!          1024: rearranging the loop. E.g., the most insidious'' example above can be
!          1025: arranged into:
!          1026: @example
!          1027: BEGIN
!          1028:   { x }
!          1029:   ... 0=
!          1030: WHILE
!          1031:   x
!          1032: REPEAT
!          1033: @end example
!          1034:
!          1035: @subsubsection How long do locals live?
!          1036:
!          1037: The right answer for the lifetime question would be: A local lives at
!          1038: least as long as it can be accessed. For a value-flavoured local this
!          1039: means: until the end of its visibility. However, a variable-flavoured
!          1040: local could be accessed through its address far beyond its visibility
!          1041: scope. Ultimately, this would mean that such locals would have to be
!          1042: garbage collected. Since this entails un-Forth-like implementation
!          1043: complexities, I adopted the same cowardly solution as some other
!          1044: languages (e.g., C): The local lives only as long as it is visible;
!          1045: afterwards its address is invalid (and programs that access it
!          1046: afterwards are erroneous).
!          1047:
!          1048: @subsubsection Programming Style
!          1049:
!          1050: The freedom to define locals anywhere has the potential to change
!          1051: programming styles dramatically. In particular, the need to use the
!          1052: return stack for intermediate storage vanishes. Moreover, all stack
!          1053: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
!          1054: determined arguments) can be eliminated: If the stack items are in the
!          1055: wrong order, just write a locals definition for all of them; then
!          1056: write the items in the order you want.
!          1057:
!          1058: This seems a little far-fetched and eliminating stack manipulations is
!          1059: unlikely to become a conscious programming objective. Still, the
!          1060: number of stack manipulations will be reduced dramatically if local
!          1061: variables are used liberally (e.g., compare @code{max} in \sect{misc}
!          1062: with a traditional implementation of @code{max}).
!          1063:
!          1064: This shows one potential benefit of locals: making Forth programs more
!          1065: readable. Of course, this benefit will only be realized if the
!          1066: programmers continue to honour the principle of factoring instead of
!          1067: using the added latitude to make the words longer.
!          1068:
!          1069: Using @code{TO} can and should be avoided.  Without @code{TO},
!          1070: every value-flavoured local has only a single assignment and many
!          1071: advantages of functional languages apply to Forth. I.e., programs are
!          1072: easier to analyse, to optimize and to read: It is clear from the
!          1073: definition what the local stands for, it does not turn into something
!          1074: different later.
!          1075:
!          1076: E.g., a definition using @code{TO} might look like this:
!          1077: @example
!          1078: : strcmp @{ addr1 u1 addr2 u2 -- n @}
!          1079:  u1 u2 min 0
!          1080:  ?do
!          1082:    if
!          1083:      unloop exit
!          1084:    then
!          1087:  loop
!          1088:  u1 u2 - ;
!          1089: @end example
!          1090: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
!          1091: every loop iteration. @code{strcmp} is a typical example of the
!          1093: @code{strcmp}, you think that @code{addr1} refers to the start of the
!          1094: string. Only near the end of the loop you realize that it is something
!          1095: else.
!          1096:
!          1097: This can be avoided by defining two locals at the start of the loop that
!          1098: are initialized with the right value for the current iteration.
!          1099: @example
!          1100: : strcmp @{ addr1 u1 addr2 u2 -- n @}
!          1102:  u1 u2 min 0
!          1103:  ?do @{ s1 s2 @}
!          1104:    s1 c@ s2 c@ - ?dup
!          1105:    if
!          1106:      unloop exit
!          1107:    then
!          1108:    s1 char+ s2 char+
!          1109:  loop
!          1110:  2drop
!          1111:  u1 u2 - ;
!          1112: @end example
!          1113: Here it is clear from the start that @code{s1} has a different value
!          1114: in every loop iteration.
!          1115:
!          1116: @subsubsection Implementation
!          1117:
!          1118: GNU Forth uses an extra locals stack. The most compelling reason for
!          1119: this is that the return stack is not float-aligned; using an extra stack
!          1120: also eliminates the problems and restrictions of using the return stack
!          1121: as locals stack. Like the other stacks, the locals stack grows toward
!          1122: lower addresses. A few primitives allow an efficient implementation:
!          1123:
!          1124: doc-@local#
!          1125: doc-f@local#
!          1127: doc-lp+!#
!          1128: doc-lp!
!          1129: doc->l
!          1130: doc-f>l
!          1131:
!          1132: In addition to these primitives, some specializations of these
!          1133: primitives for commonly occurring inline arguments are provided for
!          1134: efficiency reasons, e.g., @code{@@local0} as specialization of
!          1135: @code{@@local#} for the inline argument 0. The following compiling words
!          1136: compile the right specialized version, or the general version, as
!          1137: appropriate:
!          1138:
!          1139: doc-compile-@@local
!          1140: doc-compile-f@@local
!          1141: doc-compile-lp+!
!          1142:
!          1143: Combinations of conditional branches and @code{lp+!#} like
!          1144: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
!          1145: is taken) are provided for efficiency and correctness in loops.
!          1146:
!          1147: A special area in the dictionary space is reserved for keeping the
!          1148: local variable names. @code{@{} switches the dictionary pointer to this
!          1149: area and @code{@}} switches it back and generates the locals
!          1150: initializing code. @code{W:} etc.@ are normal defining words. This
!          1151: special area is cleared at the start of every colon definition.
!          1152:
!          1153: A special feature of GNU Forths dictionary is used to implement the
!          1154: definition of locals without type specifiers: every wordlist (aka
!          1155: vocabulary) has its own methods for searching
!          1156: etc. (@xref{dictionary}). For the present purpose we defined a wordlist
!          1157: with a special search method: When it is searched for a word, it
!          1158: actually creates that word using @code{W:}. @code{@{} changes the search
!          1159: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
!          1160: and then the wordlist for defining locals without type specifiers.
!          1161:
!          1162: The lifetime rules support a stack discipline within a colon
!          1163: definition: The lifetime of a local is either nested with other locals
!          1164: lifetimes or it does not overlap them.
!          1165:
!          1166: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
!          1167: pointer manipulation is generated. Between control structure words
!          1168: locals definitions can push locals onto the locals stack. @code{AGAIN}
!          1169: is the simplest of the other three control flow words. It has to
!          1170: restore the locals stack depth of the corresponding @code{BEGIN}
!          1171: before branching. The code looks like this:
!          1172: @format
!          1173: @code{lp+!#} current-locals-size @minus{} dest-locals-size
!          1174: @code{branch} <begin>
!          1175: @end format
!          1176:
!          1177: @code{UNTIL} is a little more complicated: If it branches back, it
!          1178: must adjust the stack just like @code{AGAIN}. But if it falls through,
!          1179: the locals stack must not be changed. The compiler generates the
!          1180: following code:
!          1181: @format
!          1182: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
!          1183: @end format
!          1184: The locals stack pointer is only adjusted if the branch is taken.
!          1185:
!          1186: @code{THEN} can produce somewhat inefficient code:
!          1187: @format
!          1188: @code{lp+!#} current-locals-size @minus{} orig-locals-size
!          1189: <orig target>:
!          1190: @code{lp+!#} orig-locals-size @minus{} new-locals-size
!          1191: @end format
!          1192: The second @code{lp+!#} adjusts the locals stack pointer from the
!          1193: level at the {\em orig} point to the level after the @code{THEN}. The
!          1194: first @code{lp+!#} adjusts the locals stack pointer from the current
!          1195: level to the level at the orig point, so the complete effect is an
!          1196: adjustment from the current level to the right level after the
!          1197: @code{THEN}.
!          1198:
!          1199: In a conventional Forth implementation a dest control-flow stack entry
!          1200: is just the target address and an orig entry is just the address to be
!          1201: patched. Our locals implementation adds a wordlist to every orig or dest
!          1202: item. It is the list of locals visible (or assumed visible) at the point
!          1203: described by the entry. Our implementation also adds a tag to identify
!          1204: the kind of entry, in particular to differentiate between live and dead
!          1205: (reachable and unreachable) orig entries.
!          1206:
!          1207: A few unusual operations have to be performed on locals wordlists:
!          1208:
!          1209: doc-common-list
!          1210: doc-sub-list?
!          1211: doc-list-size
!          1212:
!          1213: Several features of our locals wordlist implementation make these
!          1214: operations easy to implement: The locals wordlists are organised as
!          1215: linked lists; the tails of these lists are shared, if the lists
!          1216: contain some of the same locals; and the address of a name is greater
!          1217: than the address of the names behind it in the list.
!          1218:
!          1219: Another important implementation detail is the variable
!          1220: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
!          1221: determine if they can be reached directly or only through the branch
!          1222: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
!          1223: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
!          1224: definition, by @code{BEGIN} and usually by @code{THEN}.
!          1225:
!          1226: Counted loops are similar to other loops in most respects, but
!          1227: @code{LEAVE} requires special attention: It performs basically the same
!          1228: service as @code{AHEAD}, but it does not create a control-flow stack
!          1229: entry. Therefore the information has to be stored elsewhere;
!          1230: traditionally, the information was stored in the target fields of the
!          1231: branches created by the @code{LEAVE}s, by organizing these fields into a
!          1232: linked list. Unfortunately, this clever trick does not provide enough
!          1233: space for storing our extended control flow information. Therefore, we
!          1234: introduce another stack, the leave stack. It contains the control-flow
!          1235: stack entries for all unresolved @code{LEAVE}s.
!          1236:
!          1237: Local names are kept until the end of the colon definition, even if
!          1238: they are no longer visible in any control-flow path. In a few cases
!          1239: this may lead to increased space needs for the locals name area, but
!          1240: usually less than reclaiming this space would cost in code size.
!          1241:
!          1242:
!          1243: @subsection ANS Forth locals
!          1244:
!          1245: The ANS Forth locals wordset does not define a syntax for locals, but
!          1246: words that make it possible to define various syntaxes. One of the
!          1247: possible syntaxes is a subset of the syntax we used in the gforth locals
!          1248: wordset, i.e.:
!          1249:
!          1250: @example
!          1251: @{ local1 local2 ... -- comment @}
!          1252: @end example
!          1253: or
!          1254: @example
!          1255: @{ local1 local2 ... @}
!          1256: @end example
!          1257:
!          1258: The order of the locals corresponds to the order in a stack comment. The
!          1259: restrictions are:
1.1       anton    1260:
1.2     ! anton    1261: @itemize @bullet
!          1262: @item
!          1263: Locals can only be cell-sized values (no type specifers are allowed).
!          1264: @item
!          1265: Locals can be defined only outside control structures.
!          1266: @item
!          1267: Locals can interfere with explicit usage of the return stack. For the
!          1268: exact (and long) rules, see the standard. If you don't use return stack
!          1269: accessing words in a definition using locals, you will we all right. The
!          1270: purpose of this rule is to make locals implementation on the return
!          1271: stack easier.
!          1272: @item
!          1273: The whole definition must be in one line.
!          1274: @end itemize
!          1275:
!          1276: Locals defined in this way behave like @code{VALUE}s
!          1277: (@xref{values}). I.e., they are initialized from the stack. Using their
!          1278: name produces their value. Their value can be changed using @code{TO}.
!          1279:
!          1280: Since this syntax is supported by gforth directly, you need not do
!          1281: anything to use it. If you want to port a program using this syntax to
!          1282: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
!          1283: on the other system.
!          1284:
!          1285: Note that a syntax shown in the standard, section A.13 looks
!          1286: similar, but is quite different in having the order of locals
!          1287: reversed. Beware!
!          1288:
!          1289: The ANS Forth locals wordset itself consists of the following word
!          1290:
!          1291: doc-(local)
!          1292:
!          1293: The ANS Forth locals extension wordset defines a syntax, but it is so
!          1294: awful that we strongly recommend not to use it. We have implemented this
!          1295: syntax to make porting to gforth easy, but do not document it here. The
!          1296: problem with this syntax is that the locals are defined in an order
!          1297: reversed with respect to the standard stack comment notation, making
!          1298: programs harder to read, and easier to misread and miswrite. The only
!          1299: merit of this syntax is that it is easy to implement using the ANS Forth
!          1300: locals wordset.
1.1       anton    1301:
1302: @contents
1303: @bye
1304:


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