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: 
                     12: Copyright @copyright{} 1994 GNU Forth Development Group
                     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
                     50: Copyright @copyright{} 1994 GNU Forth Development Group
                     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: 
                     79: @menu
                     80: * License::             
                     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
                     93: @end menu
                     94: 
                     95: @node License, Goals, Top, Top
                     96: @unnumbered License
                     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}
                    196: Loads the Forth image @var{file} instead of the default
                    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: 
                    259: @menu
                    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::
                    272: @end menu
                    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_
                    339: Cell-aligned address
                    340: @item c_
                    341: Char-aligned address (note that a Char is two bytes in Windows NT)
                    342: @item f_
                    343: Float-aligned address
                    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: 
                    497: @subsection Address arithmetic
                    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
                    522: are cell-aligned; in addition, gforth guarantees that these addresses
                    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
                    543: doc-address-unit-bits
                    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
        !           750: ahead
        !           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: 
        !           810: @menu
        !           811: @end menu
        !           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
        !           913: options, read on.
        !           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
        !           932: AHEAD
        !           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
        !           993: about undefined locals.
        !           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 }
        !          1008: AHEAD
        !          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
        !          1081:    addr1 c@ addr2 c@ - ?dup
        !          1082:    if
        !          1083:      unloop exit
        !          1084:    then
        !          1085:    addr1 char+ TO addr1
        !          1086:    addr2 char+ TO addr2
        !          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
        !          1092: readability problems of using @code{TO}. When you start reading
        !          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 @}
        !          1101:  addr1 addr2
        !          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#
        !          1126: doc-laddr#
        !          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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