File:  [gforth] / gforth / Attic / gforth.ds
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Mon Nov 14 19:01:16 1994 UTC (26 years, 5 months ago) by anton
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    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.)
    9: @ifinfo
   10: This file documents GNU Forth 0.0
   12: Copyright @copyright{} 1994 GNU Forth Development Group
   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.
   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).
   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.
   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
   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
   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
   52: @comment !! Published by ... or You can get a copy of this manual ...
   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.
   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.
   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
   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
   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
   95: @node License, Goals, Top, Top
   96: @unnumbered License
   97: !! Insert GPL here
   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
  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:
  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
  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
  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.
  151: @node Other Books, Invocation, Goals, Top
  152: @chapter Other books on ANS Forth
  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.
  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
  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.
  172: @node Invocation, Words, Other Books, Top
  173: @chapter Invocation
  175: You will usually just say @code{gforth}. In many other cases the default
  176: GNU Forth image will be invoked like this:
  178: @example
  179: gforth [files] [-e forth-code]
  180: @end example
  182: executing the contents of the files and the Forth code in the order they
  183: are given.
  185: In general, the command line looks like this:
  187: @example
  188: gforth [initialization options] [image-specific options]
  189: @end example
  191: The initialization options must come before the rest of the command
  192: line. They are:
  194: @table @code
  195: @item --image-file @var{file}
  196: Loads the Forth image @var{file} instead of the default
  197: @file{}.
  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.
  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.
  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).
  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).
  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.
  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).
  236: @end table
  238: As explained above, the image-specific command-line arguments for the
  239: default image @file{} 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.
  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).
  256: @node Words,  , Invocation, Top
  257: @chapter Forth Words
  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
  274: @node Notation, Arithmetic, Words, Words
  275: @section Notation
  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.
  280: @quotation
  281: @var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}
  282: @var{Description}
  283: @end quotation
  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.
  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.
  302: @item pronunciation
  303: How the word is pronounced
  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.
  313: @item Description
  314: A description of the behaviour of the word.
  315: @end table
  317: The name of a stack item corresponds in the following way with its type:
  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
  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).
  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
  380: @subsection Bitwise operations
  381: doc-and
  382: doc-or
  383: doc-xor
  384: doc-invert
  385: doc-2*
  386: doc-2/
  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
  399: @subsection Double precision
  400: doc-d+
  401: doc-d-
  402: doc-dnegate
  403: doc-dabs
  404: doc-dmin
  405: doc-dmax
  407: @node Stack Manipulation,,,
  408: @section Stack Manipulation
  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).
  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
  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
  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
  466: @subsection Locals stack
  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!
  478: @node Memory access
  479: @section Memory access
  481: @subsection Stack-Memory transfers
  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!
  497: @subsection Address arithmetic
  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).
  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. 
  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.
  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.
  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.
  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
  545: @subsection Memory block access
  547: doc-move
  548: doc-erase
  550: While the previous words work on address units, the rest works on
  551: characters.
  553: doc-cmove
  554: doc-cmove>
  555: doc-fill
  556: doc-blank
  558: @node Control Structures
  559: @section Control Structures
  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.
  566: @subsection Selection
  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
  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
  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.]
  603: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
  604: can avoid using @code{?dup}.
  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
  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.
  620: @subsection Simple Loops
  622: @example
  623: BEGIN
  624:   @var{code1}
  625:   @var{flag}
  626: WHILE
  627:   @var{code2}
  628: REPEAT
  629: @end example
  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}.
  634: @example
  635: BEGIN
  636:   @var{code}
  637:   @var{flag}
  638: UNTIL
  639: @end example
  641: @var{code} is executed. The loop is restarted if @code{flag} is false.
  643: @example
  644: BEGIN
  645:   @var{code}
  646: AGAIN
  647: @end example
  649: This is an endless loop.
  651: @subsection Counted Loops
  653: The basic counted loop is:
  654: @example
  655: @var{limit} @var{start}
  656: ?DO
  657:   @var{body}
  658: LOOP
  659: @end example
  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}.
  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.
  684: There are several variations on the counted loop:
  686: @code{LEAVE} leaves the innermost counted loop immediately.
  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.:
  692: @code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}
  694: @code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}
  696: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
  698: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
  700: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
  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:
  706: @code{-2 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
  708: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0}
  710: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
  712: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
  713: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
  714: Forth standard.
  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.
  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.
  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.
  737: @subsection Arbitrary control structures
  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.
  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).
  749: if
  750: ahead
  751: then
  752: begin
  753: until
  754: again
  755: cs-pick
  756: cs-roll
  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.
  764: Some standard control structure words are built from these words:
  766: else
  767: while
  768: repeat
  770: Counted loop words constitute a separate group of words:
  772: ?do
  773: do
  774: for
  775: loop
  776: s+loop
  777: +loop
  778: next
  779: leave
  780: ?leave
  781: unloop
  782: undo
  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}).
  791: Another group of control structure words are
  793: case
  794: endcase
  795: of
  796: endof
  798: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
  799: @code{cs-roll}.
  801: @node Locals
  802: @section Locals
  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).
  810: @menu
  811: @end menu
  813: @subsection gforth locals
  815: Locals can be defined with
  817: @example
  818: @{ local1 local2 ... -- comment @}
  819: @end example
  820: or
  821: @example
  822: @{ local1 local2 ... @}
  823: @end example
  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
  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.
  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.
  846: The name of the local may be preceded by a type specifier, e.g.,
  847: @code{F:} for a floating point value:
  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
  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:
  865: @example
  866: : emit @{ C^ char* -- @}
  867:     char* 1 type ;
  868: @end example
  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.
  873: Currently there is no way to define locals with user-defined data
  874: structures, but we are working on it.
  876: GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions:
  878: @subsubsection Where are locals visible by name?
  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}.
  885: doc-scope
  886: doc-endscope
  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.
  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}).
  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?
  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.
  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.
  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
  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.
  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.
  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
  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
  992: Since the guess is optimistic, there will be no spurious error messages
  993: about undefined locals.
  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.
 1003: doc-assume-live
 1005: E.g.,
 1006: @example
 1007: { x }
 1008: AHEAD
 1010: BEGIN
 1011:   x
 1012: [ 1 CS-ROLL ] THEN
 1013:   ...
 1014: UNTIL
 1015: @end example
 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}).
 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
 1035: @subsubsection How long do locals live?
 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).
 1048: @subsubsection Programming Style
 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.
 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}).
 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.
 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.
 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.
 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.
 1116: @subsubsection Implementation
 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:
 1124: doc-@local#
 1125: doc-f@local#
 1126: doc-laddr#
 1127: doc-lp+!#
 1128: doc-lp!
 1129: doc->l
 1130: doc-f>l
 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:
 1139: doc-compile-@@local
 1140: doc-compile-f@@local
 1141: doc-compile-lp+!
 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.
 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.
 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.
 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.
 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
 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.
 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}.
 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.
 1207: A few unusual operations have to be performed on locals wordlists:
 1209: doc-common-list
 1210: doc-sub-list?
 1211: doc-list-size
 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.
 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}.
 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.
 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.
 1243: @subsection ANS Forth locals
 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.:
 1250: @example
 1251: @{ local1 local2 ... -- comment @}
 1252: @end example
 1253: or
 1254: @example
 1255: @{ local1 local2 ... @}
 1256: @end example
 1258: The order of the locals corresponds to the order in a stack comment. The
 1259: restrictions are:
 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
 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}.
 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.
 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!
 1289: The ANS Forth locals wordset itself consists of the following word
 1291: doc-(local)
 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.
 1302: @contents
 1303: @bye

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