% WEB user manual -- last updated by D E Knuth on 4 Dec 89 \input webmac \parskip 0pt plus 1pt \def\RA{\char'31 } % right arrow \def\hang{\hangindent 4em\ignorespaces} \font\ninerm=cmr9 \font\ninett=cmtt9 \font\eighttt=cmtt8 \let\mc=\ninerm % medium caps for names like SAIL \def\PASCAL{Pascal} \font\quoterm=cmssq8 \font\quoteit=cmssqi8 \def\pb{\.{|...|}} \def\lpile{\def\cr{\hfill\endline}\matrix} % I only use \lpile by itself \outer\def\section #1.{\penalty-50\vskip 12pt plus 3pt minus 3pt \noindent{\bf #1.}\quad\ignorespaces} \def\lheader{\mainfont\the\pageno\hfill\sc\runninghead\hfill} \def\rheader{\hfill\sc\runninghead\hfill\mainfont\the\pageno} \def\runninghead{{\tentt WEB} USER MANUAL} % This verbatim mode assumes that no ? appears in the text being copied. \def\verbatim{\begingroup \def\do##1{\catcode`##1=12 } \dospecials \parskip 0pt \parindent 0pt \catcode`\ =13 \catcode`\^^M=13 \tt \catcode`\?=0 \verbatimdefs \verbatimgobble} {\catcode`\^^M=13{\catcode`\ =13\gdef\verbatimdefs{\def^^M{\ \par}\let =\ }} % \gdef\verbatimgobble#1^^M{}} \centerline{\titlefont The {\ttitlefont WEB} System of Structured Documentation} \vskip 15pt plus 3pt minus 3pt \noindent This memo describes how to write programs in the \.{WEB} language; and it also includes the full \.{WEB} documentation for \.{WEAVE} and \.{TANGLE}, the programs that read \.{WEB} input and produce \TeX\ and \PASCAL\ output, respectively. The philosophy behind \.{WEB} is that an experienced system programmer, who wants to provide the best possible documentation of software products, needs two things simultaneously: a language like \TeX\ for formatting, and a language like \PASCAL\ for programming. Neither type of language can provide the best documentation by itself. But when both are appropriately combined, we obtain a system that is much more useful than either language separately. The structure of a software program may be thought of as a ``web'' that is made up of many interconnected pieces. To document such a program, we want to explain each individual part of the web and how it relates to its neighbors. The typographic tools provided by \TeX\ give us an opportunity to explain the local structure of each part by making that structure visible, and the programming tools provided by \PASCAL\ make it possible for us to specify the algorithms formally and unambiguously. By combining the two, we can develop a style of programming that maximizes our ability to perceive the structure of a complex piece of software, and at the same time the documented programs can be mechanically translated into a working software system that matches the documentation. Since \.{WEB} is an experimental system developed for internal use within the \TeX\ project at Stanford, this report is rather terse, and it assumes that the reader is an experienced programmer who is highly motivated to read a detailed description of \.{WEB}'s rules. Furthermore, even if a less terse manual were to be written, the reader would have to be warned in advance that \.{WEB} is not for beginners and it never will be: The user of \.{WEB} must be familiar with both \TeX\ and \PASCAL. When one writes a \.{WEB} description of a software system, it is possible to make mistakes by breaking the rules of \.{WEB} and/or the rules of \TeX\ and/or the rules of \PASCAL. In practice, all three types of errors will occur, and you will get different error messages from the different language processors. In compensation for the sophisticated expertise needed to cope with such a variety of languages, however, experience has shown that reliable software can be created quite rapidly by working entirely in \.{WEB} from the beginning; and the documentation of such programs seems to be better than the documentation obtained by any other known method. Thus, \.{WEB} users need to be highly qualified, but they can get some satisfaction and perhaps even a special feeling of accomplishment when they have successfully created a software system with this method. To use \.{WEB}, you prepare a file called \.{COB.WEB} (say), and then you apply a system program called \.{WEAVE} to this file, obtaining an output file called \.{COB.TEX}. When \TeX\ processes \.{COB.TEX}, your output will be a ``pretty printed'' version of \.{COB.WEB} that takes appropriate care of typographic details like page layout and the use of indentation, italics, boldface, etc.; this output will contain extensive cross-index information that is gathered automatically. You can also submit the same file \.{COB.WEB} to another system program called \.{TANGLE}, which will produce a file \.{COB.PAS} that contains the \PASCAL\ code of your \.{COB} program. The \PASCAL\ compiler will convert \.{COB.PAS} into machine-language instructions corresponding to the algorithms that were so nicely formatted by \.{WEAVE} and \TeX. Finally, you can (and should) delete the files \.{COB.TEX} and \.{COB.PAS}, because \.{COB.WEB} contains the definitive source code. Examples of the behavior of \.{WEAVE} and \.{TANGLE} are appended to this manual. Besides providing a documentation tool, \.{WEB} enhances the \PASCAL\ language by providing a rudimentary macro capability together with the ability to permute pieces of the program text, so that a large system can be understood entirely in terms of small modules and their local interrelationships. The \.{TANGLE} program is so named because it takes a given web and moves the modules from their web structure into the order required by \PASCAL; the advantage of programming in \.{WEB} is that the algorithms can be expressed in ``untangled'' form, with each module explained separately. The \.{WEAVE} program is so named because it takes a given web and intertwines the \TeX\ and \PASCAL\ portions contained in each module, then it knits the whole fabric into a structured document. (Get it? Wow.) Perhaps there is some deep connection here with the fact that the German word for ``weave'' is ``{\it web\/}'', and the corresponding Latin imperative is ``{\it texe\/}''! It is impossible to list all of the related work that has influenced the design of \.{WEB}, but the key contributions should be mentioned here.\quad (1)~Myrtle Kellington, as executive editor for ACM publications, developed excellent typographic standards for the typesetting of Algol programs during the 1960s, based on the original designs of Peter Naur; the subtlety and quality of this influential work can be appreciated only by people who have seen what happens when other printers try to typeset Algol without the advice of ACM's copy editors.\quad(2)~Bill McKeeman introduced a program intended to automate some of this task [Algorithm 268, ``Algol~60 reference language editor,'' {\sl CACM \bf8} (1965), 667--668]; and a considerable flowering of such programs has occurred in recent years [see especially Derek Oppen, ``Prettyprinting,'' {\sl ACM TOPLAS \bf2} (1980), 465--483; G.~A. Rose and J. Welsh, ``Formatted programming languages,'' {\sl SOFTWARE Practice \char`\&\ Exper.\ \bf11} (1981), 651--669].\quad(3)~The top-down style of exposition encouraged by \.{WEB} was of course chiefly influenced by Edsger Dijkstra's essays on structured programming in the late 1960s. The less well known work of Pierre-Arnoul de Marneffe [``Holon programming: A survey,'' Univ.\ de Liege, Service Informatique, Liege, Belgium, 1973; 135 pp.\null] also had a significant influence on the author as \.{WEB} was being formulated.\quad(4)~Edwin Towster has proposed a similar style of documentation in which the programmer is supposed to specify the relevant data structure environment in the name of each submodule [``A convention for explicit declaration of environments and top-down refinement of data,'' {\sl IEEE Trans.\ on Software Eng.\ \bf SE--5} (1979), 374--386]; this requirement seems to make the documentation a bit too verbose, although experience with \.{WEB} has shown that any unusual control structure or data structure should definitely be incorporated into the module names on psychological grounds.\quad(5)~Discussions with Luis Trabb~Pardo in the spring of 1979 were extremely helpful for setting up a prototype version of \.{WEB} that was called \.{DOC}.\quad (6)~Ignacio Zabala's extensive experience with \.{DOC}, in which he created a full implementation of \TeX\ in \PASCAL\ that was successfully transported to many different computers, was of immense value while \.{WEB} was taking its present form.\quad(7)~David~R. Fuchs made several crucial suggestions about how to make \.{WEB} more portable; he and Arthur~L. Samuel coordinated the initial installations of \.{WEB} on dozens of computer systems, making changes to the code so that it would be acceptable to a wide variety of \PASCAL\ compilers.\quad(8)~The name \.{WEB} itself was chosen in honor of my wife's mother, Wilda Ernestine Bates. The appendices to this report contain complete \.{WEB} programs for the \.{WEAVE} and \.{TANGLE} processors. A study of these examples, together with an attempt to write \.{WEB} programs by yourself, is the best way to understand why \.{WEB} has come to be like it is. \section General rules. A \.{WEB} file is a long string of text that has been divided into individual lines. The exact line boundaries are not terribly crucial, and a programmer can pretty much chop up the \.{WEB} file in whatever way seems to look best as the file is being edited; but string constants and control texts must end on the same line on which they begin, since this convention helps to keep errors from propagating. The end of a line means the same thing as a blank space. Two kinds of material go into \.{WEB} files: \TeX\ text and \PASCAL\ text. A programmer writing in \.{WEB} should be thinking both of the documentation and of the \PASCAL\ program that he or she is creating; i.e., the programmer should be instinctively aware of the different actions that \.{WEAVE} and \.{TANGLE} will perform on the \.{WEB} file. \TeX\ text is essentially copied without change by \.{WEAVE}, and it is entirely deleted by \.{TANGLE}, since the \TeX\ text is ``pure documentation.'' \PASCAL\ text, on the other hand, is formatted by \.{WEAVE} and it is shuffled around by \.{TANGLE}, according to rules that will become clear later. For now the important point to keep in mind is that there are two kinds of text. Writing \.{WEB} programs is something like writing \TeX\ documents, but with an additional ``\PASCAL\ mode'' that is added to \TeX's horizontal mode, vertical mode, and math mode. A \.{WEB} file is built up from units called {\sl modules\/} that are more or less self-contained. Each module has three parts: \yskip\item{1)} A \TeX\ part, containing explanatory material about what is going on in the module. \item{2)} A definition part, containing macro definitions that serve as abbreviations for \PASCAL\ constructions that would be less comprehensible if written out in full each time. \item{3)} A \PASCAL\ part, containing a piece of the program that \.{TANGLE} will produce. This \PASCAL\ code should ideally be about a dozen lines long, so that it is easily comprehensible as a unit and so that its structure is readily perceived. \yskip\noindent The three parts of each module must appear in this order; i.e., the \TeX\ commentary must come first, then the definitions, and finally the \PASCAL\ code. Any of the parts may be empty. \eject % page break inserted Dec 88 A module begins with the pair of symbols `\.{@\ }' or `\.{@*}', where `\.{\ }' denotes a blank space. A module ends at the beginning of the next module (i.e., at the next `\.{@\ }' or `\.{@*}'), or at the end of the file, whichever comes first. The \.{WEB} file may also contain material that is not part of any module at all, namely the text (if any) that occurs before the first module. Such text is said to be ``in limbo''; it is ignored by \.{TANGLE} and copied essentially verbatim by \.{WEAVE}, so its function is to provide any additional formatting instructions that may be desired in the \TeX\ output. Indeed, it is customary to begin a \.{WEB} file with \TeX\ code in limbo that loads special fonts, defines special macros, changes the page sizes, and/or produces a title page. Modules are numbered consecutively, starting with 1; these numbers appear at the beginning of each module of the \TeX\ documentation, and they appear as bracketed comments at the beginning of the code generated by that module in the \PASCAL\ program. Fortunately, you never mention these numbers yourself when you are writing in \.{WEB}. You just say `\.{@\ }' or `\.{@*}' at the beginning of each new module, and the numbers are supplied automatically by \.{WEAVE} and \.{TANGLE}. As far as you are concerned, a module has a {\sl name\/} instead of a number; such a name is specified by writing `\.{@<}' followed by \TeX\ text followed by `\.{@>}'. When \.{WEAVE} outputs a module name, it replaces the `\.{@<}' and `\.{@>}' by angle brackets and inserts the module number in small type. Thus, when you read the output of \.{WEAVE} it is easy to locate any module that is referred to in another module. For expository purposes, a module name should be a good description of the contents of that module; i.e., it should stand for the abstraction represented by the module. Then the module can be ``plugged into'' one or more other modules in such a way that unimportant details of its inner workings are suppressed. A module name therefore ought to be long enough to convey the necessary meaning. Unfortunately, however, it is laborious to type such long names over and over again, and it is also difficult to specify a long name twice in exactly the same way so that \.{WEAVE} and \.{TANGLE} will be able to match the names to the modules. To ameliorate this difficulty, \.{WEAVE} and \.{TANGLE} let you abbreviate a module name after its first appearance in the \.{WEB} file; you can type simply `\.{@<$\alpha$...@>}', where $\alpha$ is any string that is a prefix of exactly one module name appearing in the file. For example, `\.{@}' can be abbreviated to `\.{@}' if no other module name begins with the five letters `\.{Clear}'. Module names must otherwise match character for character, except that consecutive blank spaces and/or tab marks are treated as equivalent to single spaces, and such spaces are deleted at the beginning and end of the name. Thus, `\.{@< Clear { }the arrays @>}' will also match the name in the previous example. We have said that a module begins with `\.{@\ }' or `\.{@*}', but we didn't say how it gets divided up into a \TeX\ part, a definition part, and a \PASCAL\ part. The definition part begins with the first appearance of `\.{@d}' or `\.{@f}' in the module, and the \PASCAL\ part begins with the first appearance of `\.{@p}' or `\.{@<}'. The latter option `\.{@<}' stands for the beginning of a module name, which is the name of the module itself. An equals sign (\.=) must follow the `\.{@>}' at the end of this module name; you are saying, in effect, that the module name stands for the \PASCAL\ text that follows, so you say `$\langle\,$module name$\,\rangle=\null$\PASCAL\ text'. Alternatively, if the \PASCAL\ part begins with `\.{@p}' instead of a module name, the current module is said to be {\sl unnamed}. Note that module names cannot appear in the definition part of a module, because the first `\.{@<}' in a module signals the beginning of its \PASCAL\ part. But any number of module names might appear in the \PASCAL\ part, once it has started. The general idea of \.{TANGLE} is to make a \PASCAL\ program out of these modules in the following way: First all the \PASCAL\ parts of unnamed modules are copied down, in order; this constitutes the initial approximation $T_0$ to the text of the program. (There should be at least one unnamed module, otherwise there will be no program.) Then all module names that appear in the initial text $T_0$ are replaced by the \PASCAL\ parts of the corresponding modules, and this substitution process continues until no module names remain. Then all defined macros are replaced by their equivalents, according to certain rules that are explained later. The resulting \PASCAL\ code is ``sanitized'' so that it will be acceptable to an average garden-variety \PASCAL\ compiler; i.e., lowercase letters are converted to uppercase, long identifiers are chopped, and the lines of the output file are constrained to be at most 72 characters long. All comments will have been removed from this \PASCAL\ program except for the meta-comments delimited by `\.{@\{}' and `\.{@\}}', as explained below, and except for the module-number comments that point to the source location where each piece of the program text originated in the \.{WEB} file. If the same name has been given to more than one module, the \PASCAL\ text for that name is obtained by putting together all of the \PASCAL\ parts in the corresponding modules. This feature is useful, for example, in a module named `Global variables in the outer block', since one can then declare global variables in whatever modules those variables are introduced. When several modules have the same name, \.{WEAVE} assigns the first module number as the number corresponding to that name, and it inserts a note at the bottom of that module telling the reader to `See also sections so-and-so'; this footnote gives the numbers of all the other modules having the same name as the present one. The \PASCAL\ text corresponding to a module is usually formatted by \.{WEAVE} so that the output has an equivalence sign in place of the equals sign in the \.{WEB} file; i.e., the output says `$\langle\,$module name$\,\rangle\equiv\null$\PASCAL\ text'. However, in the case of the second and subsequent appearances of a module with the same name, this `$\equiv$' sign is replaced by `$\mathrel+\equiv$', as an indication that the \PASCAL\ text that follows is being appended to the \PASCAL\ text of another module. The general idea of \.{WEAVE} is to make a \.{TEX} file from the \.{WEB} file in the following way: The first line of the \.{TEX} file will be `\.{\\input webmac}'; this will cause \TeX\ to read in the macros that define \.{WEB}'s documentation conventions. The next lines of the file will be copied from whatever \TeX\ text is in limbo before the first module. Then comes the output for each module in turn, possibly interspersed with end-of-page marks. Finally, \.{WEAVE} will generate a cross-reference index that lists each module number in which each \PASCAL\ identifier appears, and it will also generate an alphabetized list of the module names, as well as a table of contents that shows the page and module numbers for each ``starred'' module. What is a ``starred'' module, you ask? A module that begins with `\.{@*}' instead of `\.{@\ }' is slightly special in that it denotes a new major group of modules. The `\.{@*}' should be followed by the title of this group, followed by a period. Such modules will always start on a new page in the \TeX\ output, and the group title will appear as a running headline on all subsequent pages until the next starred module. The title will also appear in the table of contents, and in boldface type at the beginning of its module. Caution: Do not use \TeX\ control sequences in such titles, unless you know that the \.{webmac} macros will do the right thing with them. The reason is that these titles are converted to uppercase when they appear as running heads, and they are converted to boldface when they appear at the beginning of their modules, and they are also written out to a table-of-contents file used for temporary storage while \TeX\ is working; whatever control sequences you use must be meaningful in all three of these modes. The \TeX\ output produced by \.{WEAVE} for each module consists of the following: First comes the module number (e.g., `\.{\\M123.}' at the beginning of module 123, except that `\.{\\N}' appears in place of `\.{\\M}' at the beginning of a starred module). Then comes the \TeX\ part of the module, copied almost verbatim except as noted below. Then comes the definition part and the \PASCAL\ part, formatted so that there will be a little extra space between them if both are nonempty. The definition and \PASCAL\ parts are obtained by inserting a bunch of funny looking \TeX\ macros into the \PASCAL\ program; these macros handle typographic details about fonts and proper math spacing, as well as line breaks and indentation. When you are typing \TeX\ text, you will probably want to make frequent reference to variables and other quantities in your \PASCAL\ code, and you will want those variables to have the same typographic treatment when they appear in your text as when they appear in your program. Therefore the \.{WEB} language allows you to get the effect of \PASCAL\ editing within \TeX\ text, if you place `\.|' marks before and after the \PASCAL\ material. For example, suppose you want to say something like this: $$\hbox{The characters are placed into \\{buffer}, which is a \&{packed} \&{array} $[1\to\|n]$ \&{of} \\{char}.}$$ The \TeX\ text would look like this in your \.{WEB} file: $$\.{The characters are placed into |buffer|, which is a |packed array [1..n] of char|.}$$ And \.{WEAVE} translates this into something you are glad you didn't have to type: $$\lpile{\.{The characters are placed into \\\\\{buffer\},}\cr \.{which is a \\\&\{packed\}{ }\\\&\{array\}{ }\$ [1\\to\\|n]\${ }\\\&\{of\}{ }\\\\\{char\}.}\cr}$$ Incidentally, the cross-reference index that \.{WEAVE} would make, in the presence of a comment like this, would include the current module number as one of the index entries for \\{buffer} \vadjust{\eject}% page break inserted Dec 88 and \\{char}, even though \\{buffer} and \\{char} might not appear in the \PASCAL\ part of this module. Thus, the index covers references to identifiers in the explanatory comments as well as in the program itself; you will soon learn to appreciate this feature. However, the identifiers \&{packed} and \&{array} and \|n\ and \&{of\/} would not be indexed, because \.{WEAVE} does not make index entries for reserved words or single-letter identifiers. Such identifiers are felt to be so ubiquitous that it would be pointless to mention every place where they occur. Speaking of identifiers, the author of \.{WEB} thinks that \\{IdentifiersSeveralWordsLong} look terribly ugly when they mix uppercase and lowercase letters. He recommends that \\{identifiers\_several\_words\_long} be written with underline characters to get a much better effect. The actual identifiers sent to the \PASCAL\ compiler by \.{TANGLE} will have such underlines removed, and \.{TANGLE} will check to make sure that two different identifiers do not become identical when this happens. (In fact, \.{TANGLE} even checks that the first seven characters of identifiers are unique, when lowercase letters have been converted to uppercase; the number seven in this constraint is more strict than \PASCAL's eight, and it can be changed if desired.) The \.{WEAVE} processor will properly alphabetize identifiers that have embedded underlines when it makes the index. Although a module begins with \TeX\ text and ends with \PASCAL\ text, we have noted that the dividing line isn't sharp, since \PASCAL\ text can be included in \TeX\ text if it is enclosed in `\pb'. Conversely, \TeX\ text also appears frequently within \PASCAL\ text, because everything in comments (i.e., between left and right braces) is treated as \TeX\ text. Furthermore, a module name consists of \TeX\ text; thus, a \.{WEB} file typically involves constructions like `\.{if} \.x \.= \.0 \.{then} \.{@}' where we go back and forth between \PASCAL\ and \TeX\ conventions in a natural way. \section Macros. A \.{WEB} programmer can define three kinds of macros to make the programs shorter and more readable: \yskip\hang`\.{@d} \\{identifier} \.= \\{constant}' defines a {\sl numeric\/} macro, allowing \.{TANGLE} to do rudimentary arithmetic. \yskip\hang`\.{@d} \\{identifier} \.{==} \PASCAL\ text' defines a {\sl simple\/} macro, where the identifier will be replaced by the \PASCAL\ text when \.{TANGLE} produces its output. \yskip\hang`\.{@d} \\{identifier}\.{(\#) ==} \PASCAL\ text' defines a {\sl parametric\/} macro, where the identifier will be replaced by the \PASCAL\ text and where occurrences of \.{\#} in that \PASCAL\ text will be replaced by an argument. \yskip\noindent In all three cases, the identifier must have length greater than one; it must not be a single letter. Numeric macros are subject to the following restrictions:\quad (1)~The identifier must be making its first appearance in the \.{WEB} file; a numeric macro must be defined before it is used.\quad (2)~The right-hand side of the numeric definition must be made entirely from integer constants, numeric macros, preprocessed strings (see below), and plus~signs or minus signs. No other operations or symbols are allowed, not even parentheses, except that \PASCAL-like comments (enclosed in braces) can appear. Indeed, comments are recommended, since it is usually wise to give a brief explanation of the significance of each identifier as it is defined.\quad (3)~The numeric value must be less than $2^{15}=32768$ in absolute value. (For larger values, you can use `\.{==}' in place of~`\.=', thus making use of a simple macro instead of a numeric one. Note, however, that simple macros sometimes have a different effect. For example, consider the three definitions `\.{@d n1=2 @d n2=2+n1 @d n3==2+n1}'; then `\.{x-n2}' will expand into `\.{x-4}', while `\.{x-n3}' will expand into `\.{x-2+2}' which is quite different! It is wise to include parentheses in non-numeric macros, e.g., `\.{@d n3==(2+n1)}', to avoid such errors.) When constants are connected by plus signs or minus signs in a \PASCAL\ program, \.{TANGLE} does the arithmetic before putting the constant into the output file. Therefore it is permissible to say, for example, `\&{array} $[0\,.\,.\,\\{size}-1]$' if \\{size} has been declared as a macro; note that \PASCAL\ doesn't allow this kind of compile-time arithmetic if \\{size} is a \&{constant} quantity in the program. Another use of \.{TANGLE}'s arithmetic is to make \&{case} statement labels such as `$\\{flag}+1$' legitimate. Of course, it is improper to change \.{2+2} into \.4 without looking at the surrounding context; many counterexamples exist, such as the phrases `\.{-2+2}', `\.{x/2+2}', and `\.{2+2E5}'. The program for \.{TANGLE}, in the appendix, gives precise details about this conversion, which \.{TANGLE} does only when it is safe. The right-hand sides of simple and parametric macros are required to have balanced parentheses, and the \PASCAL\ texts of modules must have balanced parentheses too. Therefore when the argument to a para\-metric macro appears in parentheses, both parentheses will belong to the same \PASCAL\ text. The appendices to this report contain hundreds of typical examples of the usefulness of \.{WEB} macros, so it is not necessary to dwell on the subject here. However, the reader should know that \.{WEB}'s apparently primitive macro capabilities can actually do a lot of rather surprising things. Here is a construction that sheds further light on what is possible: After making the definitions $$\catcode`\#=12 \lpile{\.{@d two\_cases(#)==case j of 1:#(1); 2:#(2); end}\cr \.{@d reset\_file(#)==reset(input\_file@\&#)}\cr}$$ one can write `\.{two\_cases(reset\_file)}' and the resulting \PASCAL\ output will be $$\.{case j of 1:reset(input\_file1); 2:reset(input\_file2); end}$$ (but in uppercase letters and with \.\_'s removed). The `\.{@\&}' operation used here joins together two adjacent tokens into a single token, as explained later; otherwise the \PASCAL\ file would contain a space between \.{input\_file} and the digit that followed it. This trick can be used to provide the effect of an array of files, if you are unfortunate enough to have a \PASCAL\ compiler that doesn't allow such arrays. Incidentally, the cross-reference index made by \.{WEAVE} from this example would contain the identifier \\{input\_file} but it would not contain \\{input\_file1} or \\{input\_file2}. Furthermore, \.{TANGLE} would not catch the error that \.{INPUTFILE1} and \.{INPUTFILE2} both begin with the same nine letters; one should be more careful when using `\.{@\&}'! But such aspects of the construction in this trick are peripheral to our main point, which is that a parametric macro name without arguments can be used as an argument to another parametric macro. Although \.{WEB}'s macros are allowed to have at most one parameter, the following example shows that this is not as much of a restriction as it may seem at first. Let \\{amac} and \\{bmac} be any parametric macros, and suppose that we want to get the effect of $$\catcode`\#=12 \.{@d cmac(#1,#2) == amac(#1) bmac(#2)}$$ which \.{WEB} doesn't permit. The solution is to make the definitions $$\catcode`\#=12 \lpile{\.{@d cmac(#) == amac(#) dmac}\cr \.{@d dmac(#) == bmac(#)}\cr}$$ and then to say `\.{cmac(x)(y)}'. There is one restriction in the generality of \.{WEB}'s parametric macros, however: the argument to a para\-metric macro must not come from the expansion of a macro that has not already been ``started.'' For example, here is one of the things \.{WEB} cannot handle: $$\catcode`\#=12 \lpile{\.{@d arg == (p)}\cr \.{@d identity(#) == #}\cr \.{@p identity arg}\cr}$$ In this case \.{TANGLE} will complain that the \.{identity} macro is not followed by an argument in parentheses. The \.{WEB} language has another feature that is somewhat similar to a numeric macro. A {\sl preprocessed string\/} is a string that is like a \PASCAL\ string but delimited by double-quote marks (\.") instead of single-quotes. Double-quote marks inside of such strings are indicated by giving two double-quotes in a row. If a preprocessed string is of length one (e.g., \.{"A"} or \.{""""}), it will be treated by \.{TANGLE} as equivalent to the corresponding ASCII-code integer (e.g., \.{65} or \.{34}). And if a preprocessed string is not of length one, it will be converted into an integer equal to 256 or more. A {\sl string pool\/} containing all such strings will be written out by the \.{TANGLE} processor; this string pool file consists of string 256, then string 257, etc., where each string is followed by an end-of-line and prefixed by two decimal digits that define its length. Thus, for example, the empty string \.{""} would be represented in the string pool file by a line containing the two characters `\.{00}', while the string \.{"""String"""} would be represented by `\.{08"String"}'. A given string appears at most once in the string pool; the use of such a pool makes it easier to cope with \PASCAL's restrictions on string manipulation. The string pool ends with `\.{*nnnnnnnnn}', where \.{nnnnnnnnn} is a decimal number called the {\sl string pool check sum}. If any string changes, the check sum almost surely changes too; thus, the `\.{@\$}' feature described below makes it possible for a program to assure itself that it is reading its own string pool. Here is a simple example that combines numeric macros with preprocessed strings of length one: $$\lpile{\.{@d upper\_case\_Y = "Y"}\cr \.{@d case\_difference = -"y"+upper\_case\_Y}\cr}$$ The result is to define $\\{upper\_case\_Y}=89$, $\\{case\_difference}=-32$. \section Control codes. We have seen several magic uses of `\.{@}' signs in \.{WEB} files, and it is time to make a systematic study of these special features. A \.{WEB} {\sl control code\/} is a two-character combination of which the first is `\.@'. Here is a complete list of the legal control codes. The letters $L$, $T$, $P$, $M$, $C$, and/or $S$ following each code indicate whether or not that code is allowable in limbo, in \TeX\ text, in \PASCAL\ text, in module names, in comments, and/or in strings. A bar over such a letter means that the control code terminates the present part of the \.{WEB} file; for example, $\overline L$ means that this control code ends the limbo material before the first module. \def\@#1[#2] {\yskip\hangindent 2em\noindent\.{@#1\unskip \spacefactor1000{ }}$[#2]$\quad} \def\oP{\overline P} \def\oT{\overline T\mskip1mu} \@@ [C,L,M,P,S,T] A double \.@ denotes the single character `\.@'. This is the only control code that is legal in limbo, in comments, and in strings. \@\ [\overline L,\oP,\oT] This denotes the beginning of a new (unstarred) module. A tab mark or end-of-line (carriage return) is equivalent to a space when it follows an \.@ sign. \@* [\overline L,\oP,\oT] This denotes the beginning of a new starred module, i.e., a module that begins a new major group. The title of the new group should appear after the \.{@*}, followed by a period. As explained above, \TeX\ control sequences should be avoided in such titles unless they are quite simple. When \.{WEAVE} and \.{TANGLE} read a \.{@*}, they print an asterisk on the terminal followed by the current module number, so that the user can see some indication of progress. The very first module should be starred. \@d [\oP,\oT] Macro definitions begin with \.{@d} (or \.{@D}), followed by the \PASCAL\ text for one of the three kinds of macros, as explained earlier. \@f [\oP,\oT] Format definitions begin with \.{@f} (or \.{@F}); they cause \.{WEAVE} to treat identifiers in a special way when they appear in \PASCAL\ text. The general form of a format definition is `\.{@f} \|l \.{==} \|r', followed by an optional comment enclosed in braces, where \|l and \|r are identifiers; \.{WEAVE} will subsequently treat identifier \|l as it currently treats \|r. This feature allows a \.{WEB} programmer to invent new reserved words and/or to unreserve some of \PASCAL's reserved identifiers. The definition part of each module consists of any number of macro definitions (beginning with \.{@d}) and format definitions (beginning with \.{@f}), intermixed in any order. \@p [\oP,\oT] The \PASCAL\ part of an unnamed module begins with \.{@p} (or \.{@P}). This causes \.{TANGLE} to append the following \PASCAL\ code to the initial program text $T_0$ as explained above. The \.{WEAVE} processor does not cause a `\.{@p}' to appear explicitly in the \TeX\ output, so if you are creating a \.{WEB} file based on a \TeX-printed \.{WEB} documentation you have to remember to insert \.{@p} in the appropriate places of the unnamed modules. \@< [P,\oT] A module name begins with \.{@<} followed by \TeX\ text followed by \.{@>}; the \TeX\ text should not contain any \.{WEB} control codes except \.{@@}, unless these control codes appear in \PASCAL\ text that is delimited by \pb. The module name may be abbreviated, after its first appearance in a \.{WEB} file, by giving any unique prefix followed by \.{...}, where the three dots immediately precede the closing \.{@>}. No module name should be a prefix of another. Module names may not appear in \PASCAL\ text that is enclosed in \pb, nor may they appear in the definition part of a module (since the appearance of a module name ends the definition part and begins the \PASCAL\ part). \@\' [P,T] This denotes an octal constant, to be formed from the succeeding digits. For example, if the \.{WEB} file contains `\.{@\'100}', the \.{TANGLE} processor will treat this an equivalent to `\.{64}'; the constant will be formatted as ``\O{100}'' in the \TeX\ output produced via \.{WEAVE}. You should use octal notation only for positive constants; don't try to get, e.g., $-1$ by saying `\.{@\'777777777777}'. \@" [P,T] A hexadecimal constant; `\.{@"D0D0}' tangles to \.{53456} and weaves to `\H{D0D0}'. \@\$ [P] This denotes the string pool check sum. \@\{ [P] The beginning of a ``meta comment,'' i.e., a comment that is supposed to appear in the \PASCAL\ code, is indicated by \.{@\{} in the \.{WEB} file. Such delimiters can be used as isolated symbols in macros or modules, but they should be properly nested in the final \PASCAL\ program. The \.{TANGLE} processor will convert `\.{@\{}' into `\.\{' in the \PASCAL\ output file, unless the output is already part of a meta-comment; in the latter case `\.{@\{}' is converted into `\.[', since \PASCAL\ does not allow nested comments. Incidentally, module numbers are automatically inserted as meta-comments into the \PASCAL\ program, in order to help correlate the outputs of \.{WEAVE} and \.{TANGLE} (see Appendix~C\null). Meta-comments can be used to put conditional text into a \PASCAL\ program; this helps to overcome one of the limitations of \.{WEB}, since the simple macro processing routines of \.{TANGLE} do not include the dynamic evaluation of boolean expressions. \@\} [P] The end of a ``meta comment'' is indicated by `\.{@\}}'; this is converted either into `\.\}' or `\.{]}' in the \PASCAL\ output, according to the conventions explained for \.{@\{} above. \@\& [P] The \.{@\&} operation causes whatever is on its left to be adjacent to whatever is on its right, in the \PASCAL\ output. No spaces or line breaks will separate these two items. However, the thing on the left should not be a semicolon, since a line break might occur after a semicolon. \@\^ [P,T] The ``control text'' that follows, up to the next `\.{@>}', will be entered into the index together with the identifiers of the \PASCAL\ program; this text will appear in roman type. For example, to put the phrase ``system dependencies'' into the index, you can type `\.{@\^system dependencies@>}' in each module that you want to index as system dependent. A control text, like a string, must end on the same line of the \.{WEB} file as it began. Furthermore, no \.{WEB} control codes are allowed in a control text, not even \.{@@}. (If you need an \.{@} sign you can get around this restriction by typing `\.{\\AT!}'.) \@. [P,T] The ``control text'' that follows will be entered into the index in \.{typewriter} \.{type}; see the rules for `\.{@\^}', which is analogous. \@: [P,T] The ``control text'' that follows will be entered into the index in a format controlled by the \TeX\ macro `\.{\\9}', which the user should define as desired; see the rules for `\.{@\^}', which is analogous. \@t [P] The ``control text'' that follows, up to the next `\.{@>}', will be put into a \TeX\ \.{\\hbox} and formatted along with the neighboring \PASCAL\ program. This text is ignored by \.{TANGLE}, but it can be used for various purposes within \.{WEAVE}. For example, you can make comments that mix \PASCAL\ and classical mathematics, as in `$\\{size}<2^{15}$', by typing `\.{|size < @t\$2\^\{15\}\$@>|}'. A control text must end on the same line of the \.{WEB} file as it began, and it may not contain any \.{WEB} control codes. \@= [P] The ``control text'' that follows, up to the next `\.{@>}', will be passed verbatim to the \PASCAL\ program. \@\\ [P] Force end-of-line here in the \PASCAL\ program file. \@! [P,T] The module number in an index entry will be underlined if `\.{@!}' immediately precedes the identifier or control text being indexed. This convention is used to distinguish the modules where an identifier is defined, or where it is explained in some special way, from the modules where it is used. A~reserved word or an identifier of length one will not be indexed except for underlined entries. An `\.{@!}' is implicitly inserted by \.{WEAVE} just after the reserved words \&{function}, \&{procedure}, \&{program}, and \&{var}, and just after \.{@d} and \.{@f}. But you should insert your own `\.{@!}' before the definitions of types, constants, variables, parameters, and components of records and enumerated types that are not covered by this implicit convention, if you want to improve the quality of the index that you get. \@? [P,T] This cancels an implicit (or explicit) `\.{@!}', so that the next index entry will not be underlined. \@, [P] This control code inserts a thin space in \.{WEAVE}'s output; it is ignored by \.{TANGLE}. Sometimes you need this extra space if you are using macros in an unusual way, e.g., if two identifiers are adjacent. \@/ [P] This control code causes a line break to occur within a \PASCAL\ program formatted by \.{WEAVE}; it is ignored by \.{TANGLE}. Line breaks are chosen automatically by \TeX\ according to a scheme that works 99\%\ of the time, but sometimes you will prefer to force a line break so that the program is segmented according to logical rather than visual criteria. Caution: `\.{@/}' should be used only after statements or clauses, not in the middle of an expression; use \.{@|} in the middle of expressions, in order to keep \.{WEAVE}'s parser happy. \@| [P] This control code specifies an optional line break in the midst of an expression. For example, if you have a long condition between \&{if} and \&{then}, or a long expression on the right-hand side of an assignment statement, you can use `\.{@|}' to specify breakpoints more logical than the ones that \TeX\ might choose on visual grounds. \@\# [P] This control code forces a line break, like \.{@/} does, and it also causes a little extra white space to appear between the lines at this break. You might use it, for example, between procedure definitions or between groups of macro definitions that are logically separate but within the same module. \@+ [P] This control code cancels a line break that might otherwise be inserted by \.{WEAVE}, e.g., before the word `\&{else}', if you want to put a short if-then-else construction on a single line. It is ignored by \.{TANGLE}. \@; [P] This control code is treated like a semicolon, for formatting purposes, except that it is invisible. You can use it, for example, after a module name when the \PASCAL\ text represented by that module name ends with a semicolon. \yskip\noindent The last six control codes (namely `\.{@,}', `\.{@/}', `\.{@|}', `\.{@\#}', `\.{@+}', and `\.{@;}') have no effect on the \PASCAL\ program output by \.{TANGLE}; they merely help to improve the readability of the \TeX-formatted \PASCAL\ that is output by \.{WEAVE}, in unusual circumstances. \.{WEAVE}'s built-in formatting method is fairly good, but it is incapable of handling all possible cases, because it must deal with fragments of text involving macros and module names; these fragments do not necessarily obey \PASCAL's syntax. Although \.{WEB} allows you to override the automatic formatting, your best strategy is not to worry about such things until you have seen what \.{WEAVE} produces automatically, since you will probably need to make only a few corrections when you are touching up your documentation. Because of the rules by which every module is broken into three parts, the control codes `\.{@d}', `\.{@f}', and `\.{@p}' are not allowed to occur once the \PASCAL\ part of a module has begun. \section Additional features and caveats. 1. The character pairs `\.{(*}', `\.{*)}', `\.{(.}', and `\.{.)}' are converted automatically in \PASCAL\ text as though they were `\.{@\{}', `\.{@\}}', `\.[', and `\.]', respectively, except of course in strings. Furthermore in certain installations of \.{WEB} that {\def\\#1#2{`{\tentex\char'#1#2}'}% have an extended character set, the characters \\32, \\34, \\35, \\30, \\36, \\04, \\37, \\05, and \\06} can be typed as abbreviations for `\.{<>}', `\.{<=}', `\.{>=}', `\.{:=}', `\.{==}', `\.{and}', `\.{or}', `\.{not}', and `\.{in}', respectively. However, the latter abbreviations are not used in the standard versions of \.{WEAVE.WEB} and \.{TANGLE.WEB} that are distributed to people who are installing \.{WEB} on other computers, and the programs are designed to produce only standard ASCII characters as output if the input consists entirely of ASCII characters. 2. If you have an extended character set, all of the characters listed in Appendix C of {\sl The \TeX book\/} can be used in strings. But you should stick to standard ASCII characters if you want to write programs that will be useful to all the poor souls out there who don't have extended character sets. 3. The \TeX\ file output by \.{WEAVE} is broken into lines having at most 80 characters each. The algorithm that does this line breaking is unaware of \TeX's convention about comments following `\.\%' signs on a line. When \TeX\ text is being copied, the existing line breaks are copied as well, so there is no problem with `\.\%' signs unless the original \.{WEB} file contains a line more than eighty characters long or a line with \PASCAL\ text in \pb\ that expands to more than eighty characters long. Such lines should not have `\.\%' signs. 4. \PASCAL\ text is translated by a ``bottom up'' procedure that identifies each token as a ``part of speech'' and combines parts of speech into larger and larger phrases as much as possible according to a special grammar that is explained in the documentation of \.{WEAVE}. It is easy to learn the translation scheme for simple constructions like single identifiers and short expressions, just by looking at a few examples of what \.{WEAVE} does, but the general mechanism is somewhat complex because it must handle much more than \PASCAL\ itself. Furthermore the output contains embedded codes that cause \TeX\ to indent and break lines as necessary, depending on the fonts used and the desired page width. For best results it is wise to adhere to the following restrictions: \yskip\itemitem{a)}Comments in \PASCAL\ text should appear only after statements or clauses; i.e., after semicolons, after reserved words like \&{then} and \&{do}, or before reserved words like \&{end} and \&{else}. Otherwise \.{WEAVE}'s parsing method may well get mixed up. \itemitem{b)}Don't enclose long \PASCAL\ texts in \pb, since the indentation and line breaking codes are omitted when the \pb\ text is translated from \PASCAL\ to \TeX. Stick to simple expressions or statements. \yskip 5. Comments and module names are not permitted in \pb\ text. After a `\.|' signals the change from \TeX\ text to \PASCAL\ text, the next `\.|' that is not part of a string or control text ends the \PASCAL\ text. 6. A comment must have properly nested occurrences of left and right braces, otherwise \.{WEAVE} and \.{TANGLE} will not know where the comment ends. However, the character pairs `\.{\\\{}' and `\.{\\\}}' do not count as left and right braces in comments, and the character pair `\.{\\|}' does not count as a delimiter that begins \PASCAL\ text. (The actual rule is that a character after `\.\\' is ignored; hence in `\.{\\\\\{}' the left brace {\sl does\/} count.) At present, \.{TANGLE} and \.{WEAVE} treat comments in slightly different ways, and it is necessary to satisfy both conventions: \.{TANGLE} ignores `\.|' characters entirely, while \.{WEAVE} uses them to switch between \TeX\ text and \PASCAL\ text. Therefore, a comment that includes a brace in a string in \pb---e.g., `\.{\{{ }look at this |"\{"| \}}'---will be handled correctly by \.{WEAVE}, but \.{TANGLE} will think there is an unmatched left brace. In order to satisfy both processors, one can write `\.{\{{ }look at this \\leftbrace\\{ }\}}', after setting up`\.{\\def\\leftbrace\{|"\{"|\}}'. 7. Reserved words of \PASCAL\ must appear entirely in lowercase letters in the \.{WEB} file; otherwise their special nature will not be recognized by \.{WEAVE}. You could, for example, have a macro named \\{END} and it would not be confused with \PASCAL's \&{end}. However, you may not want to capitalize macro names just to distinguish them from other identifiers. Here is a way to unreserve \PASCAL's reserved word `\&{type}' and to substitute another word `\&{mtype}' in the \.{WEB} file. $$\vbox{\halign{\tt #\hfil\cr @d type(\char'43) == mem[\char'43].t\cr @d mtype == t \char'100\char'46{} y \char'100\char'46{} p \char'100\char'46{} e\cr @f mtype == type\cr @f type == true\cr}}$$ In the output of \.{TANGLE}, the macro \.{mtype} now produces `\.{TYPE}' and the macro \.{type(x)} now produces `\.{MEM[X].T}'. In the output of \.{WEAVE}, these same inputs produce \&{mtype} and \\{type}(\|x), respectively. 8. The \.{@f} feature allows you to define one identifier to act like another, and these format definitions are carried out sequentially, as the example above indicates. However, a given identifier has only one printed format throughout the entire document (and this format will even be used before the \.{@f} that defines it). The reason is that \.{WEAVE} operates in two passes; it processes \.{@f}'s and cross-references on the first pass and it does the output on the second. 9. You may want some \.{@f} formatting that doesn't correspond to any existing reserved word. In that case, \.{WEAVE} could be extended in a fairly obvious way to include new ``reserved words'' in its vocabulary. The identifier `\&{xclause}' has in fact been included already as a reserved word, so that it can be used to format the `\&{loop}' macro, where `\&{loop}' is defined to be equivalent to `\&{while \\{true} do}'. 10. Sometimes it is desirable to insert spacing into \PASCAL\ code that is more general than the thin space provided by `\.{@,}'. The \.{@t} feature can be used for this purpose; e.g., `\.{@t\\hskip 1in@>}' will leave one inch of blank space. Furthermore, `\.{@t\\4@>}' can be used to backspace by one unit of indentation, since the control sequence \.{\\4} is defined in \.{webmac} to be such a backspace. (This control sequence is used, for example, at the beginning of lines that contain labeled statements, so that the label will stick out a little at the left.) 11. \.{WEAVE} and \.{TANGLE} are designed to work with two input files, called \\{web\_file} and \\{change\_file}, where \\{change\_file} contains data that overrides selected portions of \\{web\_file}. The resulting merged text is actually what has been called the \.{WEB} file elsewhere in this report. Here's how it works: The change file consists of zero or more ``changes,'' where a change has the form `\.{@x}$\langle$old lines$\rangle$\.{@y}$\langle$% new lines$\rangle$\.{@z}'. The special control codes \.{@x}, \.{@y}, \.{@z}, which are allowed only in change files, must appear at the beginning of a line; the remainder of such a line is ignored. The $\langle$old lines$\rangle$ represent material that exactly matches consecutive lines of the \\{web\_file}; the $\langle$new lines$\rangle$ represent zero or more lines that are supposed to replace the old. Whenever the first ``old line'' of a change is found to match a line in the \\{web\_file}, all the other lines in that change must match too. Between changes, before the first change, and after the last change, the change file can have any number of lines that do not begin with `\.{@x}', `\.{@y}', or~`\.{@z}'. Such lines are bypassed and not used for matching purposes. This dual-input feature is useful when working with a master \.{WEB} file that has been received from elsewhere (e.g., \.{TANGLE.WEB} or \.{WEAVE.WEB} or \.{TEX.WEB}), when changes are desirable to customize the program for your local computer system. You will be able to debug your system-dependent changes without clobbering the master web file; and once your changes are working, you will be able to incorporate them readily into new releases of the master web file that you might receive from time to time. \section Appendices. The basic ideas of \.{WEB} can be understood most easily by looking at examples of ``real'' programs. Appendix~A shows the \.{WEB} input that generated modules 55--59 of the \.{WEAVE} program; Appendix~B shows the corresponding \TeX\ code output by \.{WEAVE}; and Appendix~C shows excerpts from the corresponding \PASCAL\ code output by \.{TANGLE}. The complete webs for \.{WEAVE} and \.{TANGLE} appear as the bulk of this report, in Appendices D and~E. The reader should first compare Appendix~A to the corresponding portion of Appendix~D; then the same material should be compared to Appendices B and~C. Finally, if time permits, the reader may enjoy studying the complete programs in Appendices D and~E, since \.{WEAVE} and \.{TANGLE} contain several interesting aspects, and since an attempt has been made in these appendices to evolve a style of programming that makes good use of the \.{WEB} language. Finally, Appendix F is the `\.{webmac}' file that sets \TeX\ up to accept the output of \.{WEAVE}; Appendix~G discusses how to use some of its macros to vary the output formats; and Appendix~H discusses what needs to be done when \.{WEAVE} and \.{TANGLE} are installed in a new operating environment. \section Performance statistics. The programs in Appendices D and E will optionally keep statistics on how much memory they require. Here is what they once printed out when processing themselves: \def\pstat#1#2#3 #4{\yskip\noindent\.{#1} applied to \.{#2} (cpu time #3 sec)\par \halign{\quad\tt##\hfil\cr#4}} \pstat{TANGLE}{TANGLE}{15} {Memory usage statistics:\cr 456 names, 215 replacement texts;\cr 3396+3361 bytes, 6685+7329+5805 tokens.\cr} \pstat{TANGLE}{WEAVE}{30} {Memory usage statistics:\cr 692 names, 339 replacement texts;\cr 4576+4294 bytes, 10184+9875+9150 tokens.\cr} \pstat{WEAVE}{TANGLE}{45} {Memory usage statistics: 478 names, 2045 cross references, 4159+3729 bytes;\cr parsing required 684 scraps, 1300 texts, 3766 tokens, 119 levels;\cr sorting required 34 levels.\cr} \pstat{WEAVE}{WEAVE}{65} {Memory usage statistics: 737 names, 3306 cross references, 4896+4962 bytes;\cr parsing required 684 scraps, 1300 texts, 3766 tokens, 119 levels;\cr sorting required 73 levels.\cr} \yskip\noindent The cpu time for \PASCAL\ to process \.{TANGLE.PAS} was approximately 13 seconds, and \.{WEAVE.PAS} took approximately 26 seconds; thus the tangling time was slightly more than the compiling time. The cpu time for \TeX\ to process \.{TANGLE.TEX} was approximately 500 seconds, and \.{WEAVE.TEX} took approximately 750 seconds (i.e., about 7 seconds per printed page, where these pages are substantially larger than the pages in a normal book). All cpu times quoted are for a DECsystem-10. \def\K{{\mc K}} The file \.{TANGLE.WEB} is about 125\K\ characters long; \.{TANGLE} reduces it to a file \.{TANGLE.PAS} whose size is about 42\K\ characters, while \.{WEAVE} expands it to a file \.{TANGLE.TEX} of about 185\K\null. The corresponding file sizes for \.{WEAVE.WEB}, \.{WEAVE.PAS}, and \.{WEAVE.TEX} are 180\K, 89\K, and 265\K. The much larger file \.{TEX.WEB} led to the following numbers: \pstat{TANGLE}{TEX}{110} {Memory usage statistics:\cr 3750 names, 1768 replacement texts;\cr 41895+41053 bytes, 42378+45074+41091 tokens.\cr} \pstat{WEAVE}{TEX}{270} {Memory usage statistics: 3412 names, 19699 cross references, 37900+40232 bytes;\cr parsing required 685 scraps, 1303 texts, 3784 tokens, 104 levels;\cr sorting required 52 levels.\cr} \yskip\noindent \PASCAL\ did \.{TEX.PAS} in about 75 seconds; \TeX\ did \.{TEX.TEX} in about 3600. % Here is a quotation that could not really be omitted \vfill {\baselineskip9pt \halign to\hsize{\hfil\quoteit#\tabskip 0pt plus 100pt& \hfil\quoteit#\tabskip 0pt\cr O, what a tangled web we weave& O, what a tangled WEB we weave\cr When first we practise to deceive!& When \TeX\ we practise to conceive!\cr \noalign{\vskip 2pt} \quoterm ---SIR WALTER SCOTT, {\quoteit Marmion} 6:17 (1808)& \quoterm ---RICHARD PALAIS (1982)\cr }} \eject \def\runninghead{APPENDIX A --- {\tentt WEB} FILE FORMAT} \section Appendix A. This excerpt from \.{WEAVE.WEB} produced modules 55--59 in Appendix~D. Note that some of the lines are indented to show the program structure. The indentation is ignored by \.{WEAVE} and \.{TANGLE}, but users find that \.{WEB} files are quite readable if they have some such indentation. \vskip 6pt \begingroup \def\tt{\eighttt} \baselineskip9pt % Note to myself: I had to remove SAIL characters from the file here! % Also tabs replaced by double-space. The changes were made in WEAVE source. \verbatim @* Searching for identifiers. The hash table described above is updated by the |id_lookup| procedure, which finds a given identifier and returns a pointer to its index in |byte_start|. The identifier is supposed to match character by character and it is also supposed to have a given |ilk| code; the same name may be present more than once if it is supposed to appear in the index with different typesetting conventions. If the identifier was not already present, it is inserted into the table. Because of the way \.{WEAVE}'s scanning mechanism works, it is most convenient to let |id_lookup| search for an identifier that is present in the |buffer| array. Two other global variables specify its position in the buffer: the first character is |buffer[id_first]|, and the last is |buffer[id_loc-1]|. @= @!id_first:0..long_buf_size; {where the current identifier begins in the buffer} @!id_loc:0..long_buf_size; {just after the current identifier in the buffer} @# @!hash:array [0..hash_size] of sixteen_bits; {heads of hash lists} @ Initially all the hash lists are empty. @= @!h:0..hash_size; {index into hash-head array} @ @= for h:=0 to hash_size-1 do hash[h]:=0; @ Here now is the main procedure for finding identifiers (and index entries). The parameter |t| is set to the desired |ilk| code. The identifier must either have |ilk=t|, or we must have |t=normal| and the identifier must be a reserved word. @p function id_lookup(@!t:eight_bits):name_pointer; {finds current identifier} label found; var i:0..long_buf_size; {index into |buffer|} @!h:0..hash_size; {hash code} @!k:0..max_bytes; {index into |byte_mem|} @!w:0..ww-1; {row of |byte_mem|} @!l:0..long_buf_size; {length of the given identifier} @!p:name_pointer; {where the identifier is being sought} begin l:=id_loc-id_first; {compute the length} @; @; if p=name_ptr then @; id_lookup:=p; end; @ A simple hash code is used: If the sequence of ASCII codes is $c_1c_2\ldots c_m$, its hash value will be $$(2^{n-1}c_1+2^{n-2}c_2+\cdots+c_n)\,\bmod\,|hash_size|.$$ @= h:=buffer[id_first]; i:=id_first+1; while i0 DO BEGIN IF(BYTESTART[P+2]-BYTESTART[P]=L)AND((ILK[P]=T)OR((T =0)AND(ILK[P]>3)))THEN{61:}BEGIN I:=IDFIRST;K:=BYTESTART[P];W:=P MOD 2; WHILE(IMAXBYTES THEN BEGIN WRITELN(TERMOUT); WRITE(TERMOUT,'! Sorry, ','byte memory',' capacity exceeded');ERROR; HISTORY:=3;JUMPOUT;END; IF NAMEPTR+2>MAXNAMES THEN BEGIN WRITELN(TERMOUT); WRITE(TERMOUT,'! Sorry, ','name',' capacity exceeded');ERROR;HISTORY:=3; JUMPOUT;END;I:=IDFIRST;K:=BYTEPTR[W]; WHILE I}', the backslash character gets in the way, and this entry wouldn't appear in the index with the T's. The solution is to use the `\.{@:}' feature, declaring a macro that simply removes a sort key as follows: $$\.{\\def\\9\#1\{\}}$$ Now you can say, e.g., `\.{@:TeX\}\{\\TeX@>}' in your \.{WEB} file; \.{WEAVE} puts it into the index alphabetically, based on the sort key, and produces the macro call `\.{\\9\{TeX\}\{\\TeX\}}' which will ensure that the sort key isn't printed. A similar idea can be used to insert hidden material into module names so that they are alphabetized in whatever way you might wish. Some people call these tricks ``special refinements''; others call them ``kludges''. \point 12. The control sequence \.{\\modno} is set to the number of the module being typeset. \point 13. If you want to list only the modules that have changed, together with the index, put the command `\.{\\let\\maybe=\\iffalse}' in the limbo section before the first module of your \.{WEB} file. It's customary to make this the first change in your change file. \point 14. To get output in languages other than English, redefine the macros \.{\\A}, \.{\\As}, \.{\\ET}, \.{\\ETs}, \.{\\U}, \.{\\Us}, \.{\\ch}, \.{\\fin}, and \.{\\con}. \.{WEAVE} itself need not be changed. \vfill\eject \def\runninghead{APPENDIX H --- GETTING STARTED} \section Appendix H: Installing the \.{WEB} system. Suppose you want to use the \.{WEB} programs on your computer, and suppose that you can't simply borrow them from somebody else who has the same kind of machine. Here's what to do: \yskip \def\step(#1){\par\hangindent 2em\noindent\hbox to 2em{\hfil(#1) }\ignorespaces} \step(1) Get a tape that contains the files \.{WEAVE.WEB}, \.{TANGLE.WEB}, \.{TANGLE.PAS}, and \.{WEBMAC.TEX}. The tape will probably also contain an example change file \.{TANGLE.CH}. \step(2) Look at the sections of \.{TANGLE} that are listed under ``system dependencies'' in the index of Appendix~E above, and figure out what changes (if any) will be needed for your system. \step(3) Make a change file \.{TANGLE.CH} that contains the changes of~(2); do not change your copy of \.{TANGLE.WEB}, leave it intact. (The rules for change files are explained at the end of the manual just before the appendices; you may want to look at the example change file that arrived with your copy of \.{TANGLE.WEB}. It's also a good idea to define all the ``switches'' like \&{debug} and \&{gubed} to be null in your first change files; then you can sure that your compiler will handle all of the code.) \step(4) Make the changes of (2) in your copy of \.{TANGLE.PAS}. (If these changes are extensive, you might be better off finding some computer that that already has \.{TANGLE} running, and making the new \.{TANGLE.PAS} from \.{TANGLE.WEB} and your \.{TANGLE.CH}.) \step(5) Use your \PASCAL\ compiler to convert your copy of \.{TANGLE.PAS} to a running program \.{TANGLE}. \step(6) Check your changes as follows: Run \.{TANGLE} on \.{TANGLE.WEB} and your \.{TANGLE.CH}, yielding $\.{TANGLE.PAS}'$; make a running program $\.{TANGLE}'$ by applying \PASCAL\ to $\.{TANGLE.PAS}'$; run $\.{TANGLE}'$ on \.{TANGLE.WEB} and your \.{TANGLE.CH}, yielding $\.{TANGLE.PAS}''$; and check that $\.{TANGLE.PAS}''$ is identical to $\.{TANGLE.PAS}'$. Once this test has been passed, you have got a working \.{TANGLE} program. \step(7) Make a change file \.{WEAVE.CH} analogous to (3), but this time consider the system-dependent parts of \.{WEAVE} that are listed in the index to Appendix~D. \step(8) Run \.{TANGLE} on \.{WEAVE.WEB} and your \.{WEAVE.CH}, obtaining \.{WEAVE.PAS}. \step(9) Use \PASCAL\ on \.{WEAVE.PAS} to make a running \.{WEAVE} program. \step(10) Run \.{WEAVE} on \.{TANGLE.WEB} and \.{TANGLE.CH} to produce \.{TANGLE.TEX}. \step(11) Run \TeX\ on \.{TANGLE.TEX}, obtaining a listing analogous to Appendix~E. This listing will incorporate your changes. \step(12) Run \.{WEAVE} on \.{WEAVE.WEB} and your \.{WEAVE.CH} to produce \.{WEAVE.TEX}. \step(13) Run \TeX\ on \.{WEAVE.TEX}, obtaining a listing analogous to Appendix~D that incorporates your changes. \yskip\noindent This description assumes that you already have a working \TeX82 system. But what if you don't have \TeX82? Then you start with a tape that also contains \.{TEX.WEB} and \.{plain.tex}, and you refer to a hardcopy listing of the \TeX82 program corresponding to \.{TEX.WEB}. Between steps (10) and (11) you do the following: \yskip \def\substep(10.#1){\par\hangindent 4em\noindent \hbox to 4em{\hfil(10.#1) }\ignorespaces} \substep(10.1) Make a change file \.{TEX.CH} to fix the system dependent portions of \.{TEX.WEB}, in a manner analogous to step~(2). Since \TeX\ is a much more complex program than \.{WEAVE} or \.{TANGLE}, there are more system-dependent features to think about, but by now you will be good at making such modifications. Do not make any changes to \.{TEX.WEB}. \substep(10.2) Make an almost-copy of your \.{TEX.CH} called \.{INITEX.CH}; this one will have the `\&{init}' and `\&{tini}' macros redefined in order to make the initialization version of \TeX. It also might have smaller font memory and dynamic memory areas, since \.{INITEX} doesn't need as much memory for such things; by setting the memory smaller in \.{INITEX}, you guarantee that the production system will have a ``cushion.'' \substep(10.3) Run \.{TANGLE} on \.{TEX.WEB} and \.{INITEX.CH}, obtaining \.{INITEX.PAS} and \.{TEX.POOL}. \substep(10.4) Run \PASCAL\ on \.{INITEX.PAS}, obtaining \.{INITEX}. \substep(10.5) Run \.{INITEX} on \.{TEX.POOL}, during which run you type `\.{plain}' and `\.{\\dump}'. This will produce a file \.{plain.fmt} containing the data needed to initialize \TeX's memory. \substep(10.6) Run \.{TANGLE} on \.{TEX.WEB} and the \.{TEX.CH} of (10.1), obtaining \.{TEX.PAS}. \substep(10.7) Run \PASCAL\ on \.{TEX.PAS}, obtaining \.{VIRTEX}. \substep(10.8) If your operating system supports programs whose core images have been saved, run \.{VIRTEX}, type `\.{\&plain}', then save the core image and call it \TeX. Otherwise, \.{VIRTEX} will be your \TeX, and it will read `\.{plain.fmt}' (or some other \.{fmt} file) each time you run. \yskip This 21-step process may seem long, but it is actually an oversimplification, since you also need fonts and a way to print the device-independent files that \TeX\ spews out. On the other hand, the total number of steps is not quite so large when you consider that \.{TANGLE}-followed-by-\PASCAL\ and \.{WEAVE}-followed-by-\TeX\ may be regarded as single operations. If you have only the present report, not a tape, you will have to prepare files \.{WEAVE.WEB} and \.{TANGLE.WEB} by hand, typing them into the computer by following Appendices D and E. Then you have to simulate the behavior of \.{TANGLE} by converting \.{TANGLE.WEB} manually into \.{TANGLE.PAS}; with a good text editor this takes about six hours. Then you have to correct errors that were made in all this hand work; but still the whole project is not impossibly difficult, because in fact the entire development of \.{WEAVE} and \.{TANGLE} (including the writing of the programs and this manual) took less than two months of work. \vfill\end