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diff --git a/Master/texmf-dist/doc/latex/dialogl/codialog.tex b/Master/texmf-dist/doc/latex/dialogl/codialog.tex new file mode 100644 index 00000000000..75d144f20d9 --- /dev/null +++ b/Master/texmf-dist/doc/latex/dialogl/codialog.tex @@ -0,0 +1,2461 @@ +%%% ==================================================================== +%%% @LaTeX-file{ +%%% filename = "codialog.tex", +%%% version = "1.1a", +%%% date = "2-Dec-1994", +%%% time = "15:24:21 EST", +%%% checksum = "06649 2461 14834 104289", +%%% author = "Michael Downes", +%%% address = "49 Weeks Street +%%% North Smithfield, RI 02986, USA", +%%% email = "mjd@math.ams.org (Internet)", +%%% docstring = "The checksum field above contains: CRC-16 +%%% checksum, number of lines, number of words, and number of +%%% characters, as produced by Robert Solovay's checksum utility.", +%%% } +%%% ==================================================================== +\documentstyle[dialogl% +,draft% +]{article} + +\title{Carrying on Dialog with \TeX} +\author{Michael J. Downes\\ +49 Weeks Street\\ +North Smithfield, RI 02895\\ +E-mail: {\tt mjd@math.ams.org}} +\date{November 1994} + +\hyphenation{ams-tex back-slash-char base-line-skip circle circlew + cnv-units con-trol-seq end-line-char err-help err-mes-sage + error-con-text-lines escape-char Hel-vet-ica hyphen-char + hyphen-pen-alty include-only lcircle lcirclew new-line-char + open-out par-fill-skip report-math-code show-box-depth show-prog-ress + show-hyphens tables} +\allowtthyphens + +\typeout{} +\typeout{% +--- This document can be printed in landscape or portrait orientation;} +\typeout{% +--- landscape is recommended for better presentation of the examples.} +\typein[\answer]{% +--- If you need portrait, enter P; otherwise, just press <return> now:} + +\def\p{p}\def\P{P} +\ifx\p\answer \let\answer=\P\fi +\ifx\P\answer + \portraitlayout +\else + \defaultlayout % defined in dialogl.sty +\fi + +\begin{document} + +\maketitle +\thispagestyle{empty} + +\tableofcontents + +% If we don't have at least half of the current column open, go to +% the next page. +\reqspace{.5\textheight} + +\section{Introduction} + +A common task in any programming language is to send a question to the +user, and to read (and act on) the user's response. In \tex/, this +usually involves the \cw{read}, \cw{message}, and \cw{write} commands. +The use of these commands, however, is beset at every turn by odd +hindrances and technical stumbling blocks, so that even experienced +macro writers, faced with an application that requires a bit of +dialog, usually find it troublesome to make that bit of dialog +good-looking, reliable, convenient for the user, and tolerant of +typical human mistakes such as minor mistyping in a response. The +purpose of this article is to analyze the capabilities that \tex/ has +for dialog and survey all the best relevant macro-writing techniques +that fall within the scope of my experience and research.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{This is an overhauled and amplified version of my paper +``Dialog with \tex/'' in {\sl TUGboat}, vol~12 no~3, Part 2, December +1991 (proceedings of the 1991 TUG conference in Dedham, +Massachusetts).} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + +In \texbook/, near the end of Chapter 20, Knuth writes + \begin{quote} It's easy to have dialogs with the user, by using + \cw{read} together with the \cw{message} command + \end{quote} +and there follows a brief example involving reading the user's name +into a macro \verb"\myname". It's clear from this passage that Knuth +means, by \term{dialog}, ordinary communication between computer programs +and users at the early-1980s level of technology: printing +character-based information on a video screen, displaying menus, asking +questions, and handling user responses. (Speech recognition and voice +synthesis are not part of the picture for most \tex/ users, yet.) For the +purposes of this article, I define \term{dialog} as any communication +between \tex/ and the user that takes place while \tex/ is running. +Forms of communication that do not take place while \tex/ is running +are excluded: for example, the black box that \tex/ prints to indicate +an overfull line in a paragraph is useful information, but not dialog +because the communication occurs after \tex/ has stopped running.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{This distinction is blurring, however, with +the advent of software like Blue Sky Research's Lightning Textures.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +On the other hand, the {\tt Overfull \string\hbox} message +printed on screen whenever \tex/ adds a black box to the +current paragraph {\em is} dialog, because it does occur while \tex/ is +running. + +Since the entire section of \texbook/ where Knuth's dialog example appears +is marked off with double dangerous bend signs, it seems that Knuth +didn't intend his words ``It's easy'' to be taken completely +literally---particularly when we look at the next thing in that +section, Exercise 20.18, which reads, + \begin{quote} + The \cw{myname} example just given doesn't work quite right, because the + $\langle$return$\rangle$ at the end of the line gets translated + into a space. Figure out how to fix that glitch. + \end{quote} +That line-ending space is only one of a number of complications that +can hamper the efforts of macro writers to write dialog for practical +applications. In fact, the word {\em easy} is far from the first +adjective that comes to mind when I remember my own early attempts at +writing dialogical macros. Though my thrashing and floundering for +the most part took place behind the scenes, invisible to others, it +eventually reached the point of threatening my secret belief that I +was a hot-shot macro writer. That spurred me to start paying special +attention to anything related to the idea of dialog in \tex/, and +accumulating scraps and pieces of assorted useful techniques. This +article is more or less a survey of what I've learned so far. +Sections \ref{s:messages} and~\ref{s:receiving} review the functions +\tex/ provides to support dialog; sections~\ref{s:stumblewrite} +and~\ref{s:stumbleread} discuss common difficulties and how to handle +them. + +\subsection{Terminology} +Rather than assume all readers know well enough the meaning of terms +like \term{primitive}, \term{token}, or \term{control word} that will +be bandied about hereinafter, I offer a quick review of some standard +\tex/ terminology, to aid those who want it, and to be skipped by the +rest of you. + +A \tex/ command is either a \term{control sequence}---a string of +characters starting with an \term{escape character}---or a single +\term{active character}, such as \verb|~|. The usual escape character +is the \term{backslash}, \verb|\|. A control sequence that consists of +a backslash plus one nonletter character is called a \term{control +symbol}; a control sequence that consists of a backslash plus one or +more letters is called a \term{control word}. Spaces are ignored after +a control word, but not after a control symbol. A control sequence is +either a \term{\tex/ primitive}---a command built into the \tex/ +program---or a macro: a composition of primitives or other macros, +defined by the user or by a \term{macro package} such as \latex/ or +\plaintex/. A control sequence may require one or more following +\term{arguments}; an argument is a piece of text that is grabbed up by +the control sequence in order to do something with it. Arguments +typically are enclosed in \term{curly braces} \verb|{| and \verb|}|. +For example, the command \cw{sqrt} is a macro with one argument, and +is used thus: \verb|\sqrt{x^2+y^2}|, producing the printed output +$\sqrt{x^2+y^2}$. + +Macros are \term{expandable}; some primitives are +expandable, more are not. An expandable control +sequence will be replaced by its expansion if it is used inside the +argument of a \cw{message} command, or anywhere else where \tex/ is +in an expansive mood (\cw{write}, \cw{errmessage}, \cw{edef}, +\cw{xdef}, \cw{mark}, \cw{special} [\texbook/, p216]). + +The term \term{parameter} is used to mean a numeric or dimensional +variable such as \cw{hyphenpenalty}, \cw{hsize}, or \cw{baselineskip}. A +\term{token} is eithera character (with associated catcode) or a control +sequence, {\em after\/} it has been read by \tex/ from some file and +entered into \tex/'s active memory. Character tokens can only have +category codes 1--4, 6--8, or 10--13; there's no such thing as a +`character token' with category code 0, 5, 9, 14, or 15: those catcodes +only control the process of creating tokens, they aren't designed for +permanent association to a token. + +Under normal circumstances, each line in a file is understood by +\tex/ to have a \ctrl{M} character, {\sc ascii} 13, at the end of it, +even if your text editor actually puts some other character, or +no character, at the end of a line when you press the \return/ or +{\sc enter} key. + +\subsection{Basic dialog principles} +It's not hard to identify a number of principles that make for good +dialog: +\begin{enumerate} +\item When asked a yes/no question, users should +be able to enter \verb|y|, \verb|yes|, or even \verb|ye|, +in lowercase, uppercase, or even mixed case, and have the +answer understood to be ``yes''. + +\item For any menu or question, a default answer +should be provided (when this makes sense), and the default +answer should be made as easy as possible to select. + +\item Users' answers should be repeated back to them, +to allow them to verify that the program's impression +of the answer entered by the user is indeed correct. + +\item Users should be given a chance to undo mistakes, +\eg., by going back to a specified point earlier in the +dialog and starting over from there. For example, it shouldn't be necessary to +stop \tex/ and restart just to fix a typing error. + +\item When practical, users' answers should be checked to make sure +they're not nonsense; for example, if a program requests an integer, +it should check the response to make sure the user didn't enter +something else entirely, rather than assume an integer was entered and +start to perform operations on it. In \tex/ this would create a risk +of losing control to low-level errors such as +{\tt Missing number, treated as~0} or {\tt Arithmetic overflow}. + +\item Information given to users should be provided in the ``best +possible form'', where the meaning of ``best possible'' must be +determined by common sense from the circumstances of a particular +application and the targeted user group. For example, a +straightforward use of the \cw{the} command to report the value of a +\tex/ dimension parameter such as \cw{vsize} to the user will produce +the value in points, down to five or six decimal places. It will +normally be more useful to report the value rounded to the nearest +whole point, or to report it in picas, inches, or +centimeters---whatever is most convenient for the user. A +typographical designer or compositor would probably prefer picas, +while someone with little knowledge of typography would probably +prefer inches or centimeters. + +\end{enumerate} + +\section{\tex/'s Message-Sending Capabilities} +\label{s:messages} + +Table~\ref{t:sending} lists the various means in \tex/ for sending +messages to the user. + +\begin{table*} +\vspace{-\medskipamount} % needed because \caption has wrong spacing + % when used at the top rather than the bottom + % of a float environment. +\caption[]{\label{t:sending}Commands that can be used for +sending messages} +\writepage{Table} +\vskip1pc +\def\hline{\noalign{\vskip3pt\hrule\vskip2pt\relax}} +\tabskip0pt +\halign to\hsize{#\hfil\tabskip1em plus.2\hsize minus0.7em& + #\hfil& \parbox[t]{.6\hsize}{\rightskip0pt plus2em\relax#}\cr +\hline +Command& Example& Result\cr +\hline +\cw{message}& \verb'\message{Hey you}'& {\tt ... Hey you ...}\cr +\hline +\cw{write}& \verb'\immediate\write{Hey you}'& {\tt ...\newline +Hey you\newline +...}\cr +\hline +\cw{errmessage}& \verb'\errmessage{Hey you}'& +{\tt !\ Hey you.\newline +l.217 \string\errmessage\string{Hey you\string}}\cr +\hline +\cw{show}& \verb'\show\footnote'& +{\tt> \string\footnote=macro:\newline +->\string\@ifnextchar\space [\string{\string\@xfootnote\space +\string} ...\newline +l.218 \string\show\string\footnote}\cr +\hline +\cw{showthe}& \verb'\showthe\textwidth'& +{\tt> 570.93257pt.\newline +l.219 \string\showthe\string\textwidth}\cr +\hline +\cw{showbox}& \verb'\showbox 0'& +{\tt> \string\box0=\newline +\string\hbox(0.0+0.0)x15.0\newline +\null\newline +!\ OK.\newline +l.220 \string\showbox 0}\cr +\hline +\cw{showlists}& \verb'\showlists'& +\tt +\string#\string#\string# vertical mode entered at line 0\newline +\string#\string#\string# current page:\newline +\string\glue(\string\topskip) 3.75\newline +...\newline +total height 403.47491 plus 14.64996 minus~8.77498\newline + goal height 751.60756\newline +prevdepth 0.0, prevgraf 2 lines\newline +\null\newline +! OK.\newline +l.221 \string\showlists\cr +\hline +\crcr} +\end{table*} + +Although it could be argued that the token register \cw{errhelp} is +another way of sending a message, it is excluded from +Table~\ref{t:sending} on the grounds that it is passive rather than +active, unlike the other commands listed. To put it another way, +\cw{errhelp} is merely a storage area associated with \cw{errmessage}, +where auxiliary text can be placed; the user won't ever see +\cw{errhelp} except by way of \cw{errmessage}. + +\subsection{The \cw{message} primitive}\label{ss:message} + +The \cw{message} command is a \tex/ primitive that prints its argument +on screen. If the current screen position is not at the beginning of +a line, \tex/ will add a blank space at the beginning of the message +text to separate it from the preceding material---except that if there +isn't enough room on the current line to fit the entire message text, +then \tex/ will go to the next line before starting to print the +message, and not add an extra blank at the beginning. The maximum +length of message lines is controlled by the constant {\it +max\underscore print\underscore line}, which is compiled into \tex/; +the normal value is 79. (In a windowing environment the width of the +current window may also affect the maximum length of message lines.) + +Thus one way to force a message to start on a new line is to add lots +of \cw{space}'s at the end. But a better way to start a message on a +new line, or break up a long message into lines, is to indicate line +breaks with the current \cw{newlinechar} character. For example, +we can set the newline character to be \verb|+| and use it in +a message as follows: +\def\next{\begin{verbatim}\string\begingroup{ }\string\newlinechar=`+% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{Use of \cw{+} instead of {\tt+} here would normally be +recommended but the outerness of \cw{+} in \plaintex/ makes this an +exception.}% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\endgraf}% +\next +\message{+This is a+three-line+message ...} +\endgroup +\end{verbatim} + which produces on screen (regardless of the length of any immediately +preceding message) +\begin{verbatim} +This is a +three-line +message ... +\end{verbatim} + In any one message, a given character +can either produce newlines, or represent itself, but not both. +As a consequence, if we wanted a plus character in a message +to actually print on screen instead of causing a line break, +we would have to set \cw{newlinechar} to some other value +before sending the message. From this knowledge it's a short step +to the insight that for general message-sending purposes it would +be convenient to set \cw{newlinechar} to the character that +is least likely to be needed in a message text. The nonprinting +ASCII characters in the range 0--31 are obvious candidates. + +But here we encounter an inconvenient idiosyncrasy of \tex/: A control +character---such as control-J, or \verb|^^J| (using \tex/'s double +caret notation), which is the default value for \cw{newlinechar} in +\amstex/ and \latex/---doesn't ever produce line breaks +in a \cw{message}, even if it is currently selected as +\cw{newlinechar}.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{Or rather, it didn't up until version 3.141 or so of \tex/. +See also the mention of em\tex/'s {\tt/r} option (which allows you to +use control characters for output purposes) in \secref{ss:8bit}.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +Instead, it will always be printed as three characters using the +double caret convention.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{This suggests the following experiment: set +\cw{newlinechar=`\string\^} and send a \cw{message} containing a +\ctrl{J} character.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +Therefore, if you want to use \cw{message} as your normal +message-sending function, you should choose one of the seldom-used +printable characters as your default \cw{newlinechar}. One possibility +would be to use the double quote character for this purpose, since +single quote characters can normally be substituted +for double quote characters in message texts. \fn{Testfont.tex} +\cite{testfont} uses the {\tt @} character. + +Unlike \cw{message}, however, the \cw{write} command is capable of +using a control character as a newline character (see +\S\ref{ss:write}). By using \cw{write} for multiline messages, and +making \verb|^^J| the default newline character, \amstex/ and +\latex/ avoid taking any of the printable characters out of +circulation for dialog purposes. + +If no line breaks are indicated in a \cw{message} that is longer than +{\it max\underscore print\underscore line}, \tex/ will introduce +arbitrary breaks at the screen column equal to {\it max\underscore +print\underscore line}, which usually means + \[\settowidth{\dimen0}{\tt random brea} +\parbox{\dimen0}{\tt +random brea\\ +ks in the m\\ +iddle of wo\\ +rds.}\] +Those of you who use \cw{tracingmacros} will have noticed that its +output also has line breaks like this. Which can make it rather +difficult to search for instances of a given string in the trace log; +to find all instances of {\tt xyz} you need a regular expression +something like +\begin{verbatim} +x\n?y\n?z +\end{verbatim} +(\verb'\n' meaning `newline', {\tt?\@} meaning 0 or 1 occurrences of +the preceding subpattern). + +\subsection{The \cw{write} primitive} +\label{ss:write} +The \cw{write} command, like \cw{message}, just prints a message. But +the message doesn't necessarily appear on screen, because +communication with the user is not the purpose for which +\cw{write} was originally designed: Its initial purpose was to send index or +table of contents information, including the associated page number, +to an auxiliary file for later processing. Because this kind of use is +closely linked to page numbering, \cw{write} commands on the current +page are normally saved up to be executed when the page is actually +shipped out, after the page break has been determined. If such +postponement is not wanted, \cw{write} must be used with the +\cw{immediate} prefix. + +In order to allow intersequential writing to different output files, +the \cw{write} command takes an extra first argument, a number +between $-1$ and $16$ inclusive, to indicate the output file to which +the text should be sent. Output files 0--15 can be associated with a +particular file on your system by the \cw{openout} command; output +file $-1$ is the \tex/ log file, and output file $16$ is the user's +terminal screen (echoed in the log file as well). + +Line breaks in the argument of a \cw{write} command can be obtained by +inserting \cw{newlinechar} characters; unlike \cw{message} +(\secref{ss:message}) and \cw{errmessage} (\secref{ss:errmessage}), +\cw{write} will always start a new line for each newline character, +even when it is a control character such as \verb|^^J|. Also, the text +of a \cw{write} command always starts on a new line and finishes on a +new line. The existence of the final newline may be observed in the +on-screen result of a \cw{message} following a \cw{write}: the message +text will always start on the next line regardless of the total length +of the \cw{message} and \cw{write} texts, whereas a \cw{message} +following another \cw{message} or one of \tex/'s internally generated +messages (such as input file names) will not start on a new line +unless there isn't enough room remaining on the current line. + +Corollary: If you prompt the user for some input and you want the +user's input to appear on the same line as the prompt text, use +\cw{message} instead of \cw{write} to send the prompt text---or at +least the last line of the prompt text. + +\subsubsection{Nonimmediate \cw{write} messages} +Sometimes it's useful to leave off +the \cw{immediate} prefix of a \cw{write} command +even when not writing information to an index file or +table of contents file: For instance, if you are working on page breaks in +a long document and want to find out, without previewing or printing, +if a nonforcing pagebreak command had the effect that you wanted, you +could insert a nonimmediate \cw{write16} just before and just after the +intended page break: + \begin{verbatim} +\write16{Before the attempted pagebreak.} +\penalty-9999 +\write16{After the attempted pagebreak.} +\end{verbatim} + The message from a nonimmediate \cw{write16} will appear before the +closing \verb|]| of the \verb|[|\thinspace\nobreak\verb|]| pair that enclose the relevant +page number. So if all went well, one of the above messages will +appear with one page number and the next message with the +next page number, like this: + \begin{verbatim} +[4] [5 +Before the attempted pagebreak. +] [6 +After the attempted pagebreak. +] [7] [8] [9] ... +\end{verbatim} + +In producing this article (using \latex/) I had some trouble getting +good placement for the floating tables and examples; to help me +experiment, I added some code that would print on screen the page +numbers. At the beginning of Example~2 (for example) there is +a line that says +\begin{verbatim} +\write16{Example 2: Page \thepage}% +\end{verbatim} +The \cw{immediate} prefix must be omitted in order to +get the page number correct. + +\subsection{The \cw{errmessage} primitive} +\label{ss:errmessage} +The \cw{errmessage} command prints its argument on screen, starting on a +new line, with an exclamation point and a space added at the beginning, +and a period added at the end. For example, +\cw{errmessage}\verb|{Surprise}| produces +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\begin{verbatim} +! Surprise. +\end{verbatim} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + on screen. \cw{errmessage} also shows the {\em current context}, +which means the current line from the current input file, along with +the line number, and additional information if there is any (such as +the surrounding parts of current macro expansions).% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{If the parameter \cw{errorcontextlines} is set high enough.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +So the {\tt Surprise} error message would show additional information +on screen. Suppose we define +\begin{verbatim} +\def\test{\errmessage{Surprise}\relax} +\end{verbatim} +Then the additional information will look something like: +\begin{verbatim} +! Surprise. +\test ->\errmessage {Surprise} + \relax +l.454 \test + and some more text. +? +\end{verbatim} +This example may be interpreted as follows: Line 454 of the current +file consists of +\begin{verbatim} +\test and some more text. +\end{verbatim} +The line break in the context listing +means that \tex/ is processing \cw{test} and has not yet +started to typeset the word `\verb"and"'. +Above the \verb"l.454", the expansion +of \cw{test} is shown. The line break after \verb"{Surprise}" +indicates that \tex/ has not yet executed the \cw{relax} command. + + The behavior of \cw{errmessage} with respect to newline characters +and control characters is the same as for \cw{message}---i.e., +\cw{errmessage} will start a new line for each \cw{newlinechar} in +its argument [unless the current value of +\cw{newlinechar} is outside the visible {\sc ascii} range +32--126 and the version of \tex/ is less than 3.141]. + +At the end of an error message, the user is presented with +a question-mark prompt, and a choice of several possible responses. These +will be discussed later in the section on \tex/'s capabilities +for receiving user input (\S\ref{s:error-recov}). + +\subsection{The \cw{show} and \cw{showthe} primitives} + +The \cw{show} command, used for showing the current meaning of a +control sequence (or indeed of any token), is rather similar to +the \cw{errmessage} command in what it produces on screen. The +prefix is a greater-than character instead of an exclamation +point. Here's the result of \verb|\newcount\C| \verb|\show\C|: + \begin{verbatim} +> \C=\count78. +l.1 \newcount\C \show\C + +? +\end{verbatim} + As with \cw{errmessage}, \tex/ displays the surrounding context of a +\cw{show} command; it also offers the same question-mark +prompt with the same range of possible responses (well, almost---the +\verb|H| option only gives a generic help message about \cw{show}, not +specific help about the item being shown.). + +The \cw{showthe} command is like \cw{show}, but is applied +to certain kinds of things such as count registers and +token registers, that have not only a meaning but also a current value. +For instance, here's the result of \verb|\C=5| \verb|\showthe\C|, using the +counter defined above: + \begin{verbatim} +> 5. +l.3 \C=5 \showthe\C + +? +\end{verbatim} + +\subsection{The \cw{showbox} and \cw{showlists} primitives} The +commands \cw{showbox} and \cw{showlists} are similar to \cw{show} in +what they produce on screen (see Table~\ref{t:sending}). Because of +their specialized nature they don't ordinarily have much application +in dialog between \tex/ and the user. + +\subsection{Piggybacking} +Many messages printed on screen by \tex/ do not +involve any of the commands listed in Table~\ref{t:sending}. +These other messages +are emitted directly by \tex/, outside the control of +the macro writer. However, with a little imagination, +you can often find ways to attach useful information +to those ``inaccessible'' messages. This is what I mean +by \term{piggybacking}. + +For example, whenever \tex/ inputs a file, a message is printed on +screen containing the name of the file, enclosed in parentheses. So +one way to send a short message would be to create an empty file +whose name was equal to the desired message, and then input the file. + +File name messages, apart from the parentheses, behave the same as +messages produced by the \cw{message} command: in particular, a file name +message will be appended to the current line, with +a preceding space, unless the length of the +message (including the two parentheses) will cause it +to cross the {\it max\underscore print\underscore line} boundary. + +If your computer system allows longer file names +you could actually get pretty fancy with a filename message. +For example, on a Unix system suppose you have a file named +\fn{test.tex} whose contents are +\begin{verbatim} +\newwrite\msgfile +\immediate\openout\msgfile=Fred.your.fly.is.open +\immediate\write\msgfile{\relax} +\immediate\closeout\msgfile +\input Fred.your.fly.is.open +\end +\end{verbatim} +When \fn{test.tex} is processed by \tex/ the screen output will be +something like: +\begin{verbatim} +This is TeX, C Version 3.1 (format=plain 91.1.4) ... +(test.tex (Fred.your.fly.is.open) ) +No pages of output. +\end{verbatim} +However, getting spaces in the message would be problematic since +\tex/ treats a space as a file name terminator. And if your message +doesn't include a period, \tex/ is likely to add \fn{.tex} at the end +of the file name when \cw{openout} is invoked. Not to mention that +this method would quickly lead to inconvenient file clutter since +\tex/ can't delete files, only create new files or change the contents +of pre-existing ones. + +Or consider the count registers 1--9; if any of +these is nonzero, its value is reported on screen whenever \tex/ +ships out a page. One way of using this feature might be to report +the accumulation of index terms for a document, by having each index command +increment count register number~1: +\begin{verbatim} +\countdef\indexcount=1 +\def\index{... + \global\advance\indexcount 1 ...} +\end{verbatim} +Incrementing count~1 like that would result in \tex/ displaying on +screen something like +\begin{verbatim} +[1.2] [2.7] [3.14] [4.15] [5.27] [6.38] ... +\end{verbatim} +instead of the more usual +\begin{verbatim} +[1] [2] [3] [4] [5] [6] ... +\end{verbatim} +as each page is shipped out. + +These two examples don't seem extremely practical---the total number of +index terms reported in the latter example won't necessarily be correct +page for page, until the end of the document, because ordinary means +for incrementing the counter are immediate in their effect rather than +synchronized with the \cw{write} commands used for creating the index +file---but the point is to realize that the messages coming out of +\tex/'s innards aren't totally beyond reach, and they can sometimes +provide a better way of achieving a given result than ordinary methods. +A couple of better-known examples, from the hands of Donald E. Knuth, can +be found in the \cw{showhyphens} command and in {\tt hyphen.tex} at the +point where \cw{patterns} is called. + +\subsubsection{The \cw{showhyphens} command} +The \cw{showhyphens} command (defined in {\tt plain.tex}) +works by exploiting \tex/'s messages about underfull boxes. +When an underfull line +of a paragraph is reported, \tex/ prints on screen the elements of that +line, including any discretionary hyphens inserted by \tex/ +while attempting to find good line breaks. +The key insight for thinking up the \cw{showhyphens} +command is to realize that if you can typeset a one-line +paragraph, and make sure that the line is underfull, then +any word in that line will have its hyphenation points +displayed on screen. That's exactly what Knuth +defined \cw{showhyphens} to do: +\begin{verbatim} +\def\showhyphens#1{\setbox0\vbox{\parfillskip\z@skip + \hsize\maxdimen \tenrm \pretolerance\m@ne \tolerance\m@ne + \hbadness0\showboxdepth0\ #1}} +\end{verbatim} +The settings of \cw{parfillskip} and \cw{hsize} ensure that the paragraph will +be all on one line, and underfull. (For extra bullet-proofing, +\begin{verbatim} + \leftskip\z@skip \rightskip\z@skip +\end{verbatim} +should probably be included too.) + +The switch to font \cw{tenrm} makes sure---or at least reasonably +sure---that the current font is not one for which hyphenation has been +inhibited by setting \cw{hyphenchar} to an out-of-range value. The +settings of \cw{pretolerance} and \cw{tolerance} ensure that +hyphenation points will be added (in making up a paragraph, \tex/ +normally tries first to get by without adding hyphenation points, if +it can find good line breaks using only the stretchability and +shrinkability of interword glue). The setting of \cw{hbadness} ensures +that an {\tt Underfull \string\hbox} message will be sent (otherwise, +if the surrounding environment had $\hbox{\cw{hbadness}}=10000$ when +\cw{showhyphens} was called, the message would be suppressed). +The setting of \cw{showboxdepth} limits the information in the +message to top level; otherwise compound structures like accented +letters or special composites (\eg. \cw{AA}) would be shown in +full detail---more detail than the user normally wants to see. +And finally the \verb*+\ + command forces entry into horizontal mode +and, more importantly, provides a glue item before the first word, +without which it would not be hyphenated (see the rules by which +\tex/ looks for hyphenatable words, \texbook/, Appendix~H). + +Note that the values of \cw{language}, \cw{lefthyphenmin}, and +\cw{righthyphenmin} are inherited from context; and this is probably +what you want for the \cw{showhyphens} command (perhaps indeed the +current font should also be inherited from context instead of being set +always to \cw{tenrm}). + +\subsubsection{Using error context to send messages} +\label{ss:errcontextcomments} +The standard \fn{hyphen.tex} containing U.S. English hyphenation +patterns has a comment after the \cw{patterns} command:% + \begin{verbatim} +\patterns{ % just type <return> if you're not using INITEX +\end{verbatim} + Ordinarily the macro writer can't use comments to communicate +with the user, because comments within the text of a macro +are discarded by \tex/ +as the macro is defined. The beauty of the comment in \fn{hyphen.tex} +is that it appears precisely when needed, because of the +way \tex/ displays context with error messages: if you \cw{input} +\fn{hyphen.tex} when not using {\sc initex}, \tex/ will give an +error message when it encounters the \cw{patterns} command, +and as usual, will show the context around the point of the +error, like this: +\par\reqspace{3\baselineskip} + \begin{verbatim} +! Patterns can be loaded only by INITEX. +l.2 \patterns + { % just type <return> if you're not using INITEX +? +\end{verbatim} +Since learning this technique, I've had occasion more than once to +apply it in similar situations. One such application had to do with +the \latex/ circle fonts. I had an assignment to create a +\latex/ documentstyle whose installation procedures involved +rebuilding the \latex/ format file, which meant calling +for the \fn{.tfm} files of the circle fonts. +However, there was at that time (1990--1991) a bit of +confusion surrounding the names; the trend among distributors of \tex/ +appears to be away from the original names \fn{circle10} +and \fn{circlew10} and toward the names +\fn{lcircle10} and \fn{lcirclew10}. +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%\footnote{Starting with the standard Unix distribution of \tex/, +%from the University of Washington.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +The \fn{.tfm} files are the same under either names, but +I had to deal with the possibility that some users of the +documentstyle I was working on would have the fonts under the older +names, while others would have them under the new names. + +My solution was to use the newer names \fn{lcircle10} and \fn{lcirclew10} +and put comments on the same lines as the font assignments, so that +the comments would appear to the user if \tex/ were unable +to find the \fn{.tfm} files and emitted an error message. + \begin{verbatim} +\font\tencirc= +lcircle10\relax% Type x to exit; see sei.prl for further info. +\end{verbatim} +Additional comments in the file \fn{sei.prl} explained +how to change the font names to their older variants. +The \cw{relax} is necessary so that \tex/ won't proceed to +the next line, bypassing the comment, in the process of looking +for a modifier such as \verb|scaled| or \verb|at|. +And the reason for the line break after the equals sign is to fit a +few more characters in the comment, which would be elided by \tex/ +if too long. +\section{Ways for \tex/ to receive messages from the user} +\label{s:receiving} + +Table~\ref{t:receiving} lists the various means in \tex/ for reading +input from the user. The primary input facility is the \cw{read} +command; the others are special cases applicable only under limited +circumstances. + +\begin{table} +\vspace{-\medskipamount} % needed because \caption has wrong spacing + % when used at the top rather than the bottom + % of a float environment. +\caption[]{\label{t:receiving}Receiving} +\writepage{Table} +\vspace{-\medskipamount} +\begin{center} +\begin{tabular}[t]{p{.35\columnwidth}|p{.55\columnwidth}} +\hline +Method & Prompt displayed by \tex/\\ +\hline +\cw{read}& \cw{controlseq=}\\[3pt] +\rp error message interaction& \verb|?|\\[3pt] +\rp `show' message interaction& \verb|?|\\[3pt] +\rp input file not found& +\tt\rp Please type another input file name:\\[3pt] +\rp output file not writable& +\tt\rp Please type another output file name:\\[3pt] +interrupt key& none +\end{tabular} +\end{center} +\end{table} + +The purpose of getting input from the user is essentially always the +same: to give the user an opportunity to change the outcome of the \tex/ +run, which would otherwise be totally predetermined by the contents of +the files read by \tex/. (Well, and perhaps by a few system variables +such as \cw{time}, \cw{day}, etc.) + +\subsection{The \cw{read} primitive}\label{ss:readprim} +The form of the \cw{read} command is + \begin{verbatim} +\read 16 to \controlseq +\end{verbatim} +where the number 16 is the input stream number, +which might be any value from $-1$ to 16; +a number between 0 and 15 would indicate reading from a file +stored on disk, while 16 and $-1$ indicate reading from the user's +keyboard. \cw{controlseq} can be any control sequence +chosen by the macro writer. If the input stream number is +16, \tex/ will display a prompt of + \begin{verbatim} +\controlseq= +\end{verbatim} +If the input stream is $-1$, this prompt will be omitted. In either +case, a \cw{read 16} or \cw{read -1} command should normally be +preceded by a message that lets the user know what kind of input to +provide. + +The action of the \cw{read} command is similar to that +of the \cw{def} primitive. Both of them create a new +macro containing an unexpanded token list, which +must contain balanced braces. The balancing required +in the response to a \cw{read} command, however, +is slightly different than for a \cw{def}; as long +as there an equal number of opening and closing +braces, it doesn't matter how they're distributed---\tex/ +will be perfectly happy with the response + \begin{verbatim} +a}}}b{{c}{{ +\end{verbatim} + as can be verified using \cw{show}\cw{answer} after reading the +above string into \cw{answer}.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{See also \secref{ss:readouter}. +I questioned the behavior of \cw{read} with respect to extra closing +braces and outer control sequences in a letter to +Donald Knuth (December 1991); +his response was to make a change in \tex/ (version 3.141?) +that causes a \cw{read} operation to terminate decisively if an extra +{\tt\char`\}} or outer control sequence is encountered. So backwards +balancing of braces will no longer be permitted.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\tex/ will read more than one line, if +necessary, if the first line contains an unmatched brace. This is a +useful property if you {\em want\/} to read more than one line at a +time, as is sometimes the case: write the information in the form +\verb|{| \ldots \verb|}|, and you can have as many lines +between the curly braces as you want. + +\tex/ always reads line by line, rather than character by character; +unlike some other programming languages, \tex/ provides no way to read a +single character and act on it immediately; the user must always press +the \return/ key before anything will happen. + +\subsection{Error recovery}\label{s:error-recov} + As mentioned earlier, after an error message \tex/ presents the user +with a question-mark prompt. Typing a second question mark in reply +to the prompt will cause \tex/ to list the options that are +available: + \begin{verbatim} +! Error message. + ... +? ? +Type <return> to proceed, S to scroll future error messages, +R to run without stopping, Q to run quietly, +I to insert something, E to edit your file, +1 or ... or 9 to ignore the next 1 to 9 tokens of input, +H for help, X to quit. +? +\end{verbatim} + Choosing the {\tt H} option causes \tex/ to print a help message +containing additional information related to the error message. +If the error message was generated using \cw{errmessage}, then +the help message will consist of the current contents of the +token register \cw{errhelp} (which should be filled with something +useful by the macro writer, immediately before the call +to \cw{errmessage}). Otherwise, for a built-in error message, +the corresponding built-in help message (from \fn{tex.pool}) is +displayed. + +The intended use of the insertion ({\tt I}) and token deletion ({\tt +1...9}) options is for error recovery; after you look at the context +of an error, you may be able to temporarily repair the damage and +continue processing the remainder of the document, by removing some +tokens and/or inserting others. Then if any other errors are uncovered +later, they can be fixed at the same time as the first error, instead +of requiring a second \tex/ run to find them. Example~\ref{e:recovery} +illustrates this. You can actually delete up to 99 tokens at a time in +all implementations of \tex/ that I know of, even though the help +message suggests that 9 is the maximum. + +\begin{example} +\begin{examplebox}% +% +\caption[]{Using error interaction possibilities to +get past a potentially bad error: a missing {\tt\string\\} before an +\cw{hline} in a \latex/ {\tt tabular} environment.}\label{e:recovery}% +\writepage{Example} + +\begin{verbatim} +! Misplaced \noalign. +\hline ->\noalign + {\ifnum 0=`}\fi \hrule... +l.120 \hline + +? +\end{verbatim} +Let's see what the help information is: +\begin{verbatim} +? h +I expect to see \noalign only after the \cr of an alignment. +Proceed, and I'll ignore this case. + +? +\end{verbatim} +Let's try skipping one token to verify what \latex/ is going +to process next: +\begin{verbatim} +? 1 + +\hline ->\noalign { + \ifnum 0=`}\fi \hrule... +l.120 \hline + +? +\end{verbatim} +All right, the opening curly brace has just gone by. We need +to insert the \verb|\\| that was forgotten, and also replace the two tokens +\cw{noalign} and \verb|{| that have slipped by. +\begin{verbatim} +? i\\ \noalign{ +\end{verbatim} +And now \latex/ will be able to continue with the rest of the table. +\end{examplebox} +\end{example} + +The token deletion option can also serve as the basis of a crude menu +facility. The idea is to use \cw{errmessage} to present a menu with +choices labeled by numbers. If the user responds by entering, say, +\verb;3; to choose item~3, then we must arrange things so that after +\cw{errmessage} does its normal thing of deleting three tokens, the +following token that was not deleted should do something to ensure +that item~3 will be selected. This isn't too hard if we use something +like the following sequence: +\begin{verbatim} +\errmessage{...}\0\1\2\3\4\5\6\7 ...\stop +\end{verbatim} +Clearly the deletion of three tokens after the error message will leave +\verb;\3; as the next command to be executed, and so if we define +\verb;\3; to do the right thing and then skip over the following +\verb;\4\5...; corresponding to unselected menu choices, we get what we +want. + +If the user just presses \return/ without entering a number, it will +be \verb;\0; that is executed---therefore \verb|\0| should be defined +to produce the default selection. + +One thing that makes me call this crude is the fact that \tex/ pauses +after any token deletion operation instead of barging ahead. This means +that if the user chooses anything other than the default selection, +they will have to press \return/ twice after typing the number, instead +of just once. + +Some other crudities are introduced by the bits and pieces of an +\cw{errmessage} that cannot be suppressed. These include: +\begin{itemize} +\item Exclamation point and space at the beginning of the error message. +\item Period at the end of the error message. + +\item The expansion of the current macro (if the error message is + contained in a macro), on two lines with the line break immediately + after the token that was last processed by \tex/. This includes, at + the beginning, the macro name followed either by \verb"->", if the + first part of the expansion text is relatively short, or by ellipsis + dots \verb"..." plus the tail end of the first part of the expansion + text. + +\item The current line of the current file, on two lines with the break + immediately after the token that was last processed by \tex/. +\item The question mark and space prompting the user for a response. +\end{itemize} + +I would have hoped that setting \cw{errorcontextlines} to $0$ or $-1$ +would cause the `innermost' two lines of the error context to be +suppressed but apparently there is no value for \cw{errorcontextlines} +that will suppress them. The following example illustrates all of the +nonsuppressable parts. +\begin{verbatim} +! Error. +\CM ->\errmessage {Error} + \CM +l.38 \CM + % A comment in the file, line 38 +? x +\end{verbatim} +Except for the ellipsis dots alternative, that is. Here's how that +looks: +\begin{verbatim} +! Longer message text, forcing elision. +\CM ...sage {Longer message text, forcing elision} + \CM +l.38 \CM + % A comment in the file, line 38 +? +\end{verbatim} +Although the exact number can vary (depending on how your particular +version of \tex/ is configured at compile time), the maximum +length of the first line of context is normally between 40 and 50 +characters, and if the expansion text would make the line longer +than this, it is truncated at the beginning and the ellipsis +dots are inserted. + +Very well then. Since we have \verb"!", \verb".", and \verb"->" or +\verb"..." in the first two lines (the lines that will be nearest our +menu text), and we cannot get rid of them, the next best thing is to +camouflage them. One possibility is setting +\cw{newlinechar}\verb"=`\!" just before sending the \cw{errmessage}, +so that the \verb"!" character will cause a blank line rather than +printing on screen, and then putting a bunch of periods in the menu +text to camouflage the other periods. + +\ref{a:menu} exhibits \fn{fontmenu.tex}, a more extensive +working-out of this idea in which I tried to pound the recalcitrant +\cw{errmessage} into the most presentable shape possible, using every +macro hack I could think of. + +\subsection{Show message `recovery'} + +After a \cw{show}, \cw{showthe}, \cw{showbox}, or \cw{showlists} +command, \tex/ offers a question-mark prompt, and the same menu of +options as after an error message. There is only one slight +difference: The {\tt H} option provides no access to the \cw{errhelp} +token register; only a generic help message about the \cw{show...} +commands is available. + +\subsection{``Please type another input file name:''} + +When you see this prompt, displayed by \tex/ when it's unable to find +an input file, you have strayed into one of the less friendly byways +of \tex/. If you can't think up a good file name to give as an answer, +you could get stuck in an endless loop. Even simply pressing the {\sc +return} key causes \tex/, on most computer systems, to look for a file +called `\fn{.tex}' which will most likely be nonexistent. Power users +know that on many systems you can enter a file name of `\fn{nul}' to +cause \tex/ to read in an empty file named \fn{nul.tex}. But it is +precisely power users who are likely to know other ways of getting +past this prompt (for example, on some systems typing a \ctrl{Z} or +\ctrl{D} character also does something useful), and it is precisely +the users with no other clue what to try next who won't know about +\fn{nul.tex}. + +It seems that it would be useful for all standard distributions of +\tex/ to provide files named \fn{.tex}, \fn{h.tex}, \fn{help.tex}, and +\fn{?.tex} in the standard \tex/ inputs path, so that when users type +\verb'h' or \verb'help' or \verb'?' or just press \return/, they will +get the corresponding file. (Unfortunately, most operating systems +don't permit the question mark in file names, which means that only +the other three files will normally be viable.) \fn{Help.tex} and its +clones could contain something as simple as: + \begin{verbatim} +\errmessage{Type ? to see your options; X to exit} +\end{verbatim} + which would give the user access to the full menu of normal error +recovery options.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{This idea is discussed at greater length in a recent \tugboat/ +article of mine (to appear, late 1994).} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + +\subsubsection{A menu trick} + The {\tt Please type another input file name} prompt is used to +implement a sort of menu in the file \fn{lfonts.new} of the +Mittelbach/Sch\"opf font selection scheme (\latex/ version), which +has a statement \cw{input} \fn{fontdef.tex}, where the file +\fn{fontdef.tex} is normally missing, intentionally, and the user is +supposed to substitute another file name such as \fn{fontdef.ori} or +\fn{fontdef.max}. The idea of comments designed to appear through +\tex/'s display of error context (\secref{ss:errcontextcomments}) +could be used to good advantage here, to +tell the user what other file names are likely +candidates: + \begin{verbatim} +\input fontdef.tex % Try fontdef.ori or fontdef.max +\end{verbatim} +In this kind of application, additional help information in a message +preceding the \cw{input} statement could also be useful. + +\subsection{Interrupt key} + The interrupt key is a key (system-dependent, but \ctrl{C} on many +systems) that allows you to interrupt +\tex/ when it is in the middle of doing something else. The normal +reaction of \tex/ when the interrupt key is pressed is to print a +message + \begin{verbatim} +! Interruption. +... % current context +? +\end{verbatim} + in the same form as an error message, complete with a question-mark +prompt, with the usual options available to the user. + +It's stretching the concept a bit to claim that the interrupt key is +a way for users to send information to \tex/; it has the flavor of the +story about the farmer who had to whack his mule over the head with +an axe handle ``just to get his attention''. When you interrupt +\tex/ you could easily find yourself in the middle of some +complex macro where it would be inadvisable to do anything except use +the \verb|X| option to exit. However, this in itself is frequently +useful. + +\section{Stumbling blocks in the use of \cw{write} and \cw{message}} +\label{s:stumblewrite} + +\subsection{Line breaking} + +As explained elsewhere (\secref{ss:message}) it is impossible to use a +control character as a newline character in the argument of a +\cw{message} or \cw{errmessage} command. +{\em[Note (30-Oct-1993): that's no longer true, as of \tex/ version 3.141.]} +\latex/ and \amstex/ +use \cw{immediate}\cw{write} instead of \cw{message} in their +all-purpose message macros (\cw{typeout} and \cw{W@}), which allows +them to have \verb"^^J" as the default newline character, +thus leaving all of the printable characters usable in message texts. +(If a given character is the current \cw{newlinechar}, there really +is no way for \tex/ to print it on screen. Try setting +\cw{newlinechar}\verb|=`\(| and see what happens to the file name +messages for input files.) + +There is a minor inconvenience with the use of \ctrl{J} as the newline +character. Under current conditions (as of 1993), it is usually wise to +limit the length of all lines in a macro file to 72 characters, in +order to avoid truncation problems that occasionally occur in, \eg., +electronic mail transmission. When constructing a long message, if you +leave \cw{endlinechar} at its normal value of \verb;^^M; and put +\verb;^^J;'s at the end of each message line, you get four extra +characters at the end of each line, three for the \verb;^^J;% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{Although the {\tt\string ^\string ^J} combination will be +resolved to a single character by \tex/, it is three characters when +writing it and when sending it through mail. Assuming, that is, that +you use \tex/'s double caret notation and don't try to insert a +\ctrl{J} byte directly (not a good idea, because of system-dependent +interpretation of \ctrl{J}).} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +and one for a percent sign to eliminate the space that would otherwise +be produced on screen at the beginning of the next message line, by the +\verb;^^M;. This means that the effective limit on the length of each +message line is 68 characters rather than 72. + +But the clutter of four extra characters at the end of a line can be +avoided by temporarily assigning \cw{endlinechar} \verb;=; +\cw{newlinechar} while a message is being constructed. This is +assuming, however, that a useful value of \cw{newlinechar} has been +established and that the same value will be in effect when the +message is sent. If the construction and sending are simultaneous, +the code can be as simple as this: + \begin{verbatim} +\begingroup \endlinechar=\newlinechar +\immediate\write16{Line 1 +Line 2 +Line 3 +}% +\endgroup% this percent sign is necessary +\end{verbatim} + And then the actual message text in each line +can run to the full 72-character length if necessary. +If a useful value of \cw{newlinechar} has not been established +(\eg., when using \plaintex/ without modifications), then +rewrite the first line above as: +\begin{verbatim} +\begingroup \newlinechar=\endlinechar +\catcode\endlinechar=12 % Make it 'other' +\end{verbatim} +Here we make the reasonable +assumption that \cw{endlinechar} has its normal value +of 13 (\verb;^^M;); even this assumption could +be done without by adding the statement \cw{endlinechar} \verb;=; +\verb;13; before the other assignments. + +The percent sign is necessary after the \cw{endgroup} in the first +variant, because the endline character for a given line is added when +the line is initially read, before \tex/ begins to execute the line. +Therefore by the time \tex/ processes the \cw{endgroup} and reverts +to the previous value of \cw{endlinechar}, it is too late to prevent +getting a newline character at the end of the current line. + +For the same reason, a percent sign is not needed +after the line that contains +\begin{verbatim} +\endlinechar=\newlinechar +\end{verbatim} +The endline character for that line has already been tacked on +by \tex/ and cannot be changed by any statements within that line. +(The \cw{catcode}, however, +of the \cw{endlinechar} can be changed by statements within the line.) + +\subsection{Expanding control sequences}\label{s:excs} + +In messages to a \tex/ user it's frequently necessary to refer +to control sequences or characters that have special category codes. +This can sometimes be problematic because of the expansion that is +done in the argument of a \cw{message} or \cw{write} command. For +example, the line + \begin{verbatim} +\message{Beware of \footnote in a \message!} +\end{verbatim} +will typically generate a hundred or so error messages when +\tex/ reaches \cw{footnote}. (Although \plaintex/, \amstex/, \latex/, +and other macro packages define \cw{footnote} differently, +all the definitions are equally explosive inside a \cw{message}.) +And the line +\begin{verbatim} +\message{Beware of \endinput in a \message!} +\end{verbatim} + will cause the current input file to terminate immediately! +(\cw{endinput} is an {\em expandable} control sequence, for +reasons that are too technical +to be worth discussing here.) + +Thus to talk about an expandable control sequence in a message, you +must do something to inhibit the expansion. Ordinarily +you apply \cw{string} or \cw{noexpand} to the control sequence; +or you could put it into a token register and use +\cw{the}\m{token register} in the message. +Nonexpandable +control sequences can be printed in a message without special +protection, except that, if you do nothing to avoid it, you will +always get an extra space after a control word, even in +some cases where it is undesirable, as when the next +thing is punctuation. For example, the message +\begin{verbatim} +\message{Enter desired value for \hangindent: } +\end{verbatim} +will print on screen with a space before the colon: +\begin{verbatim} +Enter desired value for \hangindent : +\end{verbatim} + +Table~\ref{t:expansion} shows what happens to various sorts of things +in a \cw{message} or \cw{write} argument, as well as various methods +for suppressing expansion. + +\begin{table*} +\caption[]{Expansion of \cw{message} and \cw{write} arguments% +\label{t:expansion}} +\writepage{Table} +\smallskip +\noindent +\[\begin{tabular}{|r|l|} +\hline +This input\ \ \ & Produces this on screen\\ +\hline +\verb|\message{E}|& \verb|E|\\ +\verb|\message{&}|& \verb|&|\\ +\verb|\message{#}|& \verb|##|\\ +\verb|\message{[\relax]}|& \verb|[\relax ]|\\ +\verb|\message{[\string\relax]}|& \verb|[\relax]|\\ +\verb|\message{[\empty]}|& \verb|[]|\\ +\verb|\message{[\noexpand\empty]}|& \verb|[\empty ]|\\ +\verb|\message{[\string\empty]}|& \verb|[\empty]|\\ +\verb|\message{[\space]}|& \verb|[ ]|\\ +\verb|\message{[ ]}|& \verb|[ ]|\\ +\verb|\def\spaces{\space\space\space\space|&\\ +\verb|\space\space\space\space}|&\\ +\verb|\message{[\spaces ]}|& \verb|[ ]|\\ +\verb|\message{[\romannumeral 37]}|& \verb|[xxxvii]|\\ +\verb|\message{[\uppercase{a}]}|& \verb|[\uppercase {a}]|\\ +\verb|\message{[~]}|& \verb|[\penalty \@M \ ]|\\ +\verb|\message{[\ifnum\time<600 Too early for me|&\\ + \verb|\else Let's go\fi]}|& \verb|[Too early for me]|\\ +\hline +\end{tabular}\] +\end{table*} + +\subsection{Collapsing spaces} + +If you want to print on screen a menu or similar message consisting +of multiple columns nicely arranged, you have to deal with the fact +that \tex/ normally condenses multiple spaces and tab characters +to a single +space. The easiest way to handle this difficulty is to change the +catcode of the space character to, say, 12 +before reading the argument of a +\cw{message} or \cw{write} command. + +\subsection{Special characters} + +The space character is but one example of a larger class: +`special' characters, that is, ones that don't have category 11 or 12. +An obvious +question to ask is, ``What other special characters are difficult to +use in a message?'' Table~\ref{t:expansion} shows how a few special +characters are affected by the expansion process in a message: an +\verb;&; (category 4) passes through unharmed, a \verb;#; (category +6) gets doubled, and a \verb;~; (category 13) gets expanded.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{To be more precise, an active character like {\tt\string ~} +will be treated like a control sequence; it will be expanded if +expandable, otherwise it will be printed as is. After an +assignment such as +\cw{newcount}{ \tt\string ~}, or \cw{let}{ \tt\string ~ =} \cw{relax}, +the {\tt\string ~} is not expandable.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +Table~\ref{t:specchars} is a complete list +of the various categories of characters, +along with ways to produce those characters that cannot simply be +used as is. A few categories deserve more extensive comment. + \begin{description} + \item[Category 0---Escape Character] It's normally not a problem to +print an escape character because it usually occurs as part of a +control sequence, which can be printed using \cw{string} (and even +that may not be necessary if the control sequence is nonexpandable). +Even when the escape character is not, logically speaking, part of a +control sequence, it can be sent in a message by letting it combine +from \tex/'s point of view with the following character(s). For +example, to send the message {\tt Commands in TeX normally begin with +a `\string\' character}, the backslash doesn't need to be treated as +an isolated character; combined with the following apostrophe, it +forms a control symbol to which \cw{string} can be applied. + + The only time this fails is when the backslash must be sent as the +very last character of a message. Although this case is +extremely unlikely, the solution involves a rather useful +little macro: +\begin{verbatim} +\def\xstring{\expandafter\gobble\string} +\def\gobble#1{}% if this is not already defined +\end{verbatim} +With this definition,% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{Cf.\ the answer to Exercise 7.10 in \texbook/. The implicit +assumption that \cw{escapechar} is in the range 0--255 may not be +completely reliable.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\cw{xstring} not only turns a control sequence into a string of +characters, it also removes the leading backslash, so that +\cw{xstring}{\tt\string\\} will produce a single backslash +character, as desired. +Another solution that involves setting \cw{escapechar} +temporarily to $-1$ would also be possible, provided that the remainder +of the message doesn't need to use \cw{string} in a normal way, +with a printable escape character. +\begin{verbatim} +\begingroup \escapechar=-1 +\message{ ...\string\\}% +\endgroup +\end{verbatim} +On the other hand, +it might be useful to have a category-12 +backslash character always available through a macro, +not only for messages but for other purposes as well: +\begin{verbatim} +\edef\backslashchar{\xstring\\} +\end{verbatim} +Then \cw{backslashchar} could be used in a message instead of +\cw{xstring}{\tt\string\\}. If you needed to use it frequently you +would presumably give it a shorter name. + +\item[Categories 1 and 2] Characters of category 1 and 2 can be +printed without any problem in a message {\em if they occur +in matching pairs}. For these purposes, character codes are irrelevant; +\verb;];$_1$ and \verb;*;$_2$ match up as well +as \verb;{;$_1$ and \verb;};$_2$. +If a single, unmatched character of one of these categories +must be printed on screen, \cw{xstring} can be used with the +corresponding +control symbol, \eg., \cw{xstring}{\tt\string\{} or +\cw{xstring}{\tt\string\}}. + +\item[Categories 5, 9, 14, 15] These categories are similar to category +0. Characters of category 0 (escape), category 5 (end-of-line), 9 +(ignored), 14 (comment), and 15 (invalid character) cannot enter a +token list [\texbook/, Exercise 7.3], so that, actually, it doesn't make much +sense to ask what happens to them in the argument of a \cw{message} or +\cw{write} command, which do not deal with raw characters from an input +stream but with token lists. The question is not how to print a +character token of category 14 in a message (since that is impossible) +but, how to produce a category-12 \verb;%; when the normal catcode of +\verb;%; is 14. The answer is to +use \cw{xstring} with the corresponding control symbol, \eg., +\cw{xstring}{\tt\string\%}. + +\end{description} + +Then again, a better idea might be to pick one character, make it +active (probably {\tt\string ~} since it's already active in most +macro packages), and define it to produce category-12 +characters by their hexadecimal value. With preliminary +definitions such as: +\begin{verbatim} +\escapechar=-1 +\def\twelvechar#1#2{\csname hex#1#2\endcsname} +\expandafter\edef\csname hex5c\endcsname{\string\\} +\expandafter\edef\csname hex25\endcsname{\string\%} +\expandafter\edef\csname hex7e\endcsname{\string\~} +... +\escapechar=`\\ +\end{verbatim} +it would become possible to write, for example, +\begin{verbatim} +\begingroup +\let~=\twelvechar +\message{Printing backslash ~5e, percent ~25, and tilde ~7e.} +\endgroup +\end{verbatim} +and thus send all manner of special characters by substituting a +three-character sequence starting with {\tt\string~}. + +\begin{table} +\caption[]{Methods for incorporating various +categories of characters in a \cw{message} +or \cw{write} argument\label{t:specchars}} +\writepage{Table} +\smallskip +% +\begin{tabular}{|rrp{.57\columnwidth}|} +\hline +Catcode& Example& Method\\ +\hline +0& \verb;\;& Normally handled as part of a control +sequence, except at the very end of a message, in which case use +\cw{xstring}{\tt\string\\}\\ +% Using \mbox here to suppress a line-breaking problem +% until/unless I can get around to fixing it.{28-Nov-1992 mjd} +1& \verb;{;& \mbox{\cw{xstring}{\tt\string\{}} if unmatched\\ +2& \verb;};& \mbox{\cw{xstring}{\tt\string\}}} if unmatched\\ +3& \verb;$;& as is\\ +4& \verb;&;& as is\\ +5& \verb;^^M;& \mbox{\cw{xstring}}\verb;\^^M; (see the note below)\\ +6& \verb;#;& \mbox{\cw{string}}\verb;#; to avoid doubling, or + \cw{xstring}{\tt\string\#}\\ +7& \verb;^;& as is (except in rare combinations + like \verb;^^>;)\\ +8& \verb;_;& as is\\ +9& \verb;^^@;& \mbox{\cw{xstring}}\verb;\^^@; (see the note below)\\ +10& space& as is, except use \cw{space}'s to + produce multiple spaces\\ +11& a& as is\\ +12& /& as is\\ +13& \verb;~;& \mbox{\cw{string}{\tt\string ~}}, \cw{noexpand}\verb;~;\\ +14& \verb;%;& \mbox{\cw{xstring}{\tt\string\%}}\\ +15& \verb;^^?;& \mbox{\cw{xstring}}\verb;\^^?; (see the note below)\\ +\hline +\end{tabular} +\par +\smallskip +\small +Note: Because of the way \cw{string} operates, something like +\cw{xstring}{\tt\string\^^@} will not produce a single +character but three category-12 characters, +{\tt \string^ \string^ @}. This may normally be what you want, but it +won't be satisfactory if +the character in question has a special purpose---perhaps to cause a +newline, or to print on-screen as an accented letter. + +\end{table} + +\subsection{Space after a control word} + +In \secref{s:excs} it was pointed out that an unwanted space may be +printed at the end of a control word under some circumstances. It's +equally possible that a wanted space at the end of a control word may +disappear under other circumstances. For example, it is not uncommon +to see macro writers use +the combination \cw{string}\cw{controlseq}\cw{space} when a +control word occurs as an isolated word in the middle of a message; +the final \cw{space} is necessary because a plain space after +\cw{controlseq} would simply disappear according to \tex/'s normal +rules for finding the end of a control sequence name. + +The solution to both of these difficulties is easy: use \cw{string} +if you don't want a space after the control word, and use +\cw{noexpand} if you do want a space.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{I hadn't noticed the usefulness of \cw{noexpand} for this +purpose until Michael Spivak drew it to my attention, in a +conversation at the 1991 TUG meeting in Boston.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +Note: The character tokens produced by \verb|\string| can be passed +without harm through any number of subsequent steps, including +expansion via \cw{edef} or similar operators, while \cw{noexpand} +only protects an expandable macro through the first expansion step. + +\subsection{Outer Control Sequences} +\label{ss:outer} +You can't send an \cw{outer} control sequence in a \cw{message} +unless you do something to get around the outerness. +To illustrate, I present +a transcript of \tex/'s reaction to the following line: +\begin{verbatim} +\message{Control-L: ^^L} +\end{verbatim} +along with various attempts to recover from the resulting error +message. (\ctrl{L} is defined as an active character with the `outer' +attribute in both \plaintex/ and \latex/.) +\begin{verbatim} +Runaway text? +Control-L: +! Forbidden control sequence found while scanning text of \message. +<inserted text> + } +<to be read again> + ^^L +l.149 \message{Control-L: ^^L + } +? 1 +Type <return> to proceed, S to scroll future error messages, +R to run without stopping, Q to run quietly, +I to insert something, E to edit your file, +H for help, X to quit. +? h +I suspect you have forgotten a `}', causing me +to read past where you wanted me to stop. +I'll try to recover; but if the error is serious, +you'd better type `E' or `X' now and fix your file. +? +\end{verbatim} + +Notice that when I tried to type a \verb;1; to delete the offending +\cw{outer} token, I got instead a help message indicating that token +deletion is not an option at this point. (The reason behind this lack +of token deletion is fairly technical: \tex/ was in the middle of a +procedure called {\it get\_token} when it stumbled over the \ctrl{L} +character; but since token deletion itself involves calling {\it +get\_token}, allowing token deletion here would mean calling +{\it get\_token} +from inside itself---something it was not designed for.) + +\subsection{Semi-verbatim alternative} +An alternative way of handling message texts, that eliminates the +need to remember special methods for various kinds of +message elements, is to temporarily change the +catcodes of all special +characters while reading the argument of +a message. With the following definitions: +\begin{verbatim} +\def\verbwrite{\begingroup + \def\do##1{\catcode`##1=12}% + \do\ \dospecials + \catcode\endlinechar=12 + \newlinechar=\endlinechar + \verbcontinue} +\begingroup \lccode`\/=`\\ +\lowercase{\endgroup +\def\verbcontinue##1/}endverbwrite{% + \immediate\write16{#1}\endgroup} +\end{verbatim} +you could send messages like +\begin{verbatim} +\verbwrite !#$%$%#^%&*~@^^"<?:}{>|+ +_^+\footnote_|)\\90\ +\bye ^^L \endinput +\endverbwrite +\end{verbatim} +without regard to the contents. The main limitation of this approach +is that in order for the handling of the special characters to work, +\cw{verbwrite} has to be executed directly in a file; it cannot be +embedded in a macro. Furthermore, the message text is unalterable: it +cannot contain a context-dependent part, as in a message to display +the current font name on screen: +\begin{verbatim} +\message{Current font is \fontname\font.} +\end{verbatim} +This message could not be generated with \cw{verbwrite} because +\cw{verbwrite} would not expand \cw{fontname}. + +\subsection{Presenting information in the best possible form} + +Example: In reporting a dimension to the user, it is usually +desirable to report the value rounded to tenths or hundredths, in +units that are convenient for the user: points for a font size or +line spacing value; centimeters, picas, or inches for the height or +width of a page or of an included figure. + +The file \fn{cnvunits.tex} gives some examples of what is possible in +this vein, including conversions from points to picas, inches, and +centimeters. The conversions from points to other units are the most +important ones because when \cw{the} is applied to a dimension or +skip register \tex/ always reports the value in \verb;pt; units. (Not +counting \cw{muskip} registers, where the reported unit is \verb;mu;.) + +\reqspace{1.5in} +\verbatiminput{cnvunits.tex} + +Another example: if you want to report the \cw{mathcode} of a +particular character to the user, \cw{number}\cw{mathcode`}\cw{x} or +\cw{the}\cw{mathcode`}\cw{x} aren't too great, because they produce a +decimal number, when it would be more +convenient to get a hexadecimal number filled out to four digits, so +that the class, math family, and font position information can be +read off directly. +Extending some ideas from \fn{testfont.tex} \cite{testfont}, we can +write a quite friendly \cw{reportmathcode} function: +\begin{verbatim} +% When \meaning is applied to a \mathchar, it produces +% \mathchar"<digits> where <digits> are 1 to 4 hexadecimal digits. +% The function \gethex strips off the prefix and leaves only the +% digits. +\def\gethex#1"{} +% The function \reportmathcode takes a character or control symbol +% argument and reports the associated mathcode in hexadecimal +% form, filling out to four digits with leading zeros, if +% necessary. +\def\reportmathcode#1{% + \begingroup + \mathchardef\temp=\mathcode`#1 \relax + \edef\temp{\expandafter\gethex\meaning\temp}% + \count@="\temp\relax + \edef\temp{% +% Fill in leading zeros + \ifnum\count@<"1000 0% + \ifnum\count@<"100 0% + \ifnum\count@<"10 0\fi\fi\fi + \temp}% + \message{The mathcode of \string#1 is: "\temp}% + \endgroup} +\end{verbatim} + +\begin{example} +\begin{examplebox}% +\caption[]{The \cw{printoptions} command of \amstex/ version 1.1. +\cw{W@} is the \amstex/ abbreviation for +\cw{immediate}\cw{write}{\tt 16}}\label{amstex/syntax}% +\writepage{Example} +\hfuzz=2em +\begin{verbatim} +\def\S@{S } \def\G@{G } \def\P@{P } +\newif\ifbadans@ +\def\printoptions{\W@{Do you want S(yntax check), + G(alleys) or P(ages)?^^JType S, G or P, follow by <return>: }% + \loop \read\m@ne to\ans@ + \xdef\next@{\def\noexpand\Ans@{\ans@}}% + \uppercase\expandafter{\next@}% + \ifx\Ans@\S@\badans@false\syntax\else + \ifx\Ans@\G@\badans@false\galleys\else + \ifx\Ans@\P@\badans@false\else + \badans@true\fi\fi\fi + \ifbadans@\W@{Type S, G or P, follow by <return>: }% + \repeat} +\end{verbatim} + +\end{examplebox}% +\end{example} + +\section{Stumbling blocks in the use of \cw{read}} +\label{s:stumbleread} + +\subsection{An example: \amstex/'s \cw{printoptions} command} + +Consider the \cw{printoptions} command of \amstex/ 1.1 +(Example~\ref{amstex/syntax}): +The definition of this command shows one way of dealing +with the extra space at the end of a macro created using \cw{read}: +Define some macros consisting of the expected answers, with the extra +space included, and then use \verb|\ifx| to compare them to the +user's response. It also shows how to uppercase the user's response +so that lower- and uppercase responses will be treated identically. +This is the second method given in the answer to \texbook/'s Exercise +20.19. One more noteworthy feature of \cw{printoptions} is that it +runs a loop that doesn't quit until the user gives an acceptable +answer. + +In \cw{printoptions} since \verb|\W@| is defined to be +\verb|\immediate|\5\verb|\write16|, and the \cw{write} command always +starts a new line after its message text, we can see that the reply +typed by the user will appear on the next line instead of immediately +after the colon. This brings up the question: what if we want the +user's reply to appear on the same line? + +One way to do this is to use \cw{message} to send the last line of +the prompt message, and use \cw{write} to send the previous line(s). +For example: + \begin{verbatim} +\W@{Do you want S(yntax check), G(alleys) or P(ages)?}% +\message{Type S, G or P, follow by <return>: }% +\end{verbatim} + This idea is used in the \latex/ option \fn{checknum.sty} +\cite{checknum}. An alternative would be to put the whole prompt in a +single \cw{message} with embedded newline characters (as long as you +are careful to select a character for \cw{newlinechar} that is not +needed in the text of the message). + +\subsection{{\ntt{\char94}{\char94}M} at the end of a line} +In \label{s:endlinechar} +\cw{printoptions} separate macros \cw{S@}, \cw{G@}, and +\cw{P@} are defined for each legitimate response. If the menu +becomes more extensive, this technique is rather wasteful +of hash size, main memory, and other useful commodities. +The problem here is that the +\verb|^^M| character at the end of the user's response +is included by \cw{read} in the macro being read. +Under normal conditions +\verb|^^M| is converted to a space; however, another +possibility\Dash if the user just enters \return/ without +typing any response\Dash is that the \verb|^^M| will produce a \cw{par} +token (following the general rule that an empty line is +equivalent to \cw{par}). The best approach is +to prevent the \verb|^^M| character from getting into the +read macro in the first place. This can be done in two ways: +setting the catcode of \verb|^^M| to 9 (``ignore''), or +setting \cw{endlinechar} to $-1$. + +Unfortunately, this immediately raises another difficulty: we want to keep +the catcode change or \cw{endlinechar} change local so that it will +affect only the \cw{read}. This could be accomplished by +saving the current catcode or \cw{endlinechar} (just in case) +and restoring it after the \cw{read} is done, but it's +simpler to enclose the \cw{read} in a group: +\begin{verbatim} +\begingroup +\endlinechar=-1 +\global\read16 to\answer +\endgroup +\end{verbatim} +Here the \cw{global} prefix makes \cw{answer} +retain its definition beyond the \cw{endgroup}. + +With this modification the tests done by \cw{printoptions} could be +simplified to +\begin{verbatim} +\if\Ans@ S ...\else +\if\Ans@ G ...\else +\if\Ans@ P ...\else +... +\end{verbatim} +which renders the macros \verb|\S@|, \verb|\G@|, \verb|\P@| unnecessary. + +On the other hand, we have advanced to some splendid new +complications: \verb|\Ans@| might now be completely empty, if the +user just pressed the \return/ key, and an empty \verb|\Ans@| would +bollix up the \verb|\if| tests. This case is easy to handle, though: +add an extra branch \verb|\ifx\Ans@\empty...| at the beginning. +We have the opposite problem if the user types more than one +letter: on the true branch (the `none-of-the-above' branch, unless +the user's first two letters happen to be identical), the extra +characters could potentially cause spurious typesetting activity. As +it happens, we can kill two birds with one stone, as we'll see +in \secref{s:defaults}. + +\subsection{Uppercasing input} + +Next let's look at the procedure used by \cw{printoptions} for +uppercasing the user's reply: after reading \verb|\ans@|, \cw{xdef} +and \cw{uppercase} are applied to it as follows: + \begin{verbatim} +\xdef\next@{\def\noexpand\Ans@{\ans@}}% +\uppercase\expandafter{\next@}% +\end{verbatim} + A more economical version of the same technique would be: + \begin{verbatim} +\xdef\ans@{\uppercase{% + \gdef\noexpand\ans@{\ans@}}}% +\ans@ +\end{verbatim} + If \verb|\ans@| contains \verb|s| +to begin with, then after the \cw{xdef} has been completed, the +definition of \verb|\ans@| is \verb|\uppercase{\def\ans@{s}}|. Then calling +\verb|\ans@| causes it to redefine itself, but not before the tokens in the +argument of \cw{uppercase} are suitably uppercased.% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{Only the \verb|s| is +affected because \cw{uppercase} operates only on letters, not +on control sequences or nonletters. Well, to be more precise: only on +characters that have a nonzero \cw{uccode}; they don't have to have +catcode 11.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +With this change, the auxiliary macro \verb|\Ans@| is no longer needed. + +To simplify the structure of macros using this uppercasing process, +it could be embodied in a dedicated function of its own: +\begin{verbatim} +\def\uppermac#1{\xdef#1{\uppercase{\gdef\noexpand#1{#1}}}% + #1} +\end{verbatim} + +A nonglobal alternative may sometimes be desirable, however. +Also the full expansion may not be wanted in some cases when +the contents of the macro being uppercased are `fragile'. +An alternative that is safer with respect to expansion: +\begin{verbatim} +\def\uppermac#1{% + \uppercase\expandafter\expandafter\expandafter{% + \expandafter\toks@\expandafter{#1}}% + \edef#1{\the\toks@}} +\end{verbatim} + +\subsection{Default responses} +One \label{s:defaults} +last refinement in \cw{printoptions} would be to provide +a default response if the user's response is empty (that is, +the user only hit +the \return/ key). One method involves an auxiliary +macro like the \latex/ macro \verb|\@car|: + \begin{verbatim} +\def\@car#1#2\@nil{#1} +\end{verbatim} + +A more descriptive name (for those whose knowledge of Lisp is nil) +would be \cw{firsttoken}:% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\footnote{Using {\tt\string @} as the ending +delimiter is pretty safe if we make sure that it has catcode 11 at the +time \cw{firsttoken} is defined and some other catcode at the time of +reading user input.} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + \begin{verbatim} +\def\firsttoken#1#2@{#1} +\end{verbatim} +Then, if we want \verb|\ans@| to be +given a default value of P when it comes back empty from the user, we +do this: + \begin{verbatim} +\xdef\ans@{\expandafter\firsttoken\ans@ P@} +\end{verbatim} +At the critical intermediate step, the following cases will arise: + \begin{quote} + \begin{tabular}{l|l} +\hline +User input& Critical step\\\hline +s& \verb|\firsttoken|\,\,\verb|s|\,\verb|P|\,\verb|@|\\ +P& \verb|\firsttoken|\,\,\verb|P|\,\verb|P|\,\verb|@|\hphantom{\tt syn}\\ +\m{return}& \verb|\firsttoken|\,\,\verb|P|\,\verb|@|\\ +\hline +\end{tabular} +\end{quote} + This gives exactly what we want. + +The application of \cw{firsttoken} also gives us a nice way around the +difficulty mentioned earlier if the user types more than one character. +The case +\begin{quote} +\begin{tabular}{l|l} +\hline +synt& \verb|\firsttoken|\,\,\verb|s|\,\verb|y|\,\verb|n|\,\verb|t|\,% +\verb|P|\,\verb|@|\\ +\hline +\omit\hphantom{User input}\kern2\tabcolsep\cr +\end{tabular} +\end{quote} + will produce the same result as the first case above, because +everything after the first \verb|s|, up to the category-11 \verb|@| character, +will be discarded. + +\subsection{A new \cw{printoptions}} + +By noticing that the \cw{xdef}'s used in +the \cw{firsttoken} step and the \cw{uppercase} +step can be combined, and putting together everything +discussed so far, we come up with a new, improved version +of \cw{printoptions}: + \begin{verbatim} +\def\printoptions{% + \W@{Do you want S(yntax check), G(alleys) or P(ages)?}% + \message{Type S, G or P, follow by <return>: }% + \begingroup \endlinechar\m@ne + \global\read\m@ne to\ans@ + \endgroup + \xdef\ans@{\uppercase{% + \def\noexpand\ans@{% +% Default to `P': + \expandafter\firsttoken\ans@ P@}% + }}% +% Execute \ans@ to uppercase itself: + \ans@ + \if S\ans@ \syntax\else + \if G\ans@ \galleys\else + \if P\ans@ % fine, no action needed + \else \message{Unknown option: \ans@; + `pages' option will be used}\fi + \fi\fi +} +\end{verbatim} + The loop has been discarded in favor of simply taking the normal +default action if the user's reply is unintelligible. + +\subsection{Matching braces} +\label{ss:readbrace} +The \cw{read} command normally reads only one line, but if the first +line does not contain an equal number of left and right braces, \tex/ +will continue to read additional lines until equality is achieved +(cf.~\secref{ss:readprim}). +This could be trouble in interactive use of \cw{read}, if the user +doesn't understand what has happened---it's difficult to extricate +yourself except by getting the braces right. + + +On the other hand, if you {\em want} to enter more than one line +at a time, you can do it by entering an opening brace on the first +line and the matching closing brace on the last line; this +is illustrated in Example~\ref{ex:readbrace}. +In the example there are two things worthy of note: +(1)~The braces appear in the replacement text of the macro \cw{name}; +this may be undesirable, depending on the intended use of the +information. (2)~There's no space between +Frank and Henry in \cw{name}. + +The loss of the space has two causes. First, \cw{endlinechar} was set to +$-1$ (so that an empty line will not produce a \cw{par}---see +\secref{s:endlinechar}), and +second, the space that was typed at the beginning of the second +line of the response didn't register either, following \tex/'s +usual rule of ignoring spaces at the beginning of a line +(\texbook/, Chapter~8, double dangerous bends). + +\begin{example*} +\begin{examplebox} +\caption[]{Reading multiple lines with a single \cw{read} command}% +\label{ex:readbrace}% +\writepage{Example} +First, the input file: +\begin{verbatim} +\begingroup \newlinechar=`\& \endlinechar=-1 +\message{&Please enter your name: }\global\read-1 to\name +\message{&And your Social Security number: }\global\read-1 to\ssno +\endgroup +\show\name \show\ssno +... +\end{verbatim} +Now the log file, including the responses (a \return/ was typed after +{\tt Frank}): +\begin{verbatim} +Please enter your name: {Joe Bob Willie Clark Mark Raphael Ferguson Frank + Henry James Percival Emerson Elmo Davenport, Jr.} + +And your Social Security number: 360-60-6000 +> \name=macro: +->{Joe Bob Willie Clark Mark Raphael Ferguson FrankHenry James Percival Emerson + Elmo Davenport, Jr.}. +l.7 \show\name + \show\ssno +? +> \ssno=macro: +->360-60-6000. +l.7 \show\name \show\ssno + +? +\end{verbatim} +\end{examplebox} +\end{example*} + +\subsection{Outer macros}\label{ss:readouter} + +{\em[This section is partly obsolete as of \tex/ version 3.141; see +\secref{ss:readprim}.]} + +If you enter an \cw{outer} macro in response to a \cw{read} prompt, +\tex/ will inform you in an error message +that it has inserted a closing brace. +Unfortunately, this is rather unhelpful, since you will then have the +matching brace problem described in the previous section; your answer +now contains an unmatched right brace, and if you don't type `x' at the +question-mark prompt to exit, you could get stuck. +Fortunately, it is unlikely for anyone to ever enter an outer macro in +response to a prompt, since in \plaintex/ the set of such macros is +small and used relatively infrequently, and in \latex/ there are almost +no \cw{outer} macros at all; accidental typing of a \verb;^^L; +character (which is active and outer, in \plaintex/ and \latex/) is +perhaps the least unlikely possibility. + + +\begin{example} +\begin{examplebox} +\caption[]{Outer control sequence choking a \cw{read} operation. Compare +this to the behavior of \ctrl{L} inside \cw{message} as discussed in \secref{ss:outer}.} +\label{ex:outerread}% +\writepage{Example} +Input file: +\begin{verbatim} +\read16 to\x \show\x +\end +\end{verbatim} + +Log file: +\def\next{\begin{verbatim}\rightskip0pt plus100pt minus 100pt +\linepenalty 500 } +\next +\x=^^L +Runaway definition? +-> +! Forbidden control sequence found while scanning definition of \x. +<inserted text> + } +<read *> ^^L + +l.1 \read16 to\x + +? h +I suspect you have forgotten a `}', causing me +to read past where you wanted me to stop. +I'll try to recover; but if the error is serious, +you'd better type `E' or `X' now and fix your file. + +? 1 +Type <return> to proceed, S to scroll future error messages, +R to run without stopping, Q to run quietly, +I to insert something, E to edit your file, +H for help, X to quit. +? +a +\ +# +{ +> \x=macro: +-> } \par a \^^M## { . +l.2 \show\x + +? +\end{verbatim} + +\end{examplebox} +\end{example} + +Some observations: + +---If the user checks the help message and stops to ponder the +situation, they have the opportunity, at least, to realize that E or +X to exit is indeed the wise choice. They're not {\em really\/} +stuck unless they carelessly try to continue. + +---If the user doesn't avail him/herself of the E or X option, just +about anything else that they try will be ineffective. On some +systems even the interrupt key won't help here; that leaves +essentially two ways out: match up the closing brace, or type another +outer thing to get back to the \verb;?; prompt and the associated +error recovery opportunity. + + +\subsection{Catcodes} + +The treatment of a user's response depends on the use to which it +will be put. \latex/'s \cw{typein} command is designed to take the +response and execute it, and therefore reads the user's response +using normal category codes. On the other hand, \fn{testfont.tex} +\cite{testfont} +changes the category codes of the special characters to 12 when +reading a user response, because the response will not be executed +but will be processed as simple character data. This approach is +probably the better one for most applications, since it avoids the +possibility of problems with things like mismatched braces or outer +control sequences, and since the \cw{read} command is used more often +to read strings of ordinary characters than to read executable control +sequences. + + +\subsection{\fn{Latex.tex}: \cw{typeout} and \cw{typein}} + +An interesting aspect of the \cw{typeout} and \cw{typein} commands in +\latex/ is that they aren't private control sequences (with \verb|@| +characters in their names); they are available for use in ordinary +document files. One of the uses suggested in the \latex/ manual is for +entering an \cw{includeonly} command interactively each time a +multipart document is processed. More commonly, however, \cw{typeout} +and \cw{typein} are used internally in documentstyle files, or in +special applications such as \fn{docstrip.tex} [Mittelbach, 1991] +where the distinction between private and public control sequences is +irrelevant. + +The purpose of \cw{typein} is (a)~to print a message on screen, and +(b)~to +read a response (one line) from the user, either into the internal +macro \cw{@typein}, or into a macro chosen by the macro writer. If +\cw{@typein} is used to receive the response, it will be executed as +\cw{typein}'s final action. Otherwise the response will be stored in +the designated alternate macro, without execution. (Cf.\ the \latex/ +manual, \S4.6.) + +I found the definition of \cw{typein} in \fn{latex.tex} more difficult +to understand than almost anything else of comparable length that I +have looked at. All the complications in the definition serve two +goals: (1)~If the user simply presses the {\sc return} key, the +resulting \cw{par} token needs to be discarded, leaving the macro that +holds the user reply empty; and (2)~if the user reply is not empty, it +will usually, but not always, contain a final space which needs to be +trimmed off. The fact that the final space might be missing is the +crucial problem. + +A simpler version of \cw{@xtypein} could be written using +a temporary deassignment of \cw{endlinechar}: +\begin{verbatim} +\def\@xtypein[#1]#2{\typeout{#2}\let\@typein\relax + \begingroup \endlinechar\m@ne \global\read\z@ to#1\endgroup + \@typein} +\end{verbatim} +This solves both the \cw{par} problem and the trailing space problem. + +However, much of the initial development of \latex/ took place in 1982 +and 1983, before the ultimate release of \tex/82, version~1.0 +(officially: December 3, 1983), and in old \TeX{} there was no access +to \cw{endlinechar}. (In fact many features of \tex/82 were added by +Knuth in response to reports from Lamport about various limitations of +the language that he ran into in the course of \latex/'s development.) +If Lamport noticed later that \cw{endlinechar} could be applied in +\cw{@xtypein}, he probably invoked the principle `If it ain't broke, +don't fix it' and left it alone. + +\subsection{\fn{Docstrip.tex}: \cw{typeout}, \cw{typein}, progress reports} + +As an example of the use of \cw{typeout} and \cw{typein}, +consider \fn{docstrip.tex} +\cite{docstrip}. This is a \latex/ +utility used for processing a documented macro file to remove +comments (the stripped-down version of a large macro file +will load significantly faster at run-time, at least on +less powerful computers). The use of +\cw{typeout} in \fn{docstrip.tex} is mainly a convenience, +to avoid the more cumbersome phrase +\cw{immediate}\cw{write16}, but \cw{typein} has a more +significant advantage---it takes care of removing a space +at the end of the user response, if present. + +In the following fragment from \fn{docstrip.tex}, the user is +informed that an auxiliary file named \fn{docstrip.cmd} has been +detected, and is asked whether it should be used. The response is +read into the macro \cw{answer}. + \begin{verbatim} +\typeout{*************************************************} +\typeout{* Batch file docstrip.cmd found. Use it ? (y/n) *} +\typein[\answer] + {*************************************************} +\def\@tempa{y}% +\ifx\@tempa\answer \process@batchfile \fi + \end{verbatim} +An analysis of the technique in this fragment brings +out a few noteworthy observations. + \begin{itemize} + \item In the prompt (line 2) the acceptable responses are +listed: \verb;y; or \verb;n;. + \item Since \cw{typein} doesn't change the case +of the user's response, and there's no code here in +\fn{docstrip.tex} to change case, we can deduce that a +response of \verb;Y; will be treated as a No. + \item If the user response is \verb;yes;, then \cw{answer} +and \cw{@tempa} will have different definitions, and the +\cw{ifx} test will be false. + \item If the user is expected to enter only a single letter, +we might ask, why is an \cw{ifx} test necessary? Why not +just use \cw{if} \verb|y|\cw{answer}\cw{process@batchfile} +and omit the extra step \cw{def}\cw{@tempa}\verb|{y}|? +However, this would invite trouble if the user pressed +{\sc return} without typing anything. Then \cw{answer} would +be empty (because of the special handling with \cw{@defpar} +in \cw{typein}) and before completing the evaluation of +the \cw{if}, \tex/ would continue beyond \cw{answer} +to expand \cw{process@batchfile} in search of a character +to compare with the \verb;y;. Although this would give a correct +result for the \cw{if} test, because the +first nonexpandable +token in the definition of \cw{process@batchfile} +is not a \verb;y;, the trouble is that the token will be +absorbed by the \cw{if} operation, which will throw a wrench +into subsequent processing. + +\end{itemize} + +{\it Note.} The version of \fn{docstrip.tex} from which I took these +examples was an early, unperfected version with an explicit +disclaimer from the author that it was a quick hack, and should not be +considered an example of his best macro writing. I discuss the early +version here, however, precisely because it illustrates so well some +of the typical difficulties in writing \tex/ dialog. + +An interesting feature of \fn{docstrip} is the ability to see on +screen the progress being made in processing the current file. If +this feature is turned on (with a \cw{showprogress} command), then +\fn{docstrip} prints on screen a percent sign for each comment +removed and a period for each line of code that is kept. This kind of +visual feedback is not just frippery and ornamentation, bells and +whistles. A typical user begins to get nervous if a running program +allows several seconds to pass by without any observable change on +screen. On a fast system, it's straightforward to deduce that such a +pause means something has gone wrong, whereupon the user can press an +interrupt key to investigate the problem without wasting time. But on +a slow system the poor user faces the dilemma: Abort unnecessarily, +and a restart will be required, with all the time consumed so far +wasted; or continue waiting when the program might simply be spinning +its wheels and accomplishing nothing. So when a task might take +more than, say, five seconds, it's good programming practice to give +some sort of progress reports. + +The progress reports of \fn{docstrip} use \cw{message} which, +as discussed earlier, adds a space between consecutive messages if +the second message is not too long to fit on the current line. So +what you see on screen is something like +\begin{verbatim} +% % % % % % % % % % . . . % % . . . . . % % % . . % . % % . % . % +. . . % % . % . % % % % % % % % % . . % . % % . % % % % . . . % . +. . . +\end{verbatim} +produced by repeated \cw{message}\verb"{%}" or \cw{message}\verb"{.}". +Recalling the newline behavior of \cw{immediate}\cw{write}, picture +what would happen if it were used instead of \cw{message} to see +why \cw{message} is necessary for this application. + +\section{Miscellaneous} + +\subsection{Checking a response for validity} +Here is the (slightly condensed) text of Brian Hamilton Kelly's UK\tex/ +post that contained \fn{checknum.sty} \cite{checknum}, which asks the +user for a number and checks to make sure that a valid number was +entered. + +\verbatiminput{uktex91.1x} + +\subsection{em\tex/ 8-bit output}\label{ss:8bit} +One interesting feature of em\tex/ \cite{emtex} is the 8-bit output +option. On PCs, the characters between 176 and 223 are box-drawing +characters; with the 8-bit output option of em\tex/, putting these +characters into a \cw{message} or \cw{write} command allows you to +draw some fairly elaborate boxes on screen, for embellishing menus and +other bits of dialog. I have only experimented with this a very little. + +\subsection{User Help} +There is a good deal of room for improvement in the amount and +kind of help information available to the user from within \tex/. +Help information provided externally through general help facilities +such as Unix man pages is well and good, but any help system +that's not \tex/-based has one disadvantage: lack of +portability across the whole spectrum of computer systems that +can run \tex/. + +\begin{itemize} \item Any program should have an announcement near +the beginning of how to quit without destructive side effects; for +\tex/ this means, among other things, that each version of the \tex/ +program should have in its opening message instructions on how to +break out of an infinite loop or in general how to interrupt \tex/ +before it has finished its current run. In {\it Textures\/} this +requirement is satisfied by the `Pause' button, always visible. In +DOS versions of \tex/ the interrupt key is normally the +Control-Break or Control-C key (depending partly on the particular implementation +of \tex/); in VAX/VMS it is Control-C or Control-Y, with the latter +reserved for emergency use only, since it will leave you without a +log file for reference. + +\item Response to the prompt +\[\parbox{.9\columnwidth}{\tt\raggedright +I can't find file xxx. Please type another input file name:\endgraf}\] +The novice user should be able to type the +reasonable guesses \fn{help} and \fn{h} and \fn{?}; the easiest way +to do this would be to put files \fn{help.tex}, \fn{h.tex} or +\fn{?.tex} in the \tex/ inputs directory/folder/area. (Except that +many OS's don't allow \fn{?.tex} as a file name). Already many +systems have a file \fn{null.tex} or \fn{nul.tex} to allow you to +abort reasonably gracefully if you know enough to enter {\tt null} or +{\tt nul} in response to the prompt. Very +few \tex/ users, however, will ever think of entering \fn{nul} without reading +about it in the documentation or hearing about it from a more +experienced user. + +\item Some of \tex/'s {\em built-in\/} error/help messages are +specific to \fn{plain.tex}. Some big macro packages such as \latex/ +might prefer to change some of the wording at least. For example, +there are references in some of \tex/'s compiled-in help messages to +things like \cw{def} and \cw{eqalign} that are documented nowhere in +the \latex/ book. And if you press \return/ at a \verb'*' prompt after +getting into \tex/'s interactive mode (intentionally or accidentally), +\tex/ urges you to enter a command or type \cw{end}---the latter being +worse than useless in \latex/ (where \cw{stop} or +\verb|\end{document}| are what is required): after typing \cw{end} and +\return/, nothing happens because \latex/ is waiting for the argument +of the \cw{end} command. + +\item Long help and error messages use string pool and main memory. +Storing them in external files would provide more space (at the cost +of slower access; but of course, once you get an error message, +processing speed is scarcely relevant any longer). + +\end{itemize} + +\section{Summary} + +\subsection{Sending messages} + +Recommendations: Until versions of \tex/ earlier than 3.141 are +sufficiently phased out, you had better use \cw{immediate}\cw{write} +rather than \cw{message} for generic message-sending macros, so that +all ``printable'' characters remain available for use in the message +text. Use \cw{message} instead of \cw{immediate}\cw{write} for +producing a prompt if you want the user's response to appear on the +same line. Uncatcode all special characters while constructing the +text of a message, if the message text is completely invariant between +one use and the next. Use \cw{string} if you don't want a space after +a control word, and use \cw{noexpand} if you do want a space. + +\subsection{Reading user input} + +Recommendations: Set the catcode of \cw{endlinechar} temporarily to +$9$ while reading a response, to avoid getting an extra space at the +end from the \ctrl{M}. Uncatcode all special characters, especially +opening and closing braces. If it is not uncatcoded, remove the +outerness from \verb;^^L;, at least while reading a user response, and +similarly, if the backslash is not uncatcoded and you want to be +supremely cautious, remove the outerness from any other outer control +sequences (\eg., \cw{newif}) if they might reasonably, or even +unreasonably, turn up in a user's response. + +\begin{thebibliography}{x} + +\bibitem[Cowan, 1987]{tables} Cowan, Ray. \fn{tables.sty}. 1987. This is +derived from \fn{tables.tex}; I found it at +\fn{sun\-.soe\-.clarkson\-.edu}, directory: \fn{pub\slash tex\slash +latex-style}. + +\bibitem[Greene, 1989]{animals} Greene, Andrew Marc. +``\tex/reation\Dash Playing games with \tex/'s mind.'' +\tugboat/ 10(4), pages 691\dash 705, 1989. Includes a listing +of \fn{animals.tex}. + +\bibitem[Greene, 1990]{basix} Greene, Andrew Marc. +``BaSiX: An interpreter written in \tex/.'' \tugboat/ 11(3), +pages 381\dash 392, 1990. + +\bibitem[Hamilton Kelly, 1991]{checknum} +{Hamilton Kelly}, Brian. \fn{checknum.sty}. {\it UK\tex/} +91(1), 4 January 1991. + +\bibitem[Knuth, 1986b]{TeX} Knuth, Donald E. {\sl \tex/: The program}. +Reading, Mass.: Addison-Wesley, 1986. + +\bibitem[Knuth, 1986c]{testfont} Knuth, Donald E. \fn{testfont.tex}. +{\sl The META\-FONT\-book}, Appendix H, section 4. Reading, Mass.: +Addison-Wesley, 1986. This file is included in all standard +distributions of \mf/. + +\bibitem[Lamport, 1985]{latex} Lamport, Leslie. \fn{latex.tex}. +Version 2.09 (1985--1992). Main source file for \latex/, included with +any standard distribution. + +\bibitem[Mattes, 1992]{emtex} Mattes, Eberhard. +em\tex/. Version 3.1415. +A comprehensive suite of programs including \tex/, +\mf/, printer drivers, previewers, Bib\tex/, \dots. +Available by anonymous ftp from \fn{niord.shsu.edu} (USA) or +\fn{ftp.uni-stuttgart.de} (Europe) and other fine archives. + +\bibitem[Mittelbach, 1991]{docstrip} Mittelbach, Frank. +\fn{docstrip.tex}. Version 1.1l, 1991. This file is part of the +\fn{multicol} package available by anonymous FTP from many archives, +including \fn{ftp.uni-stuttgart.de} and +\fn{ymir.claremont.edu}. + +\end{thebibliography} + +\appendix +\renewcommand{\thesection}{Appendix \Alph{section}} +\renewcommand{\thesubsection}{\Alph{section}.\arabic{subsection}} + +\section{Animals.tex} +This I have cleaned up a bit by making uppercase and lowercase +responses equally acceptable, and by making a loop so that +you can repeat the game without restarting \tex/ each time. +Source credit is in the bibliography \cite{animals}. + +\subsection{animals.tex} +\verbatiminput{animals.tex} + +\subsection{animals2.tex} + +\verbatiminput{animals2.tex} + +\subsection{animal.dat} +A typical file \fn{animal.dat} is shown here, in part. But the nature +of the game is for you to create your own version of this file based +on the answers that you give while playing. +\begin{verbatim} +1 +Does it have four legs? +2 +3 +2 +Is it an amphibian? +4 +5 +3 +Does it have six legs? +6 +7 +4 +Does it have a tail? +36 +37 +5 +Is it a big kind of cat? +10 +11 +6 +Does it have big hind legs used for jumping? +40 +41 +7 +Is it a fish? +8 +9 +8 +... +\end{verbatim} + +\section{Basix.tex} + +Another effort by Andrew Marc Greene, with clear relevance to the +subject of dialog in \tex/, is his prototype Basic interpreter +described in \cite{basix}. I had planned to give here a closer study +of the dialog concepts used by \fn{basix.tex} but it seems I will not +have enough time. + +\section{Tables.tex} + +The file \fn{tables.tex} \cite{tables} +provides table macros with the unique property that a preamble +line specifying the format of each row is not required; the +format is determined automatically by an analysis of the table +contents. The dialog part consists of a message such as +\begin{verbatim} +[Nrows=9, Ncols=2] +\end{verbatim} +that is printed on screen for each table. This provides confirmation +at run-time for the user of the general structure of each table. +In the worst case, if the number of rows or columns is +wildly wrong, the user can press the interrupt +key and go fix up the table before trying again. + +\section{Fontmenu.tex}\label{a:menu} + +The file \fn{fontmenu.tex} demonstrates a crude menu system based on +the token deletion option after an error message. There are five +tokens \cw{ComputerModern}, \cw{Garamond}, etc., corresponding to the +five font choices. They are so defined and arranged that if the user +enters, say, 2 to select Garamond fonts, then the deletion of two +tokens will leave the \cw{Garamond} token showing on screen (as the +last token deleted by the user), and then the next token +(\cw{Helvetica}) will define the font base to be `Garamond' and gobble +the remainder of the list. The effect of this arrangement is that the +user sees the '\cw{Garamond}' on screen as a confirmation of their +selection after they enter the number and before they press the +\return/ key a second time. + +\verbatiminput{fontmenu.tex} + +And here is screen output of a typical run through \fn{fontmenu.tex}: + +\verbatiminput{fontmenu.log} + +\ifdim\textwidth>\textheight +\typeout{^^J% +*******************************************************************^^J% +[*[*[*[*[ DON'T FORGET: print using LANDSCAPE orientation ]*]*]*]*]^^J% +*******************************************************************^^J% +} +\fi + +\end{document} |