path: root/web/spiderweb/doc/cacm.tex

\documentstyle[11pt]{article}
\title{Building a Language-Independent {\tt WEB}\thanks{This research
has been sponsored in part by the USAF, Rome Air Development Center,
under contract number F30602--86--C--0071.}}
%\author{Norman Ramsey\\Department of Computer Science\\Princeton University}
\author{Norman Ramsey\thanks{Current address: Department of Computer
Science, Princeton University, Princeton, New Jersey 08544}%
\\Odyssey Research Associates}

\newcommand{\token}{\cstok}
\def\sizedboxit#1#2{\vtop{\vbox{\hrule\hbox{\vrule\kern #2%
    \vtop{\vbox{\kern #2\hbox{#1}}\kern #2}\kern #2\vrule}}\hrule}}
\def\boxit#1{\sizedboxit{#1}{3pt}}
\newcommand{\remark}{\marginpar}
\def\remark#1{\marginpar{\footnotesize\it #1}}
\def\cstok#1{\leavevmode\thinspace\hbox{\vrule\vtop{\vbox{\hrule\kern1pt
        \hbox{\vphantom{\tt/}\thinspace{\tt#1}\thinspace}}
      \kern1pt\hrule}\vrule}\thinspace} % control sequence token
\def\ttok#1{\leavevmode\thinspace\hbox{\vrule\vtop{\vbox{\hrule\kern1pt
        \hbox{\vphantom{\tt(j}\thinspace{\tt#1}\thinspace}}
      \kern1pt\hrule}\vrule}\thinspace} % token
\newcommand{\X}{}
\def\X:#1\X{\mbox{$\langle${#1}$\rangle$}}

\begin{document}
\maketitle

In the fall of~1987 I started planning the implementation of a 
suite of tools for building verified Ada
programs~\cite{ramsey:developing}.
The first tool to be built was a
verification condition generator, which was to be formally defined
using the typed lambda calculus.  
I was eager to include the definition with the code
so that it would be easy to check the code's correctness.
Using {\tt WEB} would have made this easy, but,
unfortunately, the target programming language was 
SSL (a language for specifying structure editors), and the only
languages for which {\tt WEB} implementations were 
available were  Pascal and~C. 

Writing a new {\tt WEB} from scratch didn't make sense, so
I decided to modify Silvio Levy's implementation of {\tt WEB}
in~C~\cite{levy:cweb}, 
to get a {\tt WEB} that would be written in C, but would read and
write SSL code.
From my previous experiences modifying {\tt WEB}, I knew that the
most time-consuming job would be fine-tuning the grammar that  {\tt
WEAVE} uses to prettyprint code.
I believed I could make debugging that grammar a lot less painful
if, instead of trying to make dozens of small modifications by hand, I
 wrote a simple program, perhaps an AWK script, that would read 
a description of the grammar and generate C~code for {\tt WEAVE}.
That AWK script became {\tt SPIDER}, a program that turns language
descriptions into C~code for {\tt TANGLE} and {\tt WEAVE}.
I have used {\tt SPIDER} to generate {\tt WEB}s for C, AWK, SSL, Ada,
and a couple of other languages.
I won't go into the details of {\tt SPIDER}; instead, I'll try to
describe what {\tt SPIDER} does to accomplish its mission, or 
how to take the ``essence of {\tt WEB}'' and make it
language-independent. 

\medskip

When using {\tt WEB}, a programmer writes a single source file, {\tt
foo.{\tt web}}, that holds both code and documentation.
{\tt TANGLE} and {\tt WEAVE} read that file.
{\tt TANGLE} extracts
the code from the {\tt WEB} file and rewrites it in a form suitable
for compiling.
{\tt WEAVE} passes the documentation parts to a document formatter
({\TeX}), and prettyprints the code parts.
The whole process is shown in Figure~\ref{instance}, for C~programs
written in {\tt WEB}.
The~{\S} represents files that have to be written by hand.
{\sl Slant} type is used for the names of executable programs.
{\sl CTANGLE} and {\sl CWEAVE} are the C-language versions of {\tt
TANGLE} and {\tt WEAVE}, {\sl cc}~is a C~compiler, and {\sl ld}~is a
loader. 

\begin{figure}
\caption{Processing a C~{\tt web} file}
\label{instance}
\footnotesize
\setlength{\unitlength}{2pt}
\begin{picture}(170,80)(0,-40)
\tt
\put(20,12.5){\makebox(0,0)[l]{\ \sl CTANGLE}}
\put(20,-12.5){\makebox(0,0)[l]{\ \sl CWEAVE}}

\put(0,-5){\framebox(30,10){foo.{\tt web}\smash{${}^{\S}$}}}
\put(15,5){\vector(1,2){7.5}}
\put(10,20){\framebox(30,10){foo.c}}
\put(15,-5){\vector(1,-2){7.5}}
\put(10,-30){\framebox(30,10){foo.tex}}

\put(40,-25){\vector(1,0){20}}
\put(50,-23.5){\makebox(0,0)[b]{\sl {\TeX}}}
\put(60,-30){\framebox(30,10){foo.dvi}}
\put(90,-25){\vector(1,0){20}}
\put(100,-25){\makebox(0,0){\shortstack{\strut dvi\\\strut \rm driver}}}
\put(110,-25){\makebox(0,0)[l]{\rm\strut \ Typeset documentation for
{\tt foo}}}

\put(40,25){\vector(1,0){20}}
\put(50,26.5){\makebox(0,0)[b]{\sl cc}}
\put(60,20){\framebox(30,10){foo.o}}
\put(90,25){\vector(1,0){20}}
\put(100,26.5){\makebox(0,0)[b]{\sl ld}}
\put(110,25){\makebox(0,0)[l]{\rm\strut \ Executable \sl foo}}



\end{picture}

\hrule
\end{figure}

{\tt SPIDER} is used to construct {\em instances} of {\tt TANGLE} and
{\tt WEAVE}, and these instances are used to process programs as
shown in Figure~\ref{instance}.
Code for the language-dependent parts of these instances is generated
by {\tt SPIDER} when it 
reads a language description file written by a {\tt WEB} designer.
Figure~\ref{building} shows how instances of {\tt TANGLE} and {\tt
WEAVE} are generated. 
{\tt SPIDER} converts a hand-written description of a programming language
into C~{\tt WEB} code for the language-dependent parts of {\tt TANGLE}
and {\tt WEAVE}. 
In Figure~\ref{building} the target programming language is a hypothetical
``X,'' and the description file is called ``{\tt x.spider}.''
{\tt CTANGLE} combines the code {\tt SPIDER} writes with the ``master
copies'' of {\tt tangle.web} and {\tt weave.web}, which contain the
language-independent parts of {\tt TANGLE} and {\tt WEAVE}.
The result is C~source code for {\tt XTANGLE} and {\tt XWEAVE}.
After that code is compiled and loaded with {\tt WEB}'s I/O code, the
resulting executable versions of {\tt XTANGLE} and {\tt XWEAVE} can be
used to process X-language programs written in {\tt WEB} format, as
shown around the periphery of Figure~\ref{building}.


The master copies of {\tt tangle.web} and {\tt weave.web} are about
1800 and 3200 lines long, respectively.
About one third of these lines are comments.
To illustrate the other size, suppose X is the Ada programming language.
The {\tt ada.spider} file is about 260 lines long, and from it {\tt
SPIDER} generates 
about 1400~lines of {\tt ADATANGLE} and {\tt ADAWEAVE}.
About one tenth of these lines are comments.
It is typical for {\tt SPIDER} to generate between $5n$ and $6n$ lines
of C~{\tt WEB} code from an $n$~line language description.

\begin{figure}
\caption{Building and using an instance 
of {\tt WEB} (for language X)}
\label{building}
\footnotesize
\setlength{\unitlength}{2pt}
\begin{picture}(215,115)(10,-55)
\tt
\put(185,-5){\framebox(30,10){x.spider\smash{${}^{\S}$}}}
\put(185,0){\line(-1,0){20}}
\put(174,1.5){\makebox(0,0)[b]{$\,$\sl SPIDER}}
\put(165,0){\vector(-1,1){10}}
\put(165,0){\vector(-1,-1){10}}

\put(125,5){\framebox(30,10){xt.web}}
\put(125,-15){\framebox(30,10){xw.web}}
\put(125,25){\framebox(30,10){\shortstack{{\rm Master}\\tangle.{\tt web}}}}
\put(125,-35){\framebox(30,10){\shortstack{{\rm Master}\\weave.{\tt web}}}}
\put(125,-30){\line(-1,1){10}}
\put(125,-10){\line(-1,-1){10}}
\put(115,-20){\vector(-1,0){25}}

\put(104,-18.5){\makebox(0,0)[b]{\scriptsize\sl CTANGLE}}

\put(125,10){\line(-1,1){10}}
\put(125,30){\line(-1,-1){10}}
\put(115,20){\vector(-1,0){25}}

\put(104,21.5){\makebox(0,0)[b]{\scriptsize\sl CTANGLE}}

\put(60,15){\framebox(30,10){xtangle.c}}
\put(60,-25){\framebox(30,10){xweave.c}}
\put(60,-20){\vector(-1,0){20}}
\put(50,-18.5){\makebox(0,0)[b]{\sl cc, ld}}
\put(60,20){\vector(-1,0){20}}
\put(50,21.5){\makebox(0,0)[b]{\sl cc, ld}}
\put(38,20){\makebox(0,0)[r]{\sl XTANGLE}}
\put(38,-20){\makebox(0,0)[r]{\sl XWEAVE}}

\put(-6.25,-5){\framebox(30,10){foo.{\tt web}\smash{${}^{\S}$}}}
\put(8.75,5){\vector(1,4){6.25}}
\put(0,30){\framebox(30,10){foo.x}}
\put(8.75,-5){\vector(1,-4){6.25}}
\put(0,-40){\framebox(30,10){foo.tex}}

\put(30,-35){\vector(4,-1){20}}
\put(50,-48.75){\framebox(30,10){foo.dvi}}
\put(80,-47.5){\vector(4,-1){20}}
\put(100,-52.5){\makebox(0,0)[l]{\rm\strut \ Typeset documentation for
{\tt foo}}}

\put(30,35){\vector(4,1){20}}
\put(50,38.75){\framebox(30,10){foo.o}}
\put(80,47.5){\vector(4,1){20}}
\put(100,52.5){\makebox(0,0)[l]{\rm\strut \ Executable \sl foo}}



\end{picture}

\hrule
\end{figure}

\medskip

A {\tt WEB} program is a collection of ``sections,'' each of which has a
documentation part, a definition part, and a code part.
The documentation part describes what the section is supposed to do,
and is intended to be processed by a formatter---my {\tt WEB}s use
{\TeX}, which is especially convenient for mathematical symbols like
those used in writing a formal semantics.
The definition part contains macro definitions.
Each macro may have parameters or not, as the programmer chooses.
The code in the code part is a fragment of the whole program.
It is called a ``module'' and can be named or unnamed.
When the module is named, the module name ``stands for'' that code,
just as a macro name stands for the code in its definition.
The unnamed module is special; the code in the unnamed module is
considered to be ``the program.''

Figure~\ref{fragment} shows a fragment of a {\tt WEB} program; the
fragment inverts an 
EBCDIC-to-ASCII table to obtain an ASCII-to-EBCDIC table.
The target programming language is C.
One module, \X:Invert {\it to\_ascii}, producing {\it to\_ebcdic}\X,
uses the code defined in the other, \X:Set ${\it
to\_ebcdic}[i]\leftarrow {\it UNDEFINED\_CODE}$ for all $i$\X.
The program, {\tt foo}, of which this fragment is a part, can be input
to {\tt CTANGLE} and {\tt CWEAVE}, to produce {\tt foo.c} and {\tt
foo.tex} respectively, as shown in Figure~\ref{instance}.

\begin{figure}
\caption{Table Inversion}
\label{fragment}
\begin{verbatim}
@ The array |to_ascii| converts an EBCDIC code to 
an ASCII code, or to |-1| if there is no ASCII 
equivalent to the given code.
@d UNDEFINED_CODE = -1
@<Invert |to_ascii|, producing |to_ebcdic|@>=
    @<Set |to_ebcdic[i]=UNDEFINED_CODE| for all |i|@>@;
    for (i=0; i<256; i++)
        if (to_ascii[i] != UNDEFINED_CODE)
            to_ebcdic[to_ascii[i]]=i;

@ @<Set |to_ebcdic[i]=UNDEFINED_CODE| for all |i|@>=
    for (i=0; i<128; i++) to_ebcdic[i] = UNDEFINED_CODE;
\end{verbatim}
\hrule
\end{figure}



{\tt TANGLE}'s job is to take a  collection of sections and to
produce a compilable program.
{\tt TANGLE} reads all the sections, skipping the documentation parts
completely, but storing the macro definitions from the definition parts
and the module definitions from the code parts.
After it has read all the sections, {\tt TANGLE} then
writes out the code in the unnamed module.
But whenever it encounters a module name in that code, instead of
writing out the name, it writes out the code for which this name
stands. 
That code may itself contain module names, which are replaced with the
code for which they stand, and so on until {\tt TANGLE} reaches code
which contains no occurrences of module names.
{\tt TANGLE} processes macros similarly, except that macros may
have parameters (modules may not).

As I've described it, the ``essence of tangling'' is
language-independent. 
In the full implementation of {\tt TANGLE} there are only a few
language-dependent details, and almost all of them come up only in
lexical analysis. 
During its input phase, {\tt TANGLE} converts macro definitions and
module definitions into token lists.
%%%Every token is represented using either eight or sixteen bits.
The major kinds of tokens are
module name tokens, identifier tokens, and ordinary tokens.
Identifier tokens may be expandable (when they are macro names) or
unexpandable (when they are programming-language identifiers).
Module name tokens are always expandable, and ordinary tokens are
never expandable.
{\tt TANGLE} uses a stack to write out the token list corresponding to
the unnamed module, expanding expandable tokens as it goes.
No token is ever expanded until the time comes to write that token on
the output.

To build the language-dependent part of {\tt TANGLE}, it is enough to
tell {\tt TANGLE} how to tokenize the input and how to write out a
token list.
{\tt TANGLE} uses a ``lowest common denominator'' lexical analyzer to
tokenize its input.
The set of tokens recognized by this lexical analyzer is the union of
the sets of legal tokens of many different languages.
For example, different ways of delimiting string literals are
recognized. 
Identifiers may have underscores, even though some languages
(e.g.~Pascal) may not permit underscores in identifiers, and others
(e.g.~Ada) may not permit consecutive underscores in an identifier.
In general, {\tt TANGLE} and {\tt WEAVE} do the right thing with legal
programs, but they do not detect illegalities in a program.

{\tt TANGLE}'s lexical analyzer recognizes a fixed set of tokens
representing identifiers, string literals, and numeric literals.
Any printable ASCII character which is not part of some other
token forms a token all by itself.
A {\tt WEB} builder can specify that certain strings should
be treated as single tokens, and
{\tt SPIDER} will convert the specifications into appropriate code for
{\tt TANGLE}.
For example, when building {\tt WEB} for~C, we tell {\tt SPIDER} 
that the strings {\tt ++}, {\tt ==}, and {\tt \&\&} (and many others)
should be treated as single tokens, by putting the statements
\begin{quote}\tt
token ++ ...\\
token == ...\\
token \&\&\ ...
\end{quote}
into the language description file, {\tt c.spider}.

{\tt TANGLE} discards comments, rather than attempting to tokenize
them.
Comments are assumed to begin with a special string, and to end either
with another string or with a newline.
We specify C comment conventions by telling {\tt SPIDER}
\begin{quote}\tt
comment begin <"/*"> end <"*/">
\end{quote}


On output, {\tt TANGLE} converts tokens to text by inverting the
process of lexical analysis, so, for example, the token \token{++} is
written out as ``{\tt ++}''.
{\tt TANGLE}'s output phase inserts white space between adjacent
identifiers and numeric literals, but otherwise does not write white
space.
This can cause problems in some cases; for example, in older
C~compilers the string 
``{\tt =-}'' is ambiguous.
We can solve this problem by telling {\tt SPIDER} to build a {\tt
TANGLE} that uses the text 
 ``{\tt =~}'' when writing the \token{=}:
\begin{quote}\tt
token = tangleto <"= ">
\end{quote}

In sum, we can make {\tt TANGLE} language-independent with almost no
effort.
We do this by using a lowest common denominator lexical analyzer whose
only parameter is a description of comments, and by providing a way to
fine-tune the way {\tt TANGLE} writes tokens.



\medskip
Unlike {\tt TANGLE}, {\tt WEAVE} does no rearranging of the sections;
its job is to translate its input into a prettyprinted program
listing.
The documentation part of each section is simply copied to the output,
but the definition and code parts are prettyprinted.
{\tt WEAVE}'s output is handed to a document formatter, which is
assumed to implement a prettyprinting algorithm like that described by
Oppen~\cite{oppen:prettyprinting}.
Since my {\tt WEB}s use {\TeX} as the document formatter, the back-end
prettyprinting is implemented by {\TeX} macros.

{\tt WEAVE} copies the documentation parts as texts, but it converts
definition and code parts to token lists using the same lexical
analyzer used by {\tt TANGLE}.
{\tt WEAVE}'s part of the prettyprinting task (as distinct from
{\TeX}'s part) is converting these token lists to streams of {\TeX}
text, possibly with prettyprinting instructions intercalated between
tokens. 
If you like, {\tt WEAVE}'s job is to produce the input to Oppen's
algorithm. 
For simplicity, we'll discuss only three prettyprinting instructions:
{\em indent} 
(increase the level of indentation), {\em outdent} (decrease the level
of indentation), and {\em force} (force a line break).

We tell {\tt WEAVE} how to convert tokens to {\TeX} text by specifying
a {\em translation} for each token.
Suppose we want the C token \token{!=} to be printed as~``$\ne$'',
which is produced by the {\TeX} text ``\verb+\ne+''.
Then we write
\begin{quote}
\begin{verbatim}
token != translation <"\\ne">
\end{verbatim} 
\end{quote}
(Two backslashes appear in the translation because {\tt SPIDER} uses
C~conventions for string literals.
The angle brackets {\tt <...>}  delimit translations.)
The default for translation is just as in {\tt TANGLE}, so if we want
``{\tt +}'' on input to produce ``$+$'' on output we need not specify a
translation for the token \token{+}.

The process of deciding where to put line breaks and indentation is
the most complicated part of {\tt WEAVE}.
We have to do this based on the sequence of tokens we have, but the
exact details of which token is where usually aren't needed to do
prettyprinting.
Hence we introduce the {\em scrap}, which abstracts away from the
detail not needed to do prettyprinting.
A scrap has two parts: the translation, which we have already seen,
and the {\em category}, which
 corresponds to a ``part of speech'' or a symbol in a grammar. 
{\tt WEAVE} uses categories to decide where to put
indentation and line breaks.
Since there are usually many different tokens having the same category,
prettyprinting is simplified enormously.

{\tt WEAVE} begins processing a program fragment by tokenizing the
fragment, then converting each token in the resulting token list into
a scrap. 
It then attempts to reduce the length of the resulting scrap list by
combining adjacent scraps into a single scrap, possibly intercalating
additional translations, which may include
{\em indent}, {\em outdent}, and {\em force} instructions.
The scraps are combined according to one of many {\em reduction rules}.
{\tt WEAVE} decides which adjacent scraps are eligible to be reduced
based only on the categories of the scraps and a knowledge of the
reduction rules.
The reduction rules are the productions of the {\em prettyprinting
grammar}. 
{\tt WEAVE}'s reductions of scraps are like the reductions done in
bottom-up parsing. 

To take an example, suppose that we want statements to be separated by
line breaks.
If we can guarantee that any scrap representing a statement has
category {\tt stmt}, it will be enough to specify the reduction rule
\begin{quote}\tt
stmt <force> stmt --> stmt
\end{quote}
which says ``two adjacent {\tt stmt} scraps may be reduced to a single
{\tt stmt} scrap by intercalating a forced line break between them.''

So we tell {\tt WEAVE} how to prettyprint a language by telling
how to assign a category to each token and how to combine scraps.
Here's another example: the language of C~expressions.
 Let {\tt math} be the category of expressions,
{\tt binop} be the category of binary infix operators, and
{\tt unop} be the category of both unary prefix and unary postfix
operators.
Here are some sample tokens:
\begin{quote}
\begin{verbatim} 
token identifier category math
token + category binop
token - category binop
token = category binop translation <"\\leftarrow">
token == translation <"\\equiv"> category binop
token ( category open
token ) category close
\end{verbatim}
\end{quote}
Notice we print the \token{=} token (assignment) as $\leftarrow$,
whereas we print the \token{==}~token (comparison) as $\equiv$.
This makes it a bit easier for us to see when a programmer has
mistakenly used \token{=} instead of \token{==}.


The prettyprinting grammar for C~expressions is:
\begin{quote}
\begin{verbatim}
math binop math --> math
math unop --> math
unop math --> math
open math close --> math
\end{verbatim}
\end{quote}
Using this grammar, {\tt WEAVE} can take a long expression consisting
of many scraps, and reduce it all to a single scrap of category {\tt
math}. 

What about an operator like ``{\tt *}'', which is both binary infix
and unary prefix? 
This does the job:
\begin{quote}
\begin{verbatim}
token * category unorbinop
unorbinop math --> math
math unorbinop math --> math
\end{verbatim}
\end{quote}

% No Note about context-sensitive reductions?

There is a mechanism for assigning categories and translations to
reserved words as well as to tokens, using slightly different syntax. 

To give an idea of the complexity of the grammars, the grammar
describing AWK uses 24~categories in 39~productions.
The Ada grammar uses 40~categories in 65~productions, and the
C~grammar uses 54~categories in 129~productions.

\medskip

{\tt SPIDER}-generated versions of {\tt TANGLE} and {\tt WEAVE} differ
subtly from the originals written by Donald Knuth.
The most important difference is that {\tt SPIDER}-generated {\tt WEB}
is not self-contained: where Knuth's Pascal {\tt WEB} required only a
Pascal compiler to bring up, {\tt SPIDER} would need a C~compiler and
an AWK~interpreter to generate a Pascal {\tt WEB}, and a Pascal
compiler for the resulting {\tt WEB} to be of any use.
Other differences are minor; for example, Knuth's {\tt TANGLE}
does arithmetic on constants at {\tt TANGLE} time, but {\tt
SPIDER}-generated {\tt TANGLE}s do not.
Knuth's {\tt TANGLE} provides three different kinds of macros, but
none with more than one parameter;
{\tt SPIDER}-generated {\tt TANGLE}s provide only one kind of macro,
but macros of that kind may have from zero to thirty-two parameters.



{\tt SPIDER} is a {\tt WEB} generator, akin to parser generators.
Both read formal descriptions of some part of a programming language,
and both produce code that processes programs written in that language.
Since both produce code that is part of the ``compiler,'' using them
doesn't introduce any extra steps into the processing of user programs.
{\tt SPIDER} itself is a large AWK script, written as a {\tt WEB}
program.  
{\tt spider.web} is about 2600 lines long; about a third of these are
comments. 


The major cost of using {\tt SPIDER} is the cost of learning yet
another language.
Learning this language is supposed to substitute for learning how to
modify {\tt WEB}, so it is probably not an exorbitant cost.
Some other limitations are the the need for a C~compiler and an
AWK~interpreter, and the need to use a lowest-common-denominator
lexical analyzer. 

The major benefit of using {\tt SPIDER} is the ease with which new
{\tt WEB}s can be built.
%\remark{(Here is the place to talk about Oberon.)}
The {\tt SPIDER} description of a language is much smaller than the
{\tt WEB} implementation generated from that description, and {\tt
SPIDER} descriptions of similar languages are similar.
Using {\tt SPIDER} one can build a {\tt WEB} without understanding the
details of {\tt WEB}'s implementation, and one can easily adjust that
{\tt WEB} to change as a language definition changes.
%\remark{Could mention work with Larch/Ada-88}



{\tt SPIDER}  should make one literate programming tool, {\tt WEB},
available to a much larger audience.
I hope that, by separating the language-independent parts of {\tt WEAVE}
and {\tt TANGLE}, {\tt SPIDER} will  encourage us not just to think
about what the essence of tangling and weaving is, but also about what
the essence of literate programming is.


\medskip

I enjoyed many useful discussions of {\tt WEB} with Charlie Mills.
I am grateful to
Silvio Levy for providing his {\tt CWEB} as the basis for the
``master copies'' of {\tt TANGLE} and {\tt WEAVE}, and to
Dave Hanson for comments on an earlier version of this paper.

\begin{thebibliography}{van~Knuth~999}
\bibitem[Bentley~86]{bentley:lp}
Jon L. Bentley, ``Programming Pearls,''
{\sl Communications of the ACM} {\bf 29:5} (May~1986), 364--368, and
{\bf 29:6} (June~1986), 471--483.
\bibitem[Knuth~84]{knuth:literate-programming}
Donald E. Knuth, ``Literate Programming,'' {\sl The Computer Journal}
{\bf 27:2}(1984), 97-111.
\bibitem[Levy~87]{levy:cweb}
Silvio Levy, ``{\tt WEB} Adapted to C, Another Approach,''
 {\sl TUGBoat} {\bf 8:1}(1987), 12--13.
\bibitem[Oppen~80]{oppen:prettyprinting}
Derek Oppen, ``Prettyprinting,'' TOPLAS~{\bf 2:4} (October 1980),
465--483. 
\bibitem[Ramsey~89]{ramsey:developing}
Norman Ramsey, ``Developing Formally Verified Ada Programs,''
Proceedings, {\sl Fifth International Workshop on Software
Specification and Design}, to appear.
\bibitem[Van~Wyk~87]{cvw:loom}
Christopher J. Van~Wyk, ``Literate Programming,''
{\sl Communications of the ACM} {\bf 30:7} (July~1987), 593--599, and
{\bf 30:12} (December~1987), 1000--1010.
\end{thebibliography}

\end{document}