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Literate Programming with J and noweb

by Martin Neitzel (neitzel@gaertner.de)

The integrated creation of program source and its documentation is called literate programming. This term goes back to Donald Knuth and his "WEB" system, a literate programming system for Pascal.

This article describes my first experiences with using Norman Ramsey's "noweb" system for J programming. We also take a look at a few other approaches to create nice representations of J code.


Perhaps interestingly, I took my first look at noweb when I wrote my first Vector article. I already had all the J coding done, and I just wanted to document it for Vector readers. Rather than starting a new text document and including some source snippets via cut and paste, I extended the original J script file with additional text material.

Keeping the source code and its documentation in the same file is the central idea behind literate programming. This way it becomes easy to maintain these two aspects of a program consistently. As the old adage says: nothing is more misleading than outdated comments and incorrect documentation. By tying the code and all its documentation closely together, literate programming makes it easier to keep both in sync.

The unified source is then filtered in different ways for different purposes. One can either extract the program code for the compiler or, as is the case with J, the interpreter; or, alternatively, pull out the documentation with embedded code fragments to be nicely typeset.

Noweb is a freely available WEB cousin written by Norman Ramsey. The web site http://www.eecs.harvard.edu/~nr/noweb/ is the home page of the system. Noweb's main characteristics are simplicity and language-independency. It is available for all J platforms and already comes as an installable package with all of the free Unix/Linux distributions. All in all, enough reasons for me to give it a serious try with a few small J projects.

The following are the main tools of the noweb system:

extracts just the source code proper from the literate program, i.e. a J script.

extracts documentation with embedded code prepared for some typesetting program such as TeX/LaTeX, troff, or an HTML browser; it optionally creates and formats cross references along the way.

Noweb supports a handful of programming languages and typesetting backends out of the box. For the use with J, I provided a very simple recogniser for declarations in order to get the cross referencer working. One could give explicit hints with each code chunk about the defined objects but an automatic recogniser is much more convenient.

Since I deemed only global definitions at the module level worthwhile to be indexed, this was easy. A single regular expression was all I needed, namely ^[a-zA-Z0-9_]+ *=:. This pattern matches all assignments to J names at the beginning of a line. I always indent J sentences within a verb definition, a style that in my opinion makes a script more digestable for both human readers and such simplistic analysers alike.

Noweb's way of integration of such tools is straightforward. One just needs to provide a text filter operating on a common intermediate format.

All in all, a 20-line awk script takes care of definition recognition, index collection, and some other amenities like using <<>>= for the continuation of the previous code chunk without spelling out its name again.

A Simple Example

Input (aka: The Source)

Here is a simple example. First, a noweb input file with some documented J code, presumably stored as stat.nw:

<<statistics>>= <<auxiliaries>> <<averages>> @ Averages are coming in various forms. There is the well-known [[mean]] function, for a start: <<averages>>= mean=:sum % # @ The geometric mean is another one: <<averages>>= geomean=: # %: prod @ This works with the help of some aggregating functions: <<auxiliaries>>= sum=: +/ prod=: */

See how code chunks start or are continued with names in double angle brackets. Documentation mode is entered with a single "@" character. Embedded code in documentation is written in double square brackets.

Output 1: The Code

The command

notangle -t8 -Rstatistics -filter emptydefn stat.nw > stat.ijs
will extract the J script file stat.ijs. It will look like this:

sum=: +/ prod=: */ mean=:sum % # geomean=: # %: prod

The -Rstatistics option tells noweave to start extraction at the root node statistics. This node is then expanded from its arbitrarily large tree of sub nodes. The important things to note about the nodes are:

  1. You can extract different code parts, i.e., different J scripts, from one and the same noweb source.
  2. The order of the extracted code can very well be different from the order in the noweb source which is typically the one deemed most suitable for a lucid presentation.

Output 2: The Documentation

The command

noweave -html stat.nw > stat.html
creates the documentation variant of the file. It can generate TeX, LaTeX, troff, and HTML markup out of the box. When viewed with your browser, you'd see roughly the following, including the font changes:

Defines geomean, mean, prod, sum (links are to index).

Averages are coming in various forms. There is the well-known mean function, for a start:

<averages>= (<-U) [D->]
mean=:sum % #
Defines mean (links are to index).

The geometric mean is another one:

<averages>+= (<-U) [<-D]
geomean=: # %: prod
Defines geomean (links are to index).

This works with the help of some aggregating functions:

<auxiliaries>= (<-U)
sum=: +/
prod=: */
Defines prod, sum (links are to index).

The same command with a few additional options will create hyperjumps and indexes for chunk names and definitions:

noweave -html -filter autodefs.ijs -index  stat.nw > stat.html
Check out http://juggle.gaertner.de/bnp/litprog.html to experience that version of this article.

Many references created by noweave are marked as "U" or "D", pointing to uses and definitions of the named code nodes or global J names.

The text sections in a noweb source can also use any markup commands appropriate for the noweave backend used. If you want to have chapter headings or bulleted lists in your documentation, this is the way to go.

With the output generated for the heavy-duty typesetting programs LaTeX or troff, you can make a high-quality booklet or even tome. The cross-references will go into the page margin, of course with the proper page references. I stumbled over noweb when I read the following gem: A Retargetable C Compiler: Design and Implementation by C. Fraser and D. Hanson -- 592 pages noweb-based compiler source and documentation in finest typography. The PDF for a sample chapter can be downloaded from the lcc home page http://www.cs.princeton.edu/software/lcc/.

Nitty-gritty Details

Knuth's original WEB tool was tailored to Pascal and TeX. (It was actually written to overcome some of Pascal's short-comings: the lack of a module system and the very strict order of definitions in a standard Pascal program.) The good thing about noweb is that it can be used with any source language and generate output for different text processing systems.

While the J code excerpt shown above is digestable by J, you don't want to touch it ever again. For example, if you have to fix a bug, you should do this in the stat.nw file, not in stat.ijs produced by notangle. Likewise, there is little use to put stat.ijs under version control.

I have therefore written my little load variant loadnw which notangles a script on the fly and loads the output:

NB. [chunkname] loadnw filename
NB. *loads a node from a noweb file.
loadnw_z_ =: verb define
        '*' loadnw y.
        0!:100 (2!:0) 'notangle -t8 ''-R', x., ''' -filter emptydefn ', y.
Defines loadnw_z_ (links are to index).

The chunk name * used as default left argument here is noweb's default root node. You'll often find noweb files just containing one or more <<*>> chunks and no further sub-nodes.

The option to extract different code parts from a script also comes in very handy for test code. Users of the library just want to get the definitions from the script. But when I work on the script, it helps to have an additional test chunk executed which prepares and runs some initial tests. The library code is defined as a subordinate node to the test node so that I can choose which version to extract from the script by specifying the corresponding root node. Noweb is effectively taking the role of a classic "conditional preprocessor" when used in this style.

Unfortunately, J will report errors with a line count referring just to the extracted code. It would be really helpful if the line number could refer properly to the noweb source file. After all, this is where the fix has to be made. Noweb can indicate the proper source coordinates in its notangled output. By default, it uses C preprocessor-style #line 123 stat.nw directives but it can generate any other syntax as appropriate for the programming language used. It would be really helpful if J could take such a hint -- not just for working with noweb but for any other code generator as well.

The nountangle utility is a variant of notangle. It creates a program, too, but keeps the full documentation as block comments in the extracted code. Nountangle needed to be extended to support J's NB. syntax. As luck has it, it is a shell script on Unix, so this was very simple to arrange.

Comparisons with Other Approaches

Plain Old Comments

Lines prefixed with NB.s are easy to ignore in small doses. Beyond a dozen lines, the letters can begin to irritate readers. Longer documentation also benefits from decent typesetting. After all, you seldom see a Vector article only consisting of NB'ed code.

Just for improved readability, I've seen some people using literate noun scripts to include one or more paragraphs of comments in a script. This runs as follows:

0 : 0 todayno: converts dates to day numbers. Method is to subtract dates on a calendar basis to determine integral number of months plus the exact number of days remaining. This is converted to payment periods, where # days remaining are calculated as: (# days)%365 )

The script containing the notes is simply parsed and immediately discarded again. Without any leading NB letters, it is readable with less distraction than ordinary comment blocks.


J/Windows comes with the scriptdoc utility. It will extract traditional NB. comments written according to some scriptdoc-specific markup conventions. Basically, you just have to put an asterisk and a one-letter code in a strategic place.

The comments list the names of the routines, their typical use, and perhaps some notes on their implementation. It is scriptdoc's job to extract this info and create a nice overview. This can then be used as a quick reference or manual.

Scriptdoc's job is to suppress implementation details. The output is usually biased towards the users of a library script. Noweb in turn targets other developers working on the code which is included in its entirety.


J/Windows Release 4.03 comes with a code-to-HTML converter. It uses syntax-based colouring to create nice output according to user-definable colour schemes. One would typically use HTML-Publish to save the generated output to separate web page which could then be viewed by others.


Vgrind is a traditional pretty-printing tool for the Unix nroff/troff typesetting system. Code fragments are embedded in the text within .vS/.vE start and end lines.

Vgrind will then insert the appropriate font change instructions to mark keywords, constants, and comments. It detects these elements guided by a short language description defining some patterns. The patterns will also enable vgrind to collect an index for whatever looks like a "procedure definition". You can find such a vgrind definition for J on juggle.gaertner.de .

The mixed documentation/code approach is not unlike the style used in noweb sources, but I am not aware that people ever used this system for literate programming. That is, I never saw the practice or recommendation to extract just the code for further processing. Most often vgrind is simply run on original code files without any additional text or .vS/.vE delimiters, with the pretty-printed output directly sent to the printer. This use is not much different from "HTML Publish".

The J Project Manager

The J Project Manager is not a tool concerned with documentation in any way. There is a reason why I am including it in this short list of comparisons: both the Project Manager and noweb can be used to create J scripts to be shipped to a customer.

The Project Manager usually takes the J script files belonging to the project and merges them together with the required standard library scripts into one single J script executable by a J Runtime system. Even when the code is not encrypted, the customer will have a hard time to exploit the code on his own: All comments have been stripped off, and the organisation as one huge script doesn't exactly invite you to make any changes. If you want to convince yourself, try to modify one of the PM-generated scripts in the standard library distributed with the system. You will quickly recognise that this is no fun at all.

Noweb's notangle can be used to the same effect: As a developer, you can maintain the code nicely organised and including excellent documentation. When you ship it off-site, you have the option whether you want to include the documentation (converted into classic NB. comments) and whether you want to keep the script organisation modular or not.

Another typical task of the PM is to compile a number of source scripts and referenced library scripts into a single runtime script. With noweb, you have some freedom how noweb input files relate to extracted J scripts. It is up to you which nodes you will extract into a script file. You could distil several script files from one noweb file, keep a one-to-one relationship, or collect the code from different noweb files into a single script.


Using noweb with J was simple to arrange, easy to learn, and straightforward to use. What I appreciate most of noweb is its minimal-fuzz approach to switching between code and documentation. I fell in love with the [[embedded code]] markers which are so much less obtrusive to my eyes than the \verb'foo' or <code>bar</code> or \fIbaz\fP markups known from other systems I used before to write articles.

I am a bit unsure at this moment where to draw the line between NB. comments as part of the code as opposed to comments as part of the additional documentation. My gut feeling is that only little paragraph headers and small hints "at the coding level" would remain as NB. comments.

I didn't use noweb beyond the toy level yet. My biggest nowebbed project has a whopping three J scripts. It yet remains to be seen whether programmers with Real Projects benefit from noweb. Would a developer team be happier one day with jumping around in the HTML rendition than with, say, traditional "tags" in the editor? The functionality is the same, but many people value a visually nice appearance highly. Other topics of my future investigations will be noweb's interaction with other supporting tools. For example, this article is written with noweb, too. [Ed: to produce HTML: the HTML-to-World conversion was done the old-fashioned way...]. It is also version-controlled, and its most current HTML rendition as well as its output scripts should be available at the juggle server any time.

One thing is certainly clear: noweb is fine for writing articles on J. I always have to improve or fix my code when writing an article. With noweb, I am doing the corrections in a single place and I can be sure that the code printed in the article will actually run and not be ruined by any transcription errors.


I thank Gill Smith and Christian Lindig for very helpful comments on drafts of this article.

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