]> Mark Wroth 2.7 21 October 2001 Updated based on comments from Lutz Prechelt. 2.6b 31 July 2001 Modify DSSSL style sheets to deal with the case where a programlisting is not part of an output literate program. The case of definition sections is not handled well in the resulting DSSSL code, as there is no obvious test by which we can identify such a section. 2.5 20 April 2001 This revision translates the previous release (revision 1.4) into itself (rather than a Nuweb-based literate program). Additionally, it defines several new entities intended to make SGML easier to include in the literate program. Finally, the weave style sheet has been split into two additional style sheets (in addition to SGMLWeave.dsl in order to facilitate further style customizations. 1.4 12 April 2001 This revision appears to be functional, and has been released for use (and published on the author's website). This document, and the literate programming system described herein, are copyright © 2001 by Mark Wroth. Permission is granted for the reproduction, use, and modification of this system provided that: No charge is made for the use of this system. A nominal charge for reproduction of the media on which the system is provided may be levied, however. The complete literate programming source of the system is made available to the user. Modifications of this system are clearly identified as derivative works. Provision must be made to identify the author of the changes, and to provide error reports to the modifier rather than the original author. Works created using this system (as opposed to modifications of this system itself) are subject to whatever copyright or use provisions may be imposed by the authors of those works; the use of this system shall not be the basis for any change or modification of the rights of such authors. The author of this system, in permitting its general use, makes no warranty of its suitability for any purpose nor any guarantee of its correctness. In other words, you are welcome to use this system, but you do so at your own risk. The DocBook-based Literate Programming system provides a mechanism to write literate programs using a minor extension of the SGML DocBook DTD. The system consists of two main parts: A DTD that extends DocBook to add the logic needed for literate programming. These are relatively minor extensions to the basic DTD. The details are discussed in . DSSSL style sheets that, together with a DSSSL engine that implements some of James Clark's extensions, serve as tangle and weave processors. These style sheets are discussed in and , respectively. This document also discusses the design considerations behind the implementation, and provides a short sample document that serves as an example of how the DTD is used (and serves as a simple test case).

This functional description for DocBook-based Literate Programming is written to provide: The system requirements to be satisfied which will serve as a basis for the system design. Information on performance requirements, preliminary design considerations, and user impacts including fixed and continuing costs. A basis for development of system tests.

Literate programming is a style of computer programming pioneered by Professor Donald Knuth in the early 1980's (the defining paper was published in The Computer Journal in May 1984, at which point the earliest literate programming system, WEB, was already functional). The key tenent of literate programming is that computer programs are written to be read and understood by human beings as well as computers—and the organization of the program's source code should allow the program author to explain the purpose and implementation of the code to the human audience. In Knuth's own words, Instead of imagining that our main task is to instruct a computer what to do, let us concentrate rather on explaining to human beings what we want a computer to do. Literate Programming, originally published in The Computer Journal, May 1984, quoted in p.~99 . To this end, a literate programming system typically supports several functions: Mechanisms to extract or otherwise make available to the computer the code instructions making up the computer program in a form usable by the computer (e.g. in a form suitable for submission to the compiler), and to translate the source code and documentation into appropriately rendered documentation. The ability to write code documentation using the full range of typographic features used in normal typesetting. The ability to arbitrarily intermingle code and documentation. The ability to arbitrarily reorder code fragments so that the exposition of the program design may precede in an order appropriate for the human reader, while retaining the instruction order required for the computer to correctly execute the program. The process of making the code instructions available to the computer is called the tangle phase of processing, while the process of rendering the documentation is called the weave phase. These names derive from the names of the two programs making up the original WEB system that performed these functions. Some literate programming processors, including Knuth's original WEB system, additionally pretty print the programming language instructions (for example, by setting the reserved words of the language in boldface type). This pretty print capability is not universal, and in fact is not desired by some programmers. Another capability sometimes found is the ability to define macros in the literate programming system, usually to supplement the capabilities of the underlying programming language. Since the introduction of Knuth's WEB system, which was used for the TeX and MetaFONT programs, a variety of other literate programming systems have appeared, as have a number of other mechanisms for improved commenting of source codeThe boundary between literate programming systems and improved commenting mechanisms is somewhat subjective. However, for the purposes of this discussion, a system is considered a literate programming system if it offers the capabilities listed above. . A complete review of the available systems is beyond the scope of this discussion. However, experience with the original WEB system, John Krommes' FWEB, and Preston Brigg's Nuweb literate programming systems, the doc LaTeX 2e documentation system, and Normal Walsh's DocBook-based DSSSL style sheet documentation systems (both of which may be characterized as improved commenting systems) has been a significant factor in defining the characteristics desired in this system.

The Standard Generalized Markup Language (SGML) is defined in ISO Standard ISO 8879:1986, and defines a mechanism for defining markup languages and enforcing certain relationships among the data contained in appropriately marked up documents. Among other things, SGML provides a clear mechanism for making explicit what parts of a document have what role, and does so in a way that encourages the construction of tools able to parse such documents—and hopefully do useful things with the parsed information. In the late 1990's SGML was supplemented by XML, which is a simplified SGML application designed to make it easier to construct parsers and other processing tools. In general, XML has the same basic functionality as SGML, with some of the lesser-used features of the language omitted. Tools that can process SGML documents can also usually process XML documents; it is not generally the case than an XML tool can process a general SGML document.

One of the useful features of SGML is the ability to create document type definitions (DTD) that describe the structure of documents and the markup that makes that structure explicit. This permits the creation of general document types—and the tools to process them. One such document type is DocBook, a document type created for computer-oriented technical books. DocBook is maintained by OASIS, and has evolved into a flexible and robust document definition, supported by a variety of tools. DocBook exists in both SGML and XML versions. In particular, Norman Walsh has defined a set of DSSSL style sheets that process DocBook documents and produce printed and HTML output renderings. These style sheets are both extensible and customizable, and serve as a significant base for computer-oriented documentation—such as a literate program.

This project creates a set of extensions to the DocBook SGML DTD to allow its use for literate programming markup. The resulting system shall Provide a mechanism to extract program files from the literate programming source in appropriate forms for their use as source code in the intended programming language or languages. Permit the use of existing DocBook-based tools with only minor modifications (ideally none) to produce documentation of software projects.

There are a variety of literate programming systems in use at the current time. In general, they can be described in three main categories: Language-aware systems. These systems are designed to support a single computer programming language, and are marked by the ability to do limited parsing of code sections, usually accompanied by pretty printing of the computer source code. Knuth's original WEB system falls into this category. Most language aware systems use TeX as their typesetting system. Language-independent systems. These systems attempt no parsing of the code sections. Most language-independent systems use TeX as their typesetting systems, although there is some move towards HTML as a documentation language. Comment-based systems. These systems extend the comment structures of the supported language in an attempt to provide usable documentation. Examples of such systems are the doc system used to document LaTeX 2e packages, and—I believe—the Javadoc system. Most or all of these existing systems target a specific output format, usually the printed page, rendered via the TeX typesetting system. In part, this is a historical accident; the first literate programming system used TeX. However, attempts to implement other documentation languages (notably an attempt to write a C-language literate programming system with troff as the documentation processor) indicate that the demands on the documentation branch of a literate programming system are relatively taxing. This has undoubtedly contributed to the limited number of output forms supported, although some attempts to support HTML have been made. However, I am not aware of any literate programming systems based on SGML markup of the source code There are a number of efforts related to literate programming in SGML or XML, and Robin Cover's excellent web page provides links to many of them. However, with the possible exception of SWEB, whose author disclaims publication and citation of the work, none of them appear to me to be usable general literate programming systems. . This omission seems unfortunate, given the obvious applicability of SGML markup to the process of defining a literate program. Since the original creation of DBLP at least two XML-based literate programming systems have been developed. Rafael R. Sevilla (sevillar@team.ph.inter.net) has developed an implementation that is functionally an addition to DocBook or other systems, implemented using XML namespaces. This system is at http://xml-lit.sourceforge.net/. Separately, Norman Walsh has reimplemented a literate programming system as part of the DocBook CVS source on SourceForge.

This project implements an SGML markup-based literate programming system. Literate computer programs are written using some form of text editor—preferably, but not necessarily an SGML-aware editor. The documentation and programming language code is marked up using an extension of the DocBook DTD. Once the literate program is written (partially or completely), it is processed using a variant of the Jade DSSSL engine with either a tangle style-sheet (which is a stand-alone style sheet provided by this project), or a weave style sheet (which extends the DocBook Modular Style Sheets in several small but important ways). In principle, the DTD defined here could be used by other programs implementing the weave or functionality. This would be entirely in keeping with the principles of SGML. In particular, an extension of this system to XML and re-implementation of the processors in XSL seems a natural extension to this work. However, only the DSSSL implementations mentioned above are implemented as part of this work.

Creating an SGML-based literate programming system makes it possible to exploit the wide variety of SGML (or, with minor variations, XML) tools. In particular, this makes it straightforward to produce different output formats, such as a printed version or an HTML version. The use of SGML also separates the definition of the document markup from the definition of the processing tools. In principle, this allows many different tools to be used with literate programs written with this system. This advantage is largely theoretical at this point, however.

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It is assumed that the user of this system is familiar with the use of SGML-based tools, and the DocBook DTD. The processing DSSSL style sheets assume the presence of selected DSSSL extensions implmented in James Clark's Jade engine (specifically the entity and processing-instruction flow objects). This capability is necessary to use the specific style sheets provided by this project, but the use of the DTD is not affected.

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Minimize changes to the DocBook DTD to allow processing using existing output tools. Keep the existing programlisting as the basis for a scrap (again to minimize changes needed in the output processors). Use added attributes for file output, definition and reference to continued and continuation scraps. Use of both continued and continuation markup is semantically redundant, but will make processing of the weave branch easier—I think. Use xref for reference to definition scraps, and the xreflabel attribute for the definition scrap title. Again, this is driven by a desire to minimize changes that would impact the output processing tools. Make maintaining, modifying, and adding this functionality to other document types as easy as possible (see ). ]]>

This system performs three basic functions: Provide a DTD that allows the markup of literate programs, including a flexible system for describing the purpose and implementation of the computer program (based on DocBook) and markup of the program code itself to allow the literate program to produce the computer instructions. A tangle mechanism that actually produces the computer instructions from the literate programming source code. This implementation, SGMLTangle.dsl is a DSSSL style sheet using extensions to the DSSSL standard as implemented in James Clark's Jade DSSSL engine. A weave implementation that renders the literate programming source into useful documentation. This style specification, SGMLWeave.dsl, extends Norman Walsh's Modular DocBook Style Sheets. It provides both print and HTML output.

It is the intention of this system to: Maintain the ability to update the extensions to new versions of DocBook as they are published. Make the extensions as easy as possible to to move between the SGML and XML versions. Make it as simple as possible to add the literate programming functionality to other DocBook-based DTDs. Provide a basis on which other implementations of the tangle and weave functions could be built to support other tool chains.

\label{sec:data} ]]>

The hardware required to use this system is defined by the selected software tools (in particular the SGML processor). Development and testing was accomplished on 32-bit Windows-based computers.

Effective use of this system requires three major classes of supporting software. Except as noted with regard to the tangle application, it is not necessary that the actual supporting software used in the development of this system be available.

Some form of text editor is needed to write the literate program. The minimum necessary functionality is the ability to write a plain text output file. Actually, even this is an overstatement: the editor has to be able to produce SGML input files compatible with the SGML and DSSSL processors being used. While this is usually a plain text file, there are other ways to implement SGML systems. The authoring process will be much easier, however, if the editor supports both SGML markup and the programming language or languages in which the code is being written. A customizable editor such as emacs is probably a useful choice. Most of the work on the DBLP system was done with emacs and the PSGML major mode.

The literate programming DTD extends the DocBook DTD, and therefore requires the underlying DTD. This implementation specifically uses the SGML DTD, Version 4.1 as the basis for its extension. To use this system for literate programming without additional customization, a DSSSL engine that implements James Clark's \texttt{entity} and \texttt{processing-instruction} extensions to the DSSSL standard is needed. The Jade or OpenJade engines meet this requirement. The weave style sheet is based on Norman Walsh's Modular DocBook Style Sheets , which must be available if the SGMLWeave.dsl style specification is used. As an SGML application, of course, documents marked up with this DTD can, in principle, be used by any SGML-compliant tool. While this DTD and the associated DSSSL style sheets were written for the SGML version of the DocBook DTD, it should require only minor changes to re-implement this system in XML. This has not, however, been tested.

This system was originally implemented using the Nuweb literate programming processor, which is a TeX-based system. The choice to implement the system in this way was a bootstrappingchoice; until this system achieved basic functionality, it would be difficult to implement the system in an SGML-based system. Once basic functionality has been demonstrated, this system will be published on the World-Wide Web. Future modifications and extensions may or may not be made, depending on the author's use of the system and feedback from other users (if any).

%This section shall provide a summary of cost factors for the %proposed system In accordance with DoD Instruction 7041.3, when %applicable. While the proposed system responds directly to the %project request or other initiation document, other factors may %determine the need for this system, such as requirements of %higher echelons of command, security considerations, %telecommunications considerations, and the need to interface with %other automated systems. General alternatives that may be %discussed include those for system development and system design %with consideration being given to equipment, software, supporting %telecommunications requirements, organization, operation, etc. %Reference may be made to such information contained in other %documents. ]]>

This DTD extends DocBook 4.1 to allow literate programs to be written using the DTD.

&STAGO;!-- DBLP.DTD; a literate programming DTD based on DocBook PUBLIC "-//Mark Wroth//DTD DocBook V4.1-Based Extension Literate Programming 1.1//EN" --&TAGC; &STAGO;!ENTITY % programlisting.element "IGNORE"&TAGC; &STAGO;!ENTITY % docbook PUBLIC "-//OASIS//DTD DocBook V4.1//EN"&TAGC; %docbook; We put the literalchar element definitions first, so that entity definitions will override those declared by DocBook itself, if necessary.

We will set up the programlisting element to ignore the DocBook defined definition and substitute our own. We override the original definition primarily to include the literalchar element; otherwise there are no changes to the basic element definition. &STAGO;!ELEMENT programlisting - - ((CO | LineAnnotation | literalchar | %para.char.mix;)+)&TAGC; To the attribute list, we add the attributes that we will use for literate programming: file The file name the code is to be written to. This attribute is required for the scraps which begin output files. continuedfrom The ID of the scrap this scrap continues. continuedin The ID of the scrap this file continues. We also make use of several of the attributes already defined for programlisting: ID Unique identifier for this scrap; required for scraps which are continued, continue others, or are the head of a definition section. xreflabel The title of the scrap; used for the head of a definition scrap. This fact is used in a number of places in the processing, and those additional uses depart somewhat from the standard DocBook uses. In particular, this implies that no continuation scrap shall have an xreflabel attribute. &STAGO;!ENTITY % local.programlisting.attrib " file CDATA #IMPLIED -- file name for output file -- continuedfrom IDREF #IMPLIED continuedin IDREF #IMPLIED "&TAGC; The choice to use both a continuedfrom and a continuedin attribute allows the creation of a doubly-linked list for each code section. This permits the programmer to order the code scraps in any desired sequence, while permitting the processing system to easily traverse the scraps that make up the section. While one or the other direction of the links between scraps is sematically redundant in an absolute sense (with adequate effort, the backward links could be constructed from the forward set, and vice-versa), including both sets of links makes construction of the processing tools much simpler.

The following definitions are used to provide a workaround to get an actual less than character into the SGML output. Since the character has syntactic meaning to the SGML parser, by default it is escaped when placed in the SGML output as character data. By defining an element to contain the required information, we let the DSSSL processor have access to it. Defining entity references to it simplifies the actual data entry. If particular combinations seem appropriate for a specific programming language it would make sense to define entities which make syntactic sense. This would allow one to use, for example &logicaland; instead of &&The basic suggestion to use a formatting-instruction to address the problem came from David Carlisle davidc@nag.co.uk in a post to the DSSSList, Vol 3, Number 241, although the actual implementation does not follow his suggestions precisely. . &STAGO;!ELEMENT literalchar - o EMPTY -- literal data, to be handled in the DSSSL --&TAGC; &STAGO;!ATTLIST literalchar data CDATA #REQUIRED&TAGC; &STAGO;!ENTITY lessthan "&STAGO;literalchar data='&ERO;#60;'&TAGC;" -- ``less than'' sign--&TAGC; &STAGO;!ENTITY greaterthan "&STAGO;literalchar data='&TAGC;'&TAGC;" -- ``greater than'' sign--&TAGC; &STAGO;!ENTITY ampersand "&STAGO;literalchar data='&ERO;#38;'&TAGC;" -- ``ampersand'' sign--&TAGC; We also define entities that map to the relevant SGML markup, for use when we are defining SGML systems. &STAGO;!ENTITY STAGO "&STAGO;literalchar data='<'&TAGC;" -- SGML Tag Open (``less than'' sign) --&TAGC; &STAGO;!ENTITY TAGC "&STAGO;literalchar data='&TAGC;'&TAGC;" -- SGML Tag Close (``greater than'' sign) --&TAGC; &STAGO;!ENTITY ERO "&STAGO;literalchar data='&'&TAGC;" -- SGML Entity Reference Open (``ampersand'' sign) --&TAGC;

The SGMLTangle style sheet performs the tangle phase of literate programming. In other words, it takes the literate programming code and rearranges it so that it is acceptable to the computer as a computer program (assuming, of course, that the programmer has correctly written the program!)

In order to make it as simple as possible to bring this system up on a new machine, this style sheet is presented largely as a single scrap. Notes on the program follow the implementation scrap. &STAGO;!DOCTYPE style-sheet PUBLIC "-//James Clark//DTD DSSSL Style Sheet//EN"&TAGC; &STAGO;style-sheet&TAGC; &STAGO;style-specification id = "tangle"&TAGC; &STAGO;style-specification-body&TAGC; (declare-flow-object-class entity "UNREGISTERED::James Clark//Flow Object Class::entity") (declare-flow-object-class formatting-instruction "UNREGISTERED::James Clark//Flow Object Class::formatting-instruction") (default (process-node-list (element-children))) (element programlisting (make sequence (if (attribute-string "file") (make entity system-id: (attribute-string "file") (make sequence (process-children) (if (attribute-string "continuedin") (with-mode continuation (process-element-with-id (attribute-string "continuedin"))) (empty-sosofo)))) (empty-sosofo)))) (element (programlisting xref) (with-mode definition (process-element-with-id (attribute-string "linkend")))) (mode definition (element programlisting (make sequence (process-children) (if (attribute-string "continuedin") (with-mode continuation (process-element-with-id (attribute-string "continuedin"))) (empty-sosofo))))) (mode continuation (element programlisting (make sequence (process-children) (if (attribute-string "continuedin") (with-mode continuation (process-element-with-id (attribute-string "continuedin"))) (empty-sosofo))))) (element literalchar (make sequence (make formatting-instruction data: (attribute-string "data")))) &STAGO;/style-specification-body&TAGC; &STAGO;/style-specification&TAGC; &STAGO;/style-sheet&TAGC; The (element-children snl) function finds all of the elements that are direct children of singleton nodelist snl. This is useful for filtering, processing, and counting, when you're not interested in any text, PI, or comment nodes. This function (and the default processing rule) were provided by Christopher R. Maden crism@maden.org in a message on the DSSSList dated Mon, 02 Apr 2001 22:36:33 -0700, and titled Re: (dsssl) "Default" processing rule?. (define (element-children #!optional (snl (current-node))) (select-by-class (children snl) 'element))

The SGMLWeave program (SGMLWeave.dsl) is a relatively minor customization of Norman Walsh's DSSSL stylesheets for DocBook. In fact, for minimum functionality, the only necessary change to the stylesheets is the addition of a processing rule for the literalchar element, and that processing rule (shown in ) is relatively simple. This is in fact a design goal of the DocBook-based literate programming system, as it exploits the significant efforts of a number of people to develop tools for DocBook.

We now create three related DSSSL style sheets, designed to make it as easy as possible to use the weave functions. The first of these is a master style sheet, that uses Jade ability to include multiple style sheets in a single document, selecting among them from the command line. This version, in SGMLWeave.dsl, includes a print style sheet and and HTML style sheet. &STAGO;!-- PUBLIC "-//Mark Wroth//DOCUMENT DBLP Weave Print Rules 1.0//EN" This document is intended to be a minimum DocBook style-sheet customization layer. --&TAGC; &STAGO;!DOCTYPE style-sheet PUBLIC "-//James Clark//DTD DSSSL Style Sheet//EN" [ &STAGO;!ENTITY dbprint.dsl PUBLIC "-//Norman Walsh//DOCUMENT DocBook Print Stylesheet//EN" CDATA DSSSL&TAGC; &STAGO;!ENTITY dbhtml.dsl PUBLIC "-//Norman Walsh//DOCUMENT DocBook HTML Stylesheet//EN" CDATA DSSSL&TAGC; ]&TAGC; &STAGO;style-sheet&TAGC; &STAGO;style-specification id="print" use="dbprint"&TAGC; &STAGO;style-specification-body&TAGC; &STAGO;/style-specification-body&TAGC; &STAGO;/style-specification&TAGC; &STAGO;style-specification id="html" use="dbhtml"&TAGC; &STAGO;style-specification-body&TAGC; &STAGO;/style-specification-body&TAGC; &STAGO;/style-specification&TAGC; &STAGO;external-specification id="dbprint" document="dbprint.dsl"&TAGC; &STAGO;external-specification id="dbhtml" document="dbhtml.dsl"&TAGC; &STAGO;/style-sheet&TAGC; The second style sheet is a redaction of the master sheet. It contains only the print style sheet, and is written to print.dsl. The primary motivation for having this separate file is that it is easier to write higher level customization layers for single purpose style sheets—at least at this author's level of expertise! &STAGO;!-- PUBLIC "-//Mark Wroth//DOCUMENT DBLP Weave Print Rules 1.0//EN" This document is intended to be a minimum DocBook style-sheet customization layer. --&TAGC; &STAGO;!DOCTYPE style-sheet PUBLIC "-//James Clark//DTD DSSSL Style Sheet//EN" [ &STAGO;!ENTITY dbprint.dsl PUBLIC "-//Norman Walsh//DOCUMENT DocBook Print Stylesheet//EN" CDATA DSSSL&TAGC; ]&TAGC; &STAGO;style-sheet&TAGC; &STAGO;style-specification id="print" use="dbprint"&TAGC; &STAGO;style-specification-body&TAGC; &STAGO;/style-specification-body&TAGC; &STAGO;/style-specification&TAGC; &STAGO;external-specification id="dbprint" document="dbprint.dsl"&TAGC; &STAGO;/style-sheet&TAGC; The third style sheet is also a redaction of the master sheet. It contains only the HTML style sheet, and is written to html.dsl. The primary motivation for having this separate file is, as with print.dsl, that it is easier to write higher level customization layers for single purpose style sheets. &STAGO;!-- PUBLIC "-//Mark Wroth//DOCUMENT DBLP HTML Print Rules 1.0//EN" This document is intended to be a minimum DocBook style-sheet customization layer. --&TAGC; &STAGO;!DOCTYPE style-sheet PUBLIC "-//James Clark//DTD DSSSL Style Sheet//EN" [ &STAGO;!ENTITY dbhtml.dsl PUBLIC "-//Norman Walsh//DOCUMENT DocBook HTML Stylesheet//EN" CDATA DSSSL&TAGC; ]&TAGC; &STAGO;style-sheet&TAGC; &STAGO;style-specification id="html" use="dbhtml"&TAGC; &STAGO;style-specification-body&TAGC; &STAGO;/style-specification-body&TAGC; &STAGO;/style-specification&TAGC; &STAGO;external-specification id="dbhtml" document="dbhtml.dsl"&TAGC; &STAGO;/style-sheet&TAGC;

(element literalchar (make sequence (literal (attribute-string "data"))))

The basis of our customization is the existing programlisting code from dbverb.dsl. We will modify this by adding information ahead of the basic element (by identifying the scrap by either the file name or the definition name, depending on the type of scrap), and adding information after the program listing (if the scrap is continued).

Almost everything about the print stylesheets will remain untouched. However, there are a few changes that need to be made to the processing of the printlisting element. The printlisting element itself gets some additional information added to it based on its place in the literate programming web, and the special use of xref means that we should wrap the cross reference text in some punctuation to indicate that it is a code section reference rather than actual code.

There are essentially two sets of changes we want to make to the processing of the programlisting element: adding header information identifying the code section and the particular scrap, and adding continuation information if the scrap has a continuation. Mechanically, we do this by wrapping the original code (taken from dbverb.dsl inside a make sequence and adding the code to produce the header and continuation text before and after the original code. (element programlisting (make sequence )) If the code section is part of an output literate program, the scrap header information is straight forward except for the actual title of the code section. That title is either the file name of the disk file the section will be written to, or the xreflabel of the section. In either case, the title is found in the first scrap of the section, which may or may not be the scrap we are currently processing. We deal with this as a set of nested if statements to grab the title if we are in the first scrap of the section, or processing the continuedfrom scrap with a special mode that recursively hunts back up the linked list until it gets to the first scrap.It is exactly this application that caused us to define the programlisting scraps as a doubly-linked list. The other refinement is that we will set the header and continuation information in a specific font and size intended to be distinct from the general font and size used in the document. For ease of reference, these are defined in the auxiliary information. The other potential case is that this code section is not part of a literate program. In that case, no scrap header (or continuation information) is desired. In most cases, literate programming scraps are easy to identify from the fact that they have attributes not found in non-literate programming scraps (specifically the file and continuedfrom attributes). There is, however, one ambiguous case; a scrap which contains an entire definition section has no attributes unique to the literate programming use. In this case, we will rely on the presence of the xreflabel attribute, although there are circumstances where this will produce errors. (if (or (attribute-string "file") (attribute-string "continuedfrom") (attribute-string "xreflabel")) (empty-sosofo)) (make paragraph (make sequence font-family-name: scrap-header-font font-size: scrap-header-size (literal "\left-pointing-angle-bracket") (if (attribute-string "file") (literal (attribute-string "file")) (if (attribute-string "xreflabel") (literal (attribute-string "xreflabel")) (with-mode scrap-title-mode (make sequence (process-element-with-id (attribute-string "continuedfrom")) (literal " "))))) (make sequence font-size: (* scrap-header-size 0.75) (literal " (ID: ") (literal (attribute-string "id"))) (literal ")\right-pointing-angle-bracket") (if (attribute-string "continuedfrom") (literal "+") (empty-sosofo)) (literal "\identical-to"))) The logic of the continuation listing is simpler; if there is continuation information given, we show the continuation. (if (attribute-string "continuedin") (make paragraph font-family-name: scrap-header-font font-size: scrap-header-size (make sequence (literal "Continued in ") (literal (attribute-string "continuedin")))) (empty-sosofo))

The main customization we want for the xref element is to put the cross-reference text in a running text font, and enclose it in angle brackets. In both cases, this is to distinguish a code section reference from actual code. For consistency, we will put the reference in the same font and size as the scrap header and continuation information. (element (programlisting xref) (make sequence font-family-name: scrap-header-font font-size: scrap-header-size (literal "\left-pointing-angle-bracket") (literal "\right-pointing-angle-bracket") ))

Here we define the common information such as the size and font used for the header and continuation. Additionally, we define the special mode used to find the code section title here. (define scrap-header-size 8pt) (define scrap-header-font "Georgia") The scrap-title-mode is defined to process programlisting elements and either extract the title (found in the file or xreflabel attributes, or to continue up the linked list if neither attribute is present. (mode scrap-title-mode (element programlisting (make sequence (if (attribute-string "file") (literal (attribute-string "file")) (if (attribute-string "xreflabel") (literal (attribute-string "xreflabel")) (process-element-with-id (attribute-string "continuedfrom")))))))

This is the complete DSSSL code for the programlisting element, found in dbverb.dsl. ($verbatim-display$ %indent-programlisting-lines% %number-programlisting-lines%) This is the complete DSSSL code found in dblink.dsl for the xref element. It is probably much more complex than is actually needed for the rather specialized use we are making of it, but it is easier to just reproduce it than to try to simplify it. (let* ((endterm (attribute-string (normalize "endterm"))) (linkend (attribute-string (normalize "linkend"))) (target (element-with-id linkend)) (xreflabel (if (node-list-empty? target) #f (attribute-string (normalize "xreflabel") target)))) (if (node-list-empty? target) (error (string-append "XRef LinkEnd to missing ID '" linkend "'")) (if xreflabel (make link destination: (node-list-address target) (literal xreflabel)) (if endterm (if (node-list-empty? (element-with-id endterm)) (error (string-append "XRef EndTerm to missing ID '" endterm "'")) (make link destination: (node-list-address (element-with-id endterm)) (with-mode xref-endterm-mode (process-element-with-id endterm)))) (cond ((or (equal? (gi target) (normalize "biblioentry")) (equal? (gi target) (normalize "bibliomixed"))) ;; xref to the bibliography is a special case (xref-biblioentry target)) ((equal? (gi target) (normalize "co")) ;; callouts are a special case (xref-callout target)) ((equal? (gi target) (normalize "listitem")) (xref-listitem target)) ((equal? (gi target) (normalize "question")) (xref-question target)) ((equal? (gi target) (normalize "answer")) (xref-answer target)) ((equal? (gi target) (normalize "refentry")) (xref-refentry target)) ((equal? (gi target) (normalize "glossentry")) ;; as are glossentrys (xref-glossentry target)) ((equal? (gi target) (normalize "author")) ;; and authors (xref-author target)) ((equal? (gi target) (normalize "authorgroup")) ;; and authorgroups (xref-authorgroup target)) (else (xref-general target)))))))

In the HTML style sheet, we need to make the same kinds of customization we did with the print style sheet. The details of the actual customizations differ slightly.

In the programlisting, we make the same kinds of additions (header and continuation information) as used in the print style sheet. (element programlisting (make sequence )) The code is also largely common, with some minor changes for the HTML output and its limited character repertoire. We have the same logical limitation on the identification of definition sections noted above. (if (or (attribute-string "file") (attribute-string "continuedfrom") (attribute-string "xreflabel")) (empty-sosofo)) (make element gi: "P" (make sequence (literal "<") (if (attribute-string "file") (literal (attribute-string "file")) (if (attribute-string "xreflabel") (literal (attribute-string "xreflabel")) (with-mode scrap-title-mode (make sequence (process-element-with-id (attribute-string "continuedfrom")) (literal " "))))) (make sequence (literal " (ID: ") (literal (attribute-string "id"))) (literal ")>") (if (attribute-string "continuedfrom") (literal "+") (empty-sosofo)) (literal "="))) As with the print form, we apply the simple test that if there is continuation information, we show it. (if (attribute-string "continuedin") (make element gi: "P" (make sequence (literal "Continued in ") (literal (attribute-string "continuedin")))) (empty-sosofo))

(element (programlisting xref) (make sequence (literal "&lessthan;") (literal "&greaterthan;") ))

Here we employ the same basic structure as with the print style sheet, although there is less to define; this primarily is intended to allow room for future elaboration of the style sheet. The DSSSL code for the scrap-title-mode is identical with that used in the print style sheet, so we simply re-use it.

($verbatim-display$ %indent-programlisting-lines% %number-programlisting-lines%)

(let* ((endterm (attribute-string (normalize "endterm"))) (linkend (attribute-string (normalize "linkend"))) (target (element-with-id linkend)) (xreflabel (if (node-list-empty? target) #f (attribute-string (normalize "xreflabel") target)))) (if (node-list-empty? target) (error (string-append "XRef LinkEnd to missing ID '" linkend "'")) (make element gi: "A" attributes: (list (list "HREF" (href-to target))) (if xreflabel (literal xreflabel) (if endterm (if (node-list-empty? (element-with-id endterm)) (error (string-append "XRef EndTerm to missing ID '" endterm "'")) (with-mode xref-endterm-mode (process-node-list (element-with-id endterm)))) (cond ((or (equal? (gi target) (normalize "biblioentry")) (equal? (gi target) (normalize "bibliomixed"))) ;; xref to the bibliography is a special case (xref-biblioentry target)) ((equal? (gi target) (normalize "co")) ;; callouts are a special case ($callout-mark$ target #f)) ((equal? (gi target) (normalize "listitem")) ;; listitems are a special case (if (equal? (gi (parent target)) (normalize "orderedlist")) (literal (orderedlist-listitem-label-recursive target)) (error (string-append "XRef to LISTITEM only supported in ORDEREDLISTs")))) ((equal? (gi target) (normalize "question")) ;; questions and answers are (yet another) special case (make sequence (literal (gentext-element-name target)) (literal (gentext-label-title-sep target)) (literal (question-answer-label target)))) ((equal? (gi target) (normalize "answer")) ;; questions and answers are (yet another) special case (make sequence (literal (gentext-element-name target)) (literal (gentext-label-title-sep target)) (literal (question-answer-label target)))) ((equal? (gi target) (normalize "refentry")) ;; so are refentrys (xref-refentry target)) ((equal? (gi target) (normalize "glossentry")) ;; as are glossentrys (xref-glossentry target)) ((equal? (gi target) (normalize "author")) ;; and authors (xref-author target)) ((equal? (gi target) (normalize "authorgroup")) ;; and authorgroups (xref-authorgroup target)) ; this doesn't really work very well yet ; ((equal? (gi target) (normalize "substeps")) ; ;; and substeps ; (xref-substeps target)) (else (xref-general target))))))))

This is a very simple document illustrating the used of the DBLP literate programming system. It provides an example of how to construct a simple input file for the system. &STAGO;!DOCTYPE article PUBLIC "-//Mark Wroth//DTD DocBook V4.1-Based Extension Literate Programming 1.0//EN"&TAGC; &STAGO;article id="sample-lp"&TAGC; &STAGO;title&TAGC;A Sample DocBook-Based Literate Program&STAGO;/title&TAGC; &STAGO;section&TAGC; &STAGO;title&TAGC;Introduction&STAGO;/title&TAGC; &STAGO;para&TAGC;This is a sample document illustrating basic use of the DocBook-based literate programming tool.&STAGO;/para&TAGC; &STAGO;para&TAGC;This document combines human-readable documentation of a computer program with the actual computer-readable source code. Depending on how it is processed, it becomes either printed (on-line) documentation of the program, or the actual source submitted to the computer for compilation (or interpretation, depending on the language).&STAGO;/para&TAGC; &STAGO;/section&TAGC; &STAGO;section&TAGC; &STAGO;title&TAGC;Source Code&STAGO;/title&TAGC; &STAGO;para&TAGC;The source code is divided into a number of &STAGO;quote&TAGC;scraps&STAGO;/quote&TAGC;, each containing a discrete fragment of code. These scraps are assembled into code sections by concatenating the header scrap with the various continuation scraps, in an order defined by the programmer. File sections are written to the file indicated by the programmer, while definition sections are inserted at a place or places defined by the programmer. &STAGO;/para&TAGC; &STAGO;para&TAGC;The first code scrap defines a file output, specifically to &STAGO;filename&TAGC;sample.code&STAGO;/filename&TAGC;.&STAGO;/para&TAGC; &STAGO;programlisting id="scrap1" file="sample.code" continuedin="scrap2" &TAGC; -- This is sample code in an imaginary language -- Taken from the first scrap if a &lessthan; b then &STAGO;xref linkend="scrap3"&TAGC; fi &STAGO;/programlisting&TAGC; &STAGO;para&TAGC;The next code scrap is a continuation of the first scrap.&STAGO;/para&TAGC; &STAGO;programlisting id="scrap2" continuedfrom="scrap1" &TAGC; -- This is continued code, taken from the second scrap -- set c = a &ersand; b greater than: &greaterthan; &STAGO;/programlisting&TAGC; &STAGO;para&TAGC;The following code section is an example of a definition scrap, and will be included in a file output scrap.&STAGO;/para&TAGC; &STAGO;programlisting id="scrap3" xreflabel="The Third Scrap" continuedin="scrap4"&TAGC; -- Yet more program code from the third scrap &STAGO;/programlisting&TAGC; &STAGO;para&TAGC;Finally, we have a continuation scrap continuing a definition scrap.&STAGO;/para&TAGC; &STAGO;programlisting id="scrap4" continuedfrom="scrap3" &TAGC; -- This is scrap 4, which continues scrap 3 -- It should appear where scrap 3 was inserted. &STAGO;/programlisting&TAGC; &STAGO;/section&TAGC; &STAGO;/article&TAGC; As with the DTD, the fact that this output section is written in SGML means that the literate programming file as actually written makes extensive use of entities for those characters that have syntactic meaning. This situation is one of the few where the human documentation and the computer source are noticably different.

Generally speaking, verifying the functionality of the literate programming system consists of checking that the tangle branch correctly produces the intended code, and the weave branch produces appropriate human-readable documentation.

The code output file sample.code produced from the sample document looks like this: &sample.code; This demonstrates the key functions needed in the tangle branch, namely that: Code sections are correctly assembled from the separate scraps identified in the source code. Definition sections are inserted into code sections at the location identified by the xref tag pointing to the head of the definition section. File output sections are written to disk in with the desired file names. Syntactically significant characters (to SGML) including <, >, and &, are written correctly in the output file. The handling of whitespace in the code scraps is not quite what I expected, but appears to be consistent and reasonable. It appears that whitespace is (correctly) transcribed to the output file, except that an SGML record end (CR-LF pair, under Windows) following the programlisting start tag is not transcribed to the output.

The weave DSSSL style sheets produce two sets of human-readable documentation from sample.sgm: sample.rtf and a set of HTML files. The HTML files are divided and named by the conventions of the HTML Modular DocBook Style Sheets, which (without customization) produce multiple small files. In both cases, the weave outputs reasonably produce verbatim program listings, with header and continuation information that matches the actual structure defined by the sample file. The typographic treatment of the header and continuation is acceptable, although the amount of vertical space introduced in both the print and HTML versions between the header and the body of the programlisting is larger than I would prefer.

Overall, these implementations of a DTD, tangle, and weave are functional. They do not produce really high-quality typographical output, but they do reasonably display the structure of the code sections. Pending further experience with the system, this seems to be an acceptable implementation.

DSSSL Document Style Semantics Specification Language. DSSSL is an ISO standard defining how to specify transformations from SGML documents to page-oriented output renderings. DTD Document Type Definition, Document Type Declaration HTMLHypertext Markup Language ISO International Organization for Standardization OASIS Organization for the Advancement of Structured Information Standards OSNL Optional Singleton Node List SGML Standard Generalized Markup Language SNL Singleton Node List XML Extensible Markup Language Carlisle David davidc@nag.co.uk Volume 3 Number 241 Thu, 16 Dec 1999 16:27:27 GMT Donald Knuth Center for the Study of Language and Information

Stanford, CA

1992 Christopher R. Maden crism@maden.org Volume 4 Number~143 Mon, 02 Apr 2001 22:36:33 (-0700) OASIS

http://www.oasis-open.org/docbook

Norman Walsh

http://sourceforge.net/projects/docbook

The use of catalog files makes installation of the various processing applications and DTDs much easier, by allowing the specifications of where the various files are located in the system file structure to be separated from the documents that need to use them. Using indirection in catalog files—having a catalog file point to a separate catalog file, rather than independently listing each of the system entities needed—makes the maintenance of the system much easier. A master catalog file, kept in a convenient location, points to catalog files for each of the various packages installed. These package catalog files can be kept in the installation directories. The local catalog file for a document can then reference all of the standard supporting files with a single entry. The user environment variable SGML_CATALOG_FILES may also be used to cause Jade to point to the needed catalog file or files.