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I believe that the time is ripe for significantly better documentation of programs, and that we can best achieve this by considering programs to be works of literature. Hence, my title: "Literate Programming."
Let us change our traditional attitude to the construction of programs: 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.
The practitioner of literate programming can be regarded as an essayist, whose main concern is with exposition and excellence of style. Such an author, with thesaurus in hand, chooses the names of variables carefully and explains what each variable means. He or she strives for a program that is comprehensible because its concepts have been introduced in an order that is best for human understanding, using a mixture of formal and informal methods that reinforce each other.
Donald Knuth. The CWEB System of Structure Documentation. Addison-Wesley. 1994. pg. 1.
The philosophy behind CWEB is that an experienced system programmer, who wants to provide the best possible documentation of his or her software products, needs two things simultaneously: a language like TeX for formatting, and a language like C for programming. Neither type of language can provide the best documentation by itself; but when both are appropriately combined, we obtain a system that is much more useful than either language separately.
The structure of a software program may be thought of as a "WEB" that is made up of many interconnected pieces. To document such a program we want to explain each individual part of the web and how it relates to its neighbors. The typographic tools provided by TeX give us an opportunity to explain the local structure of each part by making that structure visible, and the programming tools provided by languages like C make it possible for us to specify the algorithms formally and unambiguously. By combining the two, we can develop a style of programming that maximizes our ability to perceive the structure of a complex piece of software, and at the same time the documented programs can be mechanically translated into a working software system that matches the documentation.
Besides providing a documentation tool, CWEB enhances the C language by providing the ability to permute pieces of the program text, so that a large system can be understood entirely in terms of small sections and their local interrelationships. The CTANGLE program is so named because it takes a given web and moves the sections from their web structure into the order required by C; the advantage of programming in CWEB is that the algorithms can be expressed in "untangled" form, with each section explained separately. The CWEAVE program is so named because it takes a given web and intertwines the TeX and C portions contained in each section, then it knits the whole fabric into a structured document.
Ross Williams. FunnelWeb Tutorial Manual, pg 4.
A traditional computer program consists of a text file containing program code. Scattered in amongst the program code are comments which describe the various parts of the code.
In literate programming the emphasis is reversed. Instead of writing code containing documentation, the literate programmer writes documentation containing code. No longer does the English commentary injected into a program have to be hidden in comment delimiters at the top of the file, or under procedure headings, or at the end of lines. Instead, it is wrenched into the daylight and made the main focus. The "program" then becomes primarily a document directed at humans, with the code being herded between "code delimiters" from where it can be extracted and shuffled out sideways to the language system by literate programming tools.
The effect of this simple shift of emphasis can be so profound as to change one's whole approach to programming. Under the literate programming paradigm, the central activity of programming becomes that of conveying meaning to other intelligent beings rather than merely convincing the computer to behave in a particular way. It is the difference between performing and exposing a magic trick.
Matt Pharr and Greg Humphries. "Physically Based Rendering: From Theory to Implementation", Morgan Kaufmann, 2004
Writing a literate program is a lot more work than writing a normal program. After all, who ever documents their programs in the first place!? Moreover, who documents them in a pedagogical style that is easy to understand? And finally, who ever provides commentary on the theory and design issues behind the code as they write the documentation? All of that is here in the pages that follow.
This book presents a selection of modern rendering algorithms through the documented source code for a complete rendering system. The system, pbrt, is written using a programming methodology called literate programming that mixes prose describing the system with source code that implements it. We believe that the literate programming approach is a valuable way to introduce ideas in computer graphics and computer science in general. Often, some of the subtleties of an algorithm can be unclear or hidden until it is implemented, so seeing an actual implementation is a good way to acquire a solid understanding of that algorithm's details. Indeed we believe that deep understanding of a small number of algorithms in this manner provides a stronger base for further study of computer graphics than does a superficial understanding of many.
This book is a long literate program. This means that in the course of reading this book, you will read the full implementation of the pbrt rendering system, not just a high-level description of it. The literate programming metalanguage provides two important features. The first is the ability to mix prose with source code. This feature makes the description of the program just as important as its actual source code, encouraging careful design and documentation. Second, the language provides a mechanism for presenting program code to the reader in an entirely different order than it is supplied to the compiler. Thus the program can be described in a logical manner.
In some sense, the literate programming system is just an enhanced macro substitution package tuned to the task of rearranging source code. This may seem like a trivial change, but in fact literate programming is quite different from other ways of structuring software systems.
The fundamental logic of the WEB system encourages "top-down" programming and "structured" design. Quoting from Kernighan and Plauger, 'Top-down design and successive refinement attack a programming task by specifying it in the most general terms, then expanding these into more and more specific and detailed actions, until the whole program is complete. Structured design is the process of controlling the overall design of a system or program so the pieces fit together neatly, yet remain sufficiently decoupled that they may be independently modified. ... Each of these disciplines can materially improve programmer productivity and the quality of code produced.' The WEB system encourages you to work top-down by giving you the ability to break up your code into independent segments (called "sections").
Bart Childs. "Literate Programming, A Practitioner's View", Tugboat, December 1992, pg. 261-262.
I use the following list of requirements to imply a definition of a literate program and the minimum set of tools which are needed to prepare, use, and study the resulting code.
The high-level language code and the system documentation of the program come from the same set of source files.
The documentation and high-level language code are complementary and should address the same elements of the algorithms being written.
The literate program should have logical subdivisions. Knuth called these modules or sections.
The system should be presented in an order based upon logical considerations rather than syntactic constraints.
The documentation should include an examination of alternative solutions and should suggest future maintenance problems and extensions.
The documentation should include a description of the problem and its solution. This should include all aids such as mathematics and graphics that enhance communication of the problem statement and the understanding of its challenge.
Cross references, indices, and different fonts for text, high-level language keywords, variable names, and literals should be reasonably automatic and obvious in the source and the documentation.
WEB's design encourages writing programs in small chunks which Knuth called modules (he also used the term sections). Modules have three parts: documentation, definitions, and code. At least one of these three parts must be non-null.
The documentation portion is often a verbal description of the algorithm. It may be any textual information that aids the understanding of the problem. It is not uncommon for a WEB to have a number of `documentation only' modules. These usually describe the problem independent of the chosen language for implementation. For example, a WEB for a subprogram that solves the linear equation, Ax = b, could have discussion of singularity, condition numbers, partial pivoting, the banded nature of the expected coefficient matrices, etc. It should be an unusual but not exceptional case when a module contains no documentation.
Wayne Sewell. "Integral Pretty-printing" in Weaving a Program: Literate Programming in WEB, Van Nostrand Reinhold, 1989, pg. 7.
Listings generated by the WEB system are unlike any other form of program listings in existence. They resemble programs from computer science textbooks rather than listings from executable programs. WEB utilizes the TeX document compiler, which includes a typesetting command language capable of tremendous control over document appearance. Even if the author of a WEB program does not directly utilize TeX capabilities in the source code, the combined efforts of WEB and TeX will create beautiful documents on their own. TeX automatically handles details such as microjustification, kerning, hyphenation, ligatures, and other sophisticated operations, even when the description part of the source is simple ASCII text. WEB adds functions which are specific to computer programs, such as boldface reserved words, italicized identifiers, substitution of true mathematical symbols, and more standard pretty-printer functions such as reformatting and indentation.