Литературное программирование: подход к разработке программного обеспечения

A programming approach of software development

Literate programming is a programming paradigm introduced in 1984 by Donald Knuth in which a computer program is given as an explanation of how it works in a natural language, such as English, interspersed (embedded) with snippets of macros and traditional source code, from which compilable source code can be generated. The approach is used in scientific computing and in data science routinely for reproducible research and open access purposes. Literate programming tools are used by millions of programmers today. The literate programming paradigm, as conceived by Donald Knuth, represents a move away from writing computer programs in the manner and order imposed by the compiler, and instead gives programmers macros to develop programs in the order demanded by the logic and flow of their thoughts. Literate programs are written as an exposition of logic in more natural language in which macros are used to hide abstractions and traditional source code, more like the text of an essay. Literate programming (LP) tools are used to obtain two representations from a source file: one understandable by a compiler or interpreter, the "tangled" code, and another for viewing as formatted documentation, which is said to be "woven" from the literate source. While the first generation of literate programming tools were computer language specific, the later ones are language agnostic and exist beyond the individual programming languages.

Literate programming was first introduced in 1984 by Donald Knuth, who intended it to create programs that were suitable literature for human beings. He implemented it at Stanford University as a part of his research on algorithms and digital typography. The implementation was called "WEB" since he believed that it was one of the few three letter words of English that had not yet been applied to computing. However, it resembles the complicated nature of software delicately pieced together from simple materials. to produce the compilable source code with one command ("tangle"), and documentation with another ("weave"). The preprocessor also provides an ability to write out the content of the macros and to add to already created macros in any place in the text of the literate program source file, thereby disposing of the need to keep in mind the restrictions imposed by traditional programming languages or to interrupt the flow of thought.

Literate programming is very often misunderstood to refer only to formatted documentation produced from a common file with both source code and comments – which is properly called documentation generation – or to voluminous commentaries included with code. This is the converse of literate programming: well documented code or documentation extracted from code follows the structure of the code, with documentation embedded in the code; while in literate programming, code is embedded in documentation, with the code following the structure of the documentation. This misconception has led to claims that comment extraction tools, such as the Perl Plain Old Documentation or Java Javadoc systems, are "literate programming tools". However, because these tools do not implement the "web of abstract concepts" hiding behind the system of natural language macros, or provide an ability to change the order of the source code from a machine imposed sequence to one convenient to the human mind, they cannot properly be called literate programming tools in the sense intended by Knuth.

In 1986, Jon Bentley asked Knuth to demonstrate the concept of literate programming for his Programming Pearls column in the Communications of the ACM, by writing a program in WEB. Knuth sent him a program for a problem previously discussed in the column (that of sampling M random numbers in the range 1 N), and also asked for an "assignment". Bentley gave him the problem of finding the K most common words from a text file, for which Knuth wrote a WEB program that was published together with a review by Douglas McIlroy of Bell Labs. McIlroy praised the intricacy of Knuth's solution, his choice of a data structure (a variant of Frank M. Liang's hash trie), and the presentation. He criticized some matters of style, such as the fact that the central idea was described late in the paper, the use of magic constants, and the absence of a diagram to accompany the explanation of the data structure. McIlroy also used the review to critique the programming task itself, pointing out that in Unix (developed at Bell Labs), utilities for text processing (tr, sort, uniq and sed) had been written previously that were "staples", and a solution that was easy to implement, debug and reuse could be obtained by combining these utilities in a six line shell script. In response, Bentley wrote that:
[McIlroy] admires the execution of the solution, but faults the problem on engineering grounds. (That is, of course, my responsibility as problem assigner; Knuth solved the problem he was given on grounds that are important to most engineers—the paychecks provided by their problem assigners.) McIlroy later admitted that his critique was unfair, since he criticized Knuth's program on engineering grounds, while Knuth's purpose was only to demonstrate the literate programming technique. In 1987, Communications of the ACM published a followup article which illustrated literate programming with a C program that combined artistic approach of Knuth with engineering approach of McIlroy, with a critique by John Gilbert.

A classic example of literate programming is the literate implementation of the standard Unix wc word counting program. Knuth presented a CWEB version of this example in Chapter 12 of his Literate Programming book. The same example was later rewritten for the noweb literate programming tool. This example provides a good illustration of the basic elements of literate programming.

Axiom, which is evolved from scratchpad, a computer algebra system developed by IBM. It is now being developed by Tim Daly, one of the developers of scratchpad, Axiom is totally written as a literate program.