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Literate programming

From Academic Kids

Literate programming is the writing of computer programs in the form of documents designed for casual human reading, similar to a work of literature; hence the name "literate programming."

Documentation and source code are written into one source file. Both the complete source code and its documentation can be extracted from this file with specific utilities. The information is written and presented in a reading order suitable for human consumption with detailed explanations. The code is automatically rearranged for ordinary processing by other computer tools, such as compilers or interpreters.

Contents

Origin

The first published literate programming environment was WEB, introduced by Donald Knuth in 1981 for his TeX typesetting system; it uses Pascal as its underlying programming language and TeX for typesetting of the documentation.

The complete commented TeX source code was published in Knuth's TeX: The program, volume B of his 5-volume Computers and Typesetting. Knuth had internally used a literate programming system called DOC as early as 1979; he was inspired by the ideas of Pierre Arnoul de Marneffe. The free CWEB, written by Knuth and Levy, is WEB adapted for C, C++ and Java, runs on most operating systems and can produce TeX and PDF documentation. Other implementations of the concept are noweb and FunnelWeb.

Variants

Less powerful systems to integrate documentation and code are also sometimes labeled literate programming; examples are pod for perl, doc++ for C, C++ and Java, javadoc for Java, and Doxygen for many languages. These however do not quite follow the literate programming philosophy since they typically just produce documentation about the program, such as specifications of functions and parameters, and not documentation of the program source code itself. They also do not allow rearrangement of presentation order, which is critical to the effectiveness of literate programming. See documentation generator.

Haskell is a modern language that makes use of a limited form of literate programming: this semi-literate style does not allow code re-ordering or multiple expansion of definitions but lets the programmer intersperse documentation and code freely.

It is the fact that documentation can be written freely whereas code must be marked in a special way (see the example below) that makes the difference between semi-literate programming and excessive documenting, where the documentation is embedded into the code as comments.

Example of a simple literate program

This whole article is, in fact, a simple program that can be run using the example literate interpreter on the interpreter page.

For this particular interpreter, all the program code must be written on lines starting with a dash. Everything else, i.e., the bulk of this article, is ignored by the interpreter. This does not support some important aspects of true literate programming like code rearrangements or multiple expansion and so should only be called semi-literate.

Program and interspersed documentation

Firstly, in the interests of putting the user at ease, the program will simulate personal interest in the user by asking for their name, accepting the input and generating a greeting based on the input text.

- cls
- print text Please type your name:
- store input
- print Hello there,
- print value
- print .  Nice to meet you.
- newline
- newline

Continuing the "query-response" mode of operation, prompt the user for the radius of a circle, which is then used to calculate the area of a circle using the standard formula for the area of a circle: <math>A = \pi r^2<math>. Due to syntax limitations, this is done by multiplying the input value by itself, then by <math>\pi<math>;. This calculated value is returned to the user. Note: the value of <math>\pi<math>; used is an approximation that is sufficiently accurate for our purposes.

- print text Let's work out the area of a circle.
- newline
- print text Please enter the radius of the circle in furlongs:
- store input
- multiplyby value
- multiplyby 3.14159
- print Thank you
- newline
- print text The area of the circle is
- print value
- print text square furlongs.
- newline
- newline

Finally the user is asked for the required information, and the area of the rectangle is worked out using the standard width by height formula.

- print text Now let's work out the area of a rectangle.
- newline
- print text Please enter the width of the rectangle in ells:
- store input
- print text Please enter the length of the rectangle in ells:
- multiplyby input
- print Thank you
- newline
- print text The area of the rectangle is
- print value
- print text square ells.
- newline
- newline
- print text Goodbye,

And that is the finished program.

References

  • Donald E. Knuth, Literate Programming, Stanford, California: Center for the Study of Language and Information, 1992, CSLI Lecture Notes, no. 27.
  • Pierre Arnoul de Marneffe, Holon Programming. Univ. de Liege, Service d'Informatique (December, 1973).
  • Literate Programming (http://www.literateprogramming.com/)
  • news:comp.programming.literate
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