Literate programming
Literate programming is a programming paradigm introduced by Donald Knuth in which a computer program is given an explanation of its logic in a natural language, such as English, interspersed 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 Knuth, represents a move away from writing computer programs in the manner and order imposed by the computer, and instead enables programmers to develop programs in the order demanded by the logic and flow of their thoughts. Literate programs are written as an uninterrupted exposition of logic in an ordinary human language, much like the text of an essay, in which macros are included to hide abstractions and traditional source code.
Literate programming tools are used to obtain two representations from a literate source file: one suitable for further compilation or execution by a computer, 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 above the programming languages.
History and philosophy
Literate programming was first introduced by Knuth in 1984. The main intention behind this approach was to treat a program as literature understandable to human beings. This approach was implemented at Stanford University as a part of research on algorithms and digital typography. This implementation was called "WEB" by Knuth since he believed that it was one of the few three-letter words of English that hadn't already been applied to computing. However, it correctly resembles the complicated nature of software delicately pieced together from simple materials.Concept
Literate programming is writing out the program logic in a human language with included code snippets and macros. Macros in a literate source file are simply title-like or explanatory phrases in a human language that describe human abstractions created while solving the programming problem, and hiding chunks of code or lower-level macros. These macros are similar to the algorithms in pseudocode typically used in teaching computer science. These arbitrary explanatory phrases become precise new operators, created on the fly by the programmer, forming a meta-language on top of the underlying programming language.A preprocessor is used to substitute arbitrary hierarchies, or rather "interconnected 'webs' of macros", to produce the compilable source code with one command, and documentation with another. 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.
Advantages
According to Knuth,literate programming provides higher-quality programs, since it forces programmers to explicitly state the thoughts behind the program, making poorly thought-out design decisions more obvious. Knuth also claims that literate programming provides a first-rate documentation system, which is not an add-on, but is grown naturally in the process of exposition of one's thoughts during a program's creation. The resulting documentation allows the author to restart his own thought processes at any later time, and allows other programmers to understand the construction of the program more easily. This differs from traditional documentation, in which a programmer is presented with source code that follows a compiler-imposed order, and must decipher the thought process behind the program from the code and its associated comments. The meta-language capabilities of literate programming are also claimed to facilitate thinking, giving a higher "bird's eye view" of the code and increasing the number of concepts the mind can successfully retain and process. Applicability of the concept to programming on a large scale, that of commercial-grade programs, is proven by an edition of TeX code as a literate program.
Knuth also claims that Literate Programming can lead to easy porting of software to multiple environments, and even cites the implementation of TeX as an example.
Contrast with documentation generation
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.
Critique
In 1986, Jon Bentley asked Knuth to demonstrate the concept of literate programming by writing a program in WEB. Knuth came up with an 8-pages long monolithic listing that was published together with a critique by Douglas McIlroy of Bell Labs. McIlroy praised intricacy of Knuth's solution, his choice of a data structure, but noted that more practical, much faster to implement, debug and modify solution of the problem takes only six lines of shell script by reusing standard Unix utilities. McIlroy concluded: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.
Workflow
Implementing literate programming consists of two steps:- Weaving: Generating a comprehensive document about the program and its maintenance.
- Tangling: Generating machine executable code
Latex
A classic example of literate programming is the literate implementation of the standard Unixwc 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.Creation of macros
The following snippet of thewc literate program shows how arbitrary descriptive phrases in a natural language are used in a literate program to create macros, which act as new "operators" in the literate programming language, and hide chunks of code or other macros. The mark-up notation consists of double angle brackets that indicate macros, the "@" symbol which indicates the end of the code section in a noweb file. The "<<*>>" symbol stands for the "root", topmost node the literate programming tool will start expanding the web of macros from. Actually, writing out the expanded source code can be done from any section or subsection, so one literate program file can contain several files with machine source code.The purpose of wc is to count lines, words, and/or characters in a list of files. The
number of lines in a file is......../more explanations/
Here, then, is an overview of the file wc.c that is defined by the noweb program wc.nw:
<<*>>=
<
<
<
<
<
@
We must include the standard I/O definitions, since we want to send formatted output
to stdout and stderr.
<
#include
@
The unraveling of the chunks can be done in any place in the literate program text file, not necessarily in the order they are sequenced in the enclosing chunk, but as is demanded by the logic reflected in the explanatory text that envelops the whole program.
Program as a web—macros are not just section names
Macros are not the same as "section names" in standard documentation. Literate programming macros can hide any chunk of code behind themselves, and be used inside any low-level machine language operators, often inside logical operators such as "if", "while" or "case". This is illustrated by the following snippet of the wc literate program.The present chunk, which does the counting, was actually one of
the simplest to write. We look at each character and change state if it begins or ends
a word.
<
while
@
In fact, macros can stand for any arbitrary chunk of code or other macros, and are thus more general than top-down or bottom-up "chunking", or than subsectioning. Knuth says that when he realized this, he began to think of a program as a web of various parts.
Order of human logic, not that of the compiler
In a noweb literate program besides the free order of their exposition, the chunks behind macros, once introduced with "<<...>>=", can be grown later in any place in the file by simply writing "<>= " and adding more content to it, as the following snippet illustrates.If we made these variables local to main, we would have to do this initialization
explicitly; however, C globals are automatically zeroed.
<
long tot_word_count, tot_line_count,
tot_char_count;
/* total number of words, lines, chars */
@
Record of the train of thought
The documentation for a literate program is produced as part of writing the program. Instead of comments provided as side notes to source code a literate program contains the explanation of concepts on each level, with lower level concepts deferred to their appropriate place, which allows for better communication of thought. The snippets of the literatewc above show how an explanation of the program and its source code are interwoven. Such exposition of ideas creates the flow of thought that is like a literary work. Knuth wrote a "novel" which explains the code of the interactive fiction game Colossal Cave Adventure.Remarkable examples
- 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.
Literate programming practices
There are various other implementations of the literate programming concept :
| Name | Supported Languages | Written in | Markup Language | Comments |
| WEB | Pascal | Pascal | TeX | The first published literate programming environment. |
| CWEB | C++ and C | C | TeX | Is WEB adapted for C and C++. |
| NoWEB | Any | C, AWK, and Icon | LaTeX, TeX, HTML and troff | It is well known for its simplicity and it allows for text formatting in HTML rather than going through the TeX system. |
| Any | D | Markdown | Supports TeX equations. Compatible with Vim | |
| Any | C | HTML and TeX | It has more complicated markup, but has many more flexible options | |
| Any | C++ | LaTeX | It can translate a single LP source into any number of code files. It does it in a single invocation; it does not have separate weave and tangle commands. It does not have the extensibility of noweb | |
| Any | Python | ReStructuredText | Respects indentation which makes usable for the languages like Python, though you can use it for any programming language. | |
| Any | Perl | HTML | Aims to modernize and scale it with "folding HTML" and "virtual views" on code. It uses "noweb" markup for the literate source files. | |
| Codnar | Ruby | It is an inverse literate programming tool available as a Ruby Gem. Instead of the machine-readable source code being extracted out of the literate documentation sources, the literate documentation is extracted out of the normal machine-readable source code files. | ||
| Emacs org-mode | Any | Emacs Lisp | Plain text | Requires Babel, which allows embedding blocks of source code from multiple programming languages within a single text document. Blocks of code can share data with each other, display images inline, or be parsed into pure source code using the noweb reference syntax. |
| CoffeeScript | CoffeeScript | CoffeeScript, JavaScript | Markdown | CoffeeScript supports a "literate" mode, which enables programs to be compiled from a source document written in Markdown with indented blocks of code. |
| Maple worksheets | Maple | XML | Maple worksheets are a platform-agnostic literate programming environment that combines text and graphics with live code for symbolic computation. | |
| Wolfram Notebooks | Wolfram Language | Wolfram Language | Wolfram notebooks are a platform-agnostic literate programming method that combines text and graphics with live code. | |
| Swift | Provides an interactive programming environment that evaluates each statement and displays live results as the code is edited. Playgrounds also allow the user to add Markup language along with the code that provide headers, inline formatting and images. | |||
| Jupyter Notebook, formerly IPython Notebook | Python and any with a Jupyter Kernel | HTML | Works in the format of notebooks, which combine headings, text, plots, etc. with the written code. | |
| Julia | Supports the iJulia mode of development which was inspired by iPython. | |||
| Agda | Supports a limited form of literate programming out of the box. | |||
| Programs are primarily prose. Eve combines variants of Datalog and Markdown with a live graphical development environment. | ||||
| R Markdown Notebooks | R, Python, Julia and SQL | PDF, Microsoft Word, LibreOffice and presentation or slide show formats plus interactive formats like HTML widgets |
Other useful tools include
- The Leo text editor is an outlining editor which supports optional noweb and CWEB markup. The author of Leo mixes two different approaches: first, Leo is an outlining editor, which helps with management of large texts; second, Leo incorporates some of the ideas of literate programming, which in its pure form is possible only with some degree of inventiveness and the use of the editor in a way not exactly envisioned by its author. However, this and other extensions make outline programming and text management successful and easy and in some ways similar to literate programming.
- The Haskell programming language has native support for semi-literate programming. The compiler/interpreter supports two file name extensions:
.hsand.lhs; the latter stands for literate Haskell.
\begin
fact 0 = 1
fact = * fact n
\end
here more text
\newenvironment
\begin
comp :: -> ->
x = g
\end
- The Web 68 Literate Programming system used Algol 68 as the underlying programming language, although there was nothing in the pre-processor 'tang' to force the use of that language.
- The customization mechanism of the Text Encoding Initiative which enables the constraining, modification, or extension of the TEI scheme enables users to mix prose documentation with fragments of schema specification in their One Document Does-it-all format. From this prose documentation, schemas, and processing model pipelines can be generated and Knuth's Literate Programming paradigm is cited as the inspiration for this way of working.