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Literate programming

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Literate programming is the writing of computer programs primarily for human beings to read, similar to a work of literature; hence the name "literate programming." This contrasts with the traditional view that a programmer's primary creation is source code to be read by a computer.

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

History and current implementations

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 and C++, runs on most operating systems and can produce TeX and PDF documentation. Other implementations of the concept are noweb and FunnelWeb.

Related concepts

Outlining

Outlining editors are sometimes seen as providing a variant of the original concept of literate programming as used by Knuth. In particular, [Leo] combines outlining with interfaces to noweb and CWEB processors.

Embedded documentation

There are also less powerful systems to integrate documentation and code than 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 and interpreter

Program

This section contains a literate program, which can be run using the example literate interpreter in the Interpreter section

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

Program to calculate the area of a circle and rectangle

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. 

- clearscreen
- 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: [A = \pi r^2]. Due to syntax limitations, this is done by multiplying the input value by itself, then by [\pi]. This calculated value is returned to the user.

Note: the value of [\pi] 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,

Interpreter

The following simple interpreter program is written using BASIC. When compiled using the QuickBASIC compiler it is a straightforward interpreter but when run on the QBASIC interpreter, it is an example of an interpreted interpreter.

DECLARE SUB SplitFirst (aFirst AS STRING, aRest AS STRING)
LET Q$ = "TESTPROG.TXT"
LET F = FREEFILE
OPEN Q$ FOR INPUT AS #F
DO WHILE NOT EOF(F)

LINE INPUT #F, FileInput$
LET FileInput$ = LTRIM$(FileInput$)
SplitFirst KeyWord$, FileInput$
SELECT CASE KeyWord$
CASE "-"
SplitFirst KeyWord$, FileInput$
GOSUB InterpretKeyword
END SELECT

LOOP
CLOSE #F
SYSTEM


InterpretKeyword:

SELECT CASE UCASE$(KeyWord$)
CASE "STORE"
GOSUB AssignToValue
CASE "ADD"
GOSUB AddToValue
CASE "MULTIPLYBY"
GOSUB MultiplyWithValue
CASE "PRINT"
GOSUB PutOutput
CASE "CLEARSCREEN"
GOSUB ClearScreen
CASE "NEWLINE"
PRINT
CASE ELSE
PRINT
PRINT "I don't know what "; KeyWord$; " "; FileInput$; " means."
END SELECT
RETURN


AssignToValue:

GOSUB GetArg
LET Value$ = Arg$
RETURN


AddToValue:

GOSUB GetArg
LET Value$ = LTRIM$(STR$(VAL(Value$) + VAL(Arg$)))
RETURN


MultiplyWithValue:

GOSUB GetArg
LET Value$ = LTRIM$(STR$(VAL(Value$) * VAL(Arg$)))
RETURN


GetArg:

Split KeyWord$, FileInput$
SELECT CASE UCASE$(KeyWord$)
CASE "INPUT"
GOSUB GetInput
LET Arg$ = UserInput$
CASE "VALUE"
LET Arg$ = Value$
CASE "TEXT"
LET Arg$ = FileInput$
CASE ELSE
LET Arg$ = KeyWord$ + " " + FileInput$
END SELECT
RETURN


GetInput:

LINE INPUT "", UserInput$
RETURN


PutOutput:

GOSUB GetArg
PRINT Arg$; " ";
RETURN


ClearScreen:

CLS
RETURN


NewLine:

PRINT
RETURN


SUB SplitFirst (aFirst AS STRING, aRest AS STRING)



DIM J AS INTEGER



LET J = INSTR(aRest + " ", " ")
LET aFirst = LTRIM$(LEFT$(aRest, J - 1))
LET aRest = LTRIM$(MID$(aRest, J))


END SUB

To try with the above program, save the interpreter as INTERP.BAS and then save the program section as TESTPROG.TXT in the same folder. Run INTERP.BAS with either QBASIC or QuickBASIC.

See also

References

External links

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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