How to mark up code for literate programming.

§1. The title of a section. In any section file, there will be a few lines at the top which occur before the first paragraph of code begins. (The first paragraph begins on the first line which starts with an @ character.)

The first line should be the title of the section, followed by a full stop. For example:

    The Sieve of Eratosthenes.


A section title must contain only filename-safe characters, and it's probably wise to make them filename-safe on all platforms: so don't include either kind of slash, or a colon, and in general go easy on punctuation marks.

Optionally, a section heading can also specify its own range abbreviation, which must be given in round brackets and followed by a colon:

    (S/sieve): The Sieve of Eratosthenes.


If this is not done (and usually it is not), Inweb will construct a range abbreviation itself: in this case, it comes up with S/tsoe.

Subsequent lines of text are then taken as the optional description of the purpose of the code in this section. (This is used on contents pages.) For example:

    A fairly fast way to determine if small numbers are prime, given storage.


§2. Paragraphing. A standard paragraph is introduced with an @ command, which must place that magic character in the first column of the line:

    @ This is some comment at the start of a new paragraph, which...


A fancier paragraph with a subheading attached is introduced using the @h or @heading command instead. (This is simply a long and short version of the same command.) The text of the subheading then follows, up to the first full stop.

    @heading Reflections on the method.


Paragraphs can contain three ingredients, all optional, but if given then given in this order: comment, definitions, and code. The following example shows all three being used:

    @h Primality.
We provide this as a function which determines whether a number
is prime:

@d TRUE 1
@d FALSE 0

=
int isprime(int n) {
if (n <= 1) return FALSE;
for (int m = 2; m*m <= n; m++)
if (n % m == 0)
return FALSE;
return TRUE;
}


§3. Definitions are made using one of three commands: @d or @define; or @e or @enum; or @default, which is rarely used and has no abbreviation. These create new constants in the program, with the values given: they are the equivalent of a #define directive in C. @define is the simpler form. For example,

    @define ENIGMATIC_NUMBER 90125


sets ENIGMATIC_NUMBER to 90125. Unlike in the C preprocessor, multi-line definitions are automatically handled, so for example:

    @ The following macro defines a function:
@d EAT_FRUIT(variety)
int consume_by_##variety(variety *frp) {
return frp->eat_by_date;
}
=
banana my_banana; /* initialised somewhere else, let's suppose */
EAT_FRUIT(banana) /* expands with the definition above */
void consider_fruit(void) {
printf("The banana has an eat-by date of %d.", consume_by_banana(&my_banana));
}


In fact, a definition continues until the next definition, or until the code part of the paragraph begins, or until the paragraph ends, whichever comes first.

Enumerations with @enum are a convenience to define enumerated constants. For example,

    @enum JANUARY_MNTH from 0
@enum FEBRUARY_MNTH
@enum MARCH_MNTH


and so on, is equivalent to

    @define JANUARY_MNTH 0
@define FEBRUARY_MNTH 1
@define MARCH_MNTH 2


What happens is that @enum looks at the tail of the name, from the last underscore to the end: in this case, _MNTH. The first time an enumerated value is asked for with this tail, from is used to specify the lowest number to be used - in the above case, months begin counting from 0. With each subsequent _MNTH request, @enum allocates the next unused value.

All symbols defined with @define or @enum are global, and can be used from anywhere in the web, including in sections or paragraphs earlier than the ones in which they are defined. (The tangler automatically arranges code as necessary to make this work.)

A symbol defined with @default has the given value only if some other use of @d or @e in the web has not already defined it. For example, if the web contains:

    @default MAX_HEADROOM 100


or

    @d MAX_HEADROOM 99


then the value is 99, but if only

    @default MAX_HEADROOM 100


then the value is 100.

§4. Finally, a paragraph can contain code. This is introduced with an equals sign: in some sense, the value of the paragraph is the code it contains. In many paragraphs, as in the example above, the divider is just

    =


and this means that the rest of the paragraph is part of the program. Ordinarily, this must appear in column 1, but a special abbreviation is allowed for paragraphs with no comment and no definitions:

    @ =


This is exactly equivalent to:

    @

=


We can tell the tangler to place the code early in the tangled program, rather than at its natural place in the sequence, by annotating

    = (early code)


instead of just =. (This is occasionally useful where, for example, it's necessary to create global variables which will be referred to in other sections of code.) The more extreme = (very early code) can be used in C for complicated header file inclusions, but should be kept to an absolute minimum, if only for clarity.

§5. One last feature, but it's the most important. Some code extracts are given names, in angle brackets. If so, then the paragraph is the definition of that extract. For example:

    @<Dramatic finale@> =
printf("I'm ruined! Ruined, I say!\n");
exit(1);


Notice that the equals sign is still there: it's just that the chunk of code is given a name, written inside @< and @> "brackets". (This notation goes all the way back to Knuth's original WEB.)

What does the tangler do with this? It doesn't place the code as the next item in the program. Instead, it expands any mention of @<Dramatic finale@> elsewhere in the section with this block of code. It can be expanded as many times as necessary, but only within the same section. Another section would be quite free to define its own @<Dramatic finale@>, but it would not be able to see this one.

Why is this important? One of the points of literate programming is to subdivide the program on conceptual lines, even within single functions. For example:

    @<Perform the sieve@> =
for (int n=2; n*n <= RANGE; n++)
if (still_in_sieve[n])
@<Shake out multiples of n@>;
sieve_performed = TRUE;


This is easier to understand than writing the function all in one go, and more practicable than breaking it up into smaller functions.

Named paragraphs behave, in some ways, like macro definitions, and those have a bad name nowadays - probably fairly. But Inweb makes them much safer to use than traditional macros, because it tangles them into code blocks, not just into runs of statements. A variable defined inside a named paragraph has, as its scope, just that paragraph. And this:

            if (still_in_sieve[n])
@<Shake out multiples of n@>;


works safely because @<Shake out multiples of n@> is, thanks to being a code block, semantically a single statement.

Finally, note that if there are no commentary or definitions attached to the paragraph then it's not necessary to type the initial @. That is, this:

    @

@<Prepare to exit@> =


...is not necessary, and it's sufficient to type just:

    @<Prepare to exit@> =


§6. Conditional compilation. In some languages, especially C, it's very hard to write a program which will run on multiple operating systems without some use of conditional compilation: that is, putting some code or definitions inside #ifdef clauses or the like.

Inweb can't alter this sad fact of life, but it can make the process tidier. If a paragraph has the tag ^"ifdef-SYMBOL", then any material in it will be tangled in such a way that it takes effect only if SYMBOL is defined. For example, in a C-language web with the paragraph:

    @h Windows stuff. ^"ifdef-PLATFORM_WINDOWS"

=
...
}


...the definition of THREADS_AVAILABLE and the function start_threads would be made only inside a #ifdef PLATFORM_WINDOWS clause; the same would happen for any typedefs or #includes made.

Similarly, tagging a paragraph ^"ifndef-SYMBOL" causes it to have effect only if SYMBOL is undefined. A paragraph can have any number of such conditions applied to it, and if so then all of the conditions must be met.

Note that since tags can be applied to entire sections of a web, at the Contents listing, it's straightforward to give, say, two versions of a section file, one with effect on MacOS, one with effect on Windows.

§7. Commentary. The comment part of a paragraph is ignored by the tangler, and appears only in weaves. For the most part, the text is simply copied over verbatim: but Inweb quietly tries to improve the appearance of what it copies, and a few special notations are allowed, to help with this.

§8. A doubled hyphen becomes an em-rule; double-quotation marks automatically smarten (in TeX format, at least).

§9. Lines beginning with what look like bracketed list numbers or letters are set as such, running on into little indented paragraphs. Thus

    (a) Intellectual property has the shelf life of a banana. (Bill Gates)
(b) He is the very pineapple of politeness! (Richard Brinsley Sheridan)
(c) Harvard takes perfectly good plums as students, and turns them into
prunes. (Frank Lloyd Wright)


will be typeset thus:

• (a) Intellectual property has the shelf life of a banana. (Bill Gates)
• (b) He is the very pineapple of politeness! (Richard Brinsley Sheridan)
• (c) Harvard takes perfectly good plums as students, and turns them into prunes. (Frank Lloyd Wright)

A line which begins (...) will be treated as a continuation of indented matter (following on from some break-off such as a source quotation). A line which begins (-X) will be treated as if it were (X), but indented one tab stop further in, like so:

• (c) Harvard blah, blah, blah. (Frank Lloyd Wright)
• (d) Pick a song and sing a yellow nectarine. (Scott Weiland)

Finally, bracketed asterisks are considered to be bullets. Thus:

    (*) Do I dare to eat a peach? (T. S. Eliot)
(*) You sit on the veranda drinking tea and your ducklings swim on the pond,
and everything smells good... and there are gooseberries. (Anton Chekhov)


becomes:

• ● Do I dare to eat a peach? (T. S. Eliot)
• ● You sit on the veranda drinking tea and your ducklings swim on the pond, and everything smells good... and there are gooseberries. (Anton Chekhov)

§10. Text placed between vertical strokes will be set in a fixed-space, code style font, thus. This paragraph appears in the web you are reading thus:

    @ Text placed between vertical strokes will be set in a fixed-space, code
style font, |thus|. This paragraph appears in the web you are reading thus:


This notation may be inconvenient if you need the vertical stroke character for something else, especially as the notation is used both in code comments and in paragraph commentary. But both notations can be configured in the Contents page of a web, thus:

Code In Code Comments Notation: Off
Code In Commentary Notation: %%


This example would turn off the feature in code comments, but allow it in paragraph commentary; we would then need to write...

    @ Text placed between vertical strokes will be set in a fixed-space, code
style font, %%thus%%. This paragraph appears in the web you are reading thus:


§11. A line written thus:

    >> The monkey carries the blue scarf.


is typeset as an extract of text thus:

The monkey carries the blue scarf.

(This is a feature used for Inform 7 "code" samples, those being essentially natural language text.)

§12. Code samples and other extraneous matter. When is code not code? When it's an extract of text being displayed for documentation reasons, is the answer. We can include this like so:

    = (text)
Here is my sample bit of text.
= (undisplayed text)


This is assumed to be plain text, and is syntax-coloured (or rather, not) as such, but otherwise it's woven as code. Using the word undisplayed before text tells Inweb to do so less showily, on HTML weaves:

    = (undisplayed text)


Sometimes, though, we do want syntax colouring. If in fact it is a hypothetical piece of code from the program — for example, a demonstration of an API, but for reading and not to be compiled — we can instead write:

    = (text as code)


and the text will then be treated visually exactly as the surrounding program is. If, on the other hand, it's a sample piece of code from a different language altogether, we can specify which:

    = (text as ACME)


This will then be syntax-coloured following the rules for ACME (or any other language supported by Inweb).

Note that if your web is written in, for example, C, then these are subtly different:

    = (text as C)
= (text as code)


The difference is that syntax-colouring in the first case doesn't know the names of any surrounding functions or data structures; in the second case, it knows the names of all those in your program.

Samples of code are, uniquely, allowed to end mid-way in a paragraph (unlike real code): placing a = on the left margin allows the commentary to resume. For example,

    = (text as ACME)
=
...which is essential in order to restore the state of


§13. Extract files. Many programs can only properly function if accompanied by a configuration file of some kind: a set of default preferences, for example, or some other associated data. This is not part of the program, and will instead be read in every time the program runs.

To explain such a program properly, one really needs to explain this sidekick file as well. So Inweb provides a feature for including these files inside the body of a web, as what are called "extracts". For example:

    = (text to magic-settings.txt)
top-hat-capacity = 6 rabbits
cabinet-trapdoor = closed
=


The result weaves like so:

    top-hat-capacity = 6 rabbits
cabinet-trapdoor = closed

• This is part of the extract file magic-settings.txt.

When the web is tangled, the file magic-settings.txt will be created with these contents and placed alongside the main tangled output, i.e., usually in the web's Tangled directory.

There can be up to 10 differently-named extract files. If there are multiple extracts naming the same file — for example, if we also have:

    = (text to magic-settings.txt)
marked-card = 6 of clubs
=


which weaves like so:

    marked-card = 6 of clubs

• This is part of the extract file magic-settings.txt.

then the extracts are tangled together into one file. So the result of the two example extracts above, after tangling, would be a single file which reads:

    top-hat-capacity = 6 rabbits
cabinet-trapdoor = closed
marked-card = 6 of clubs


§14. Links. URLs in the web are automatically recognised and a weave to HTML will make them into links. For example:

    For further reading, see: https://en.wikipedia.org/wiki/How_to_Avoid_Huge_Ships.


Note that URLs are considered to continue to the next white space, except that any final full stops, question or exclamation marks, commas, brackets, semicolons, or colons are disregarded. (This is why the above sentence ended with a full stop and yet the full stop wasn't part of the reference URL.)

URLs will also be recognised in any text extract marked as hyperlinked. For example,

    Compare: https://en.wikipedia.org/wiki/Crocheting_Adventures_with_Hyperbolic_Planes!


produces:

    Compare: https://en.wikipedia.org/wiki/Crocheting_Adventures_with_Hyperbolic_Planes!


§15. Cross-references. These are like links, but are internal. These are normally written within // signs and are only available in the commentary of a web. They allow us to place cross-references like so:

    To see how cross-references are implemented, see //Format Methods//,
or more generally the whole of //Chapter 5//; to decipher the text,
Inweb uses code from //foundation// at //foundation: Web Modules//.


To see how cross-references are implemented, see Format Methods, or more generally the whole of Chapter 5: Formats; to decipher the text, Inweb uses code from foundation at Web Modules (in foundation).

What happened in that last sentence is that Inweb noticed the following:

• (a) "Format Methods" is the name of a section of code in the Inweb web;
• (b) The web also has a "Chapter 5";
• (c) It uses a module called "foundation";
• (d) And that module has a section called "Web Modules".

Inweb then made links accordingly. Chapters, which can be referred to either numerically, link to the first section in them; modules likewise. Errors are thrown if these references to sections are in any way ambiguous. They are not case sensitive.

§16. Sometimes we want to make a link without literally showing the destination. This is simple: for example,

    First //the program has to configure itself -> Configuration//, then...


produces: "First the program has to configure itself, then..."; the text "the program has to configure itself" links to Configuration. This is especially useful if the destination is given as an explicit URL, which is also allowed:

    See //this biographical note -> http://mathshistory.st-andrews.ac.uk/Biographies/Gauss.html//.


§17. It's also possible to reference function names and type names, provided that the language definition supports these (see Supporting Programming Languages): this is certainly the case for C-like languages. For example,

    Individual lines of a web are stored in //source_line// structures,


produces: Individual lines of a web are stored in source_line structures, and mostly created by Reader::read_file. And that should link to the structure definition and function of these names inside the Inweb program.

Lastly, cross-references can even be made to webs quite separate from the current one, but this requires the use of a Colony file. See Making Weaves into Websites.

§18. Cross-references also work inside text extracts marked as hyperlinked.

    = (hyperlinked text)
See the //Manual// for more on this.
=


produces:

    See the Manual for more on this.


§19. Cross-references must begin after white space, or a punctuation mark (other than a colon), and must end to be followed by more white space or another punctuation mark (this time allowing a colon). In practice, that reduces the risk of misunderstanding a // occurring in the commentary for some other reason. All the same, you might want a different notation, so this can be configured in the Contents page of a web, say like so:

Cross-References Notation: &&&


It's also possible to disable cross-referencing entirely with:

Cross-References Notation: Off


§20. Figures. Images to be included in weaves of a web are called "Figures", as they would be in a printed book. These images should ideally be in PNG, JPG or PDF format and placed in a subdirectory of the web called Figures: for instance, the weaver would seek Fig_2_3.pdf at pathname Figures/Fig_2_3.pdf.

To embed an image, we write like so:

    = (figure mars.jpg)


With results like so:

Inweb also has some limited ability to control the dimensions of an image:

    = (figure Whatever.jpg at width 500)
= (figure Something.jpg at height 2cm)


Dimensions given in cm are scaled at 72 times dimensions given without a measurement; in practice, rendering to TeX produces roughly the number of centimeters asked for, and rendering to HTML makes the image width or height correspond. If you really want to monkey with the aspect ratio,

    = (figure Whatever.jpg at 20 by 100)


§21. Carousels. A carousel is a slide-show of (usually but not always) figures; there's a set of slides with captions, only one of which is visible at a time.

That carousel was produced by:
    = (carousel "Royal Albert Hall, London: King Crimson's 50th Anniversary Concert")
= (figure rah.jpg)
= (carousel "Brighton Beach")
= (figure brighton.jpg)
= (carousel "Roman Amphitheatre, Pula")
= (figure pula.jpg)
= (carousel "St Mark's Basilica, Venice")
= (figure venice.jpg)
= (carousel end)


That carousel contained only figures, but almost any material can go into the slides, paragraph breaks excepted. For example:

This carousel has differently placed captions, too: that's because the slide lines were typed as:
    = (carousel "Stage 2 - Developed tree" above)


and the like. By default, a caption overlaps slightly with the content; but it can also be above or below. A slide can also have no caption at all:

    = (carousel)
= (figure anonymous.jpg)
= (carousel)
= (figure furtive.jpg)
= (carousel end)


§22. Video and audio. To include audio samples, place them as MP3 files in the subdirectory Audio of the web. For example, in the present web,

    = (audio SP014.mp3)


produces Space Patrol episode 14, from 1953: "Brain Bank And Space Binoculars" —

Similarly,

    = (video DW014.mp4)


produces Doctor Who episode 14, from 1963: "The Roof of the World". Still, video takes up space, so for economy's sake a demonstration is omitted from this manual.

§23. Embedded video and audio. One way to get around such space limitations is to embed players for video or audio hosted on some external service. For example:

    = (embedded YouTube video GR3aImy7dWw)


With results like so:

The YouTube ID number GR3aImy7dWw can be read from its Share URL, which in this case was https://youtu.be/GR3aImy7dWw.

Similarly for Vimeo:

    = (embedded Vimeo video 204519)


With results like so:

For audio, you may like to try SoundCloud:

    = (embedded SoundCloud audio 42803139)


With results like so:

§24. Adding width and height is straightforward; by default the dimensions are 720 by 405.

    = (embedded Vimeo video 204519 at 400 by 300)
= (embedded SoundCloud audio 42803139 at height 200)


The latter sets just the height (of the displayed waveform, that is — arguably music has width and not height, but SoundCloud thinks otherwise).

§25. Downloads. Occasional small downloads may be useful as a way to present examples to try with a program being documented. These are very simple:

    = (download alice.crt "certificate file")


produces:

The file to download, in this case alice.crt, must be placed in a Downloads subdirectory of the web. The explanatory text — usually just an indication of what sort of file this is — is optional.

§26. Raw HTML snippets. Finally, it's possible to include a chunk of raw HTML code, though of course this will only be viewable if the web is being woven to HTML.

    = (html fireworks.html)


incorporates the contents of the file from the subdirectory HTML of the web.

§27. Mathematics notation. Literate programming is a good technique to justify code which hangs on unobvious pieces of mathematics or computer science, and which must therefore be explained carefully. Formulae or equations are a real convenience for that.

For example, it's known that the average running time of Euclid's GCD algorithm on $$a$$ and numbers coprime to $$a$$ is: $$\tau (a)={\frac {12}{\pi ^{2}}}\ln 2\ln a+C+O(a^{-1/6-\varepsilon })$$ where $$C$$ is Porter's constant, $$C=-{\frac {1}{2}}+{\frac {6\ln 2}{\pi ^{2}}}\left(4\gamma - {\frac {24}{\pi ^{2}}}\zeta'(2)+3\ln 2-2\right)\approx 1.467$$ which involves evaluating Euler's constant $$\gamma$$ and the first derivative of the Riemann zeta function $$\zeta'(z)$$ at $$z=2$$.

That passage was achieved by typing this as the Inweb source:

    For example, it's known that the average running time of Euclid's GCD
algorithm on $a$ and numbers coprime to $a$ is:
$$\tau (a)={\frac {12}{\pi ^{2}}}\ln 2\ln a+C+O(a^{-1/6-\varepsilon })$$
where $C$ is Porter's constant,
$$C=-{\frac {1}{2}}+{\frac {6\ln 2}{\pi ^{2}}} \left(4\gamma - {\frac {24}{\pi^{2}}}\zeta'(2)+3\ln 2-2\right)\approx 1.467$$
which involves evaluating Euler's constant $\gamma$ and the first derivative
of the Riemann zeta function $\zeta'(z)$ at $z=2$.


Mathematical formulae is typed in TeX notation between dollar signs, as usual for TeX formulae. If those notations are inconvenient, they can be changed. The defaults are:

    TeX Mathematics Notation: $TeX Mathematics Displayed Notation:$\$


Changing these to None causes Inweb to disregard mathematics entirely, and treat it as any other text would be treated.

§28. Footnotes. Not everyone likes footnotes,1 but sometimes they're a tidy way to make references.2

• 1 But see Anthony Grafton, "The Footnote: A Curious History" (Harvard University Press, 1999).

• 2 For example, to cite Donald Knuth, "Evaluation of Porter's constant", Computers & Mathematics with Applications, 2, 137-39 (1976).

§29. The content of that sentence was typed as follows:

    Not everyone likes footnotes,[1] but sometimes they're a tidy way to make
references.[2]

[1] But see Anthony Grafton, "The Footnote: A Curious History" (Harvard
University Press, 1999).
[2] For example, to cite Donald Knuth, "Evaluation of Porter's constant",
Computers & Mathematics with Applications, 2, 137-39 (1976).


Note that footnotes should be numbered upwards from 1 in each individual paragraph; Inweb automatically renumbers them for each woven section, but we don't have to worry about that when typing.

If you're reading this as a web page (with Javascript on), then you should have seen clickable footnote blobs, which reveal the text. If Javascript is off, there's a more conventionally textual presentation.

Once again, notation may be an issue, and so it's controllable. By default, we have:

    Footnote Begins Notation: [
Footnote Ends Notation: ]


but if you need squares for something else in your commentary, then perhaps:

    Footnote Begins Notation: [fn
Footnote Ends Notation: ]


would be sensible. The "cue" between these notations is required to be a string of digits; each must occur just once in its section; and each must have a text and a cue which match up correctly.