5 Scribbling Documentation

This section describes good style for Racket documentation writing.

5.1 Prose and Terminology

In the descriptive body of defform, defproc , etc., do not start with “This ...” Instead, start with a sentence whose implicit subject is the form or value being described (but only start the first sentence that way). Capitalize the first word. Thus, the description will often start with “Returns” or “Produces.” Refer to arguments and sub-forms by name.

Do not use the word “argument” to describe a sub-form in a syntactic form; use the term “sub-form” instead, reserving “argument” for values or expressions in a function call. Refer to libraries and languages as such, rather than as “modules” (even though the form to typeset a library or language name is called racketmodname). Do not call an identifier (i.e., a syntactic element) a “variable” or a “symbol.” Do not use the word “expression” for a form that is a definition or might be a definition; use the word “form,” instead. Prefer “function” to “procedure.”

Use the word “list” only when you mean a run-time value consisting of the empty list and cons cells; use the word “sequence” in other cases, if you must use any word. For example, do not write that begin has a “list of sub-forms;” instead, it has a “sequence of sub-forms.” Similarly, do not refer to a “list of arguments” in a function call; just write “arguments” if possible, or write “sequence of argument expressions.” (Unfortunately, “sequence” has acquired a specific run-time meaning, too, but the collision is less severe than the historical confusion between lists and other entities in Lisp.)

Avoid cut-and-paste for descriptive text. If two functions are similar, consider documenting them together with deftogether. To abstract a description, consider using explicit prose abstraction, such as “x is like y, except that ...,” instead of abstracting the source and instantiating it multiple times; often, a prose abstraction is clearer to the reader than a hidden abstraction in the document implementation.

Hyphenate the words “sub-form” and “sub-expression.”

Use “Windows,” “Mac OS,” and “Unix” for the three “platforms” (as opposed to “systems”) on which Racket runs. Use “Unix” as a generic term for Unix-like operating systems—notably including Linux—other than Mac OS. Use “Unix” even when “Gtk” or “the X11 windowing system” would be more precisely correct, but use “X11” as adjective when necessary, such as “X11 display.” Racket runs “on” a platform, as opposed to “under” a platform.

Avoid using a predicate as a noun that stands for a value satisfying the predicate. Instead, use tech and deftech to establish a connection between an English word or phrase that describes the class of values and the predicate (or contract). For example, avoid “supply a path-string?”; prefer “supply a path or string.”

5.2 Typesetting Code

Use id or a name that ends -id in defform to mean an identifier, not identifier, variable, name, or symbol. Similarly, use expr or something that ends -expr for an expression position within a syntactic form. Use body for a form (definition or expression) in an internal-definition position—always followed by ...+ in a grammar description. Do not use expr for something that isn’t exactly an expression, id for something that isn’t exactly an identifier, etc.; instead, use defform/subs to define a new non-terminal.

Beware of using deftogether to define multiple variants of a syntactic form or procedure, because each defform or defproc creates a definition point, but each form or procedure should have a single definition point. (Scribble issues a warning when a binding has multiple definition points.) Instead, use defproc* or defform*.

For function arguments, use v as the meta-variable for “any value.” Use x as a meta-variable only for numerical values. Other conventions include lst for a list and proc for a procedure.

Pay attention to the difference between identifiers and meta-variables when using racket, especially outside of defproc or defform. Prefix a meta-variable with _; for example,

would be the wrong way to refer to the grammar of a function call, because it produces (rator-expr rand-expr ...), where rator-expr and rand-expr are typeset as variables. The correct description is

which produces (rator-expr rand-expr ...), where rator-expr and rand-expr are typeset as meta-variables. The defproc, defform, etc. forms greatly reduce this burden in descriptions, since they automatically set up meta-variable typesetting for non-literal identifiers. In defform, be sure to include literal identifiers (i.e., those not meant as variables, other than the form name being defined) in a #:literals clause.

To typeset an identifier with no particular interpretation—syntax, variable, meta-variable, etc.—use racketidfont (e.g., as in rand-expr above). Otherwise, use litchar, not merely racketfont or verbatim, to refer to a specific sequence of characters.

When a syntactic form synthesizes an identifier from a given identifier, use a combination of racketidfont and racket to describe the identifiers. For example, if id is combined with is- and ? to form is-id?, then implement that identifier as @racketidfont{is-}@racket[id]@racketidfont{?}.

When using defform to describe a syntactic form, don’t confuse the #:contracts clause with a grammar specification. Use #:contracts only for expressions within the syntactic form, and the contract is a run-time constraint—not a syntactic constraint, such as requiring a sub-form to be an identifier. Use defform/subs for syntactic constraints.

See also the scribble/example library and Examples.

Use four dots, ...., in place of omitted code, since ... means repetition.

5.3 Typesetting Prose

Refrain from referring to documentation “above” or “below,” and instead have a hyperlink point to the right place.

In prose, use `` and '' quotation marks instead of ". Use --- for an em dash, and do not include spaces on either side. Use American style for quotation marks and punctuation at the end of quotation marks (i.e., a sentence-terminating period goes inside the quotation marks). Of course, this rule does not apply for quotation marks that are part of code.

Do not use a citation reference (as created by cite) as a noun; use it as an annotation.

Do not start a sentence with a Racket variable name, since it is normally lowercase. For example, use “The thing argument is...” instead of “thing is...”

Use etc for “etc.” when it does not end a sentence, and include a comma after “etc.” unless it ends a sentence that is followed by other punctuation (such as a parenthesis).

5.4 Section Titles

Capitalize all words except articles (“the,” “a,” etc.), prepositions, and conjunctions that are not at the start of the title.

A manual title should normally start with a suitable keyword or key phrase (such as “Scribble” for this manual) that is in boldface. If the key word is primarily an executable name, use exec instead of bold. Optionally add further descriptive text in the title after a colon, where the text starting with the colon is not in boldface.

5.5 Indexing

Document and section titles, identifiers that are documented with defproc, defform, etc. are automatically indexed, as are terms defined with deftech.

Symbols are not indexed automatically. Use indexed-racket instead of racket for the instance of a symbol that roughly defines the use. For an example, try searching for “truncate” to find 'truncate as used with open-output-file. Do not use something like (index "'truncate") to index a symbol, because it will not typeset correctly (i.e., in a fixed-width font with the color of a literal).

Use index, as-index, and section-index as a last resort. Create index entries for terms that are completely different from terms otherwise indexed. Do not try to index minor variations of a term or phrase in an attempt to improve search results; if search fails to find a word or phrase due to a minor variation, then the search algorithm should be fixed, not the index entry.

5.6 Examples

Strive to include examples (using examples) with the documentation of every function and syntactic form. When writing examples, refrain from using nonsense words like “foo” and “bar.” For example, when documenting member, resist the temptation to write

> (member "foo" '("bar" "foo" "baz"))

'("foo" "baz")

and instead write something like

> (member "Groucho" '("Harpo" "Groucho" "Zeppo"))

'("Groucho" "Zeppo")