Keywords are similar to symbols. They are used mainly for specifying keyword arguments.

Historically keywords have been self-evaluating (you did not need to quote them). This has changed: you must quote a keyword if you want a literal keyword value, and not quote it if it is used as a keyword argument.

keyword ::= identifier:
  | #:identifier

The two syntaxes have the same meaning: The former is nicer-looking; the latter is more portable (and required if you use the --r7rs command-line flag).

Details: In r7rs and other Scheme standards the colon character does not have any special meaning, so foo: or foo:bar are just regular identifiers. Therefore some other Scheme variants that have keywords (including Guile and Racket) use the #: syntax. Kawa has some hacks so that most standard Scheme programs that have colons in identifiers will work. However, for best compatibility, use the --r7rs command-line flag (which turns colon into a regular character in a symbol), and the #: syntax.

A keyword is a single token; therefore no whitespace is allowed between the identifier and the colon or after the #:; these characters are not considered part of the name of the keyword.

Procedure: keyword? obj

Return #t if obj is a keyword, and otherwise returns #f.

Procedure: keyword->string keyword

Returns the name of keyword as a string. The name does not include the final #\:.

Procedure: string->keyword string

Returns the keyword whose name is string. (The string does not include a final #\:.)