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.
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 #\:.)
