Keywords are similar to symbols. The main difference is that keywords are self-evaluating and therefore do not need to be quoted in expressions. They are used mainly for specifying keyword arguments.
keyword
::= identifier
:
An alternative syntax, with the colon first, is supported for compatibility with Common Lisp and some other Scheme implementations:
keyword
::= :identifier
Putting the colon first has exactly the same effect as putting it last; putting is last is recommended, and is how keywords are printed.
A keyword is a single token; therefore no whitespace is allowed between
the identifier
and the colon (which is not considered part
of the name of the keyword).