2.1 KiB
Keywords
A keyword value is similar to a symbol see \[missing\], but its
printed form is prefixed with #:.
+[missing] in [missing] documents the fine points of the syntax of keywords.
Examples:
> (string->keyword "apple")
'#:apple
> '#:apple
'#:apple
> (eq? '#:apple (string->keyword "apple"))
#t
More precisely, a keyword is analogous to an identifier; in the same way
that an identifier can be quoted to produce a symbol, a keyword can be
quoted to produce a value. The same term “keyword” is used in both
cases, but we sometimes use keyword value to refer more specifically
to the result of a quote-keyword expression or of string->keyword. An
unquoted keyword is not an expression, just as an unquoted identifier
does not produce a symbol:
Examples:
> not-a-symbol-expression
not-a-symbol-expression: undefined;
cannot reference an identifier before its definition
in module: top-level
> #:not-a-keyword-expression
eval:2:0: #%datum: keyword misused as an expression
at: #:not-a-keyword-expression
Despite their similarities, keywords are used in a different way than
identifiers or symbols. Keywords are intended for use unquoted as
special markers in argument lists and in certain syntactic forms. For
run-time flags and enumerations, use symbols instead of keywords. The
example below illustrates the distinct roles of keywords and symbols.
Examples:
> (define dir (find-system-path 'temp-dir)) ; not '#:temp-dir
> (with-output-to-file (build-path dir "stuff.txt")
(lambda () (printf "example\n"))
; optional #:mode argument can be 'text or 'binary
#:mode 'text
; optional #:exists argument can be 'replace, 'truncate, ...
#:exists 'replace)