3.7 Keywords
A keyword value is similar to a symbol (see Symbols), but its printed form is prefixed with #:.
Reading Keywords in Reference: Racket 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 |
reference to undefined identifier: not-a-symbol-expression |
> #:not-a-keyword-expression |
eval:2:0: #%datum: keyword used as an expression in: |
#: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 | ||||||
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