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