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42 lines
1.4 KiB
Markdown
42 lines
1.4 KiB
Markdown
5 years ago
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# Void and Undefined
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Some procedures or expression forms have no need for a result value. For
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example, the `display` procedure is called only for the side-effect of
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writing output. In such cases the result value is normally a special
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constant that prints as `#<void>`. When the result of an expression is
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simply `#<void>`, the REPL does not print anything.
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The `void` procedure takes any number of arguments and returns
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`#<void>`. \(That is, the identifier `void` is bound to a procedure that
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returns `#<void>`, instead of being bound directly to `#<void>`.\)
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Examples:
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```racket
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> (void)
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> (void 1 2 3)
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> (list (void))
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'(#<void>)
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```
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The `undefined` constant, which prints as `#<undefined>`, is sometimes
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used as the result of a reference whose value is not yet available. In
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previous versions of Racket \(before version 6.1\), referencing a local
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binding too early produced `#<undefined>`; too-early references now
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raise an exception, instead.
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> The `undefined` result can still be produced in some cases by the
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> `shared` form.
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```racket
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(define (fails)
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(define x x)
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x)
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```
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```racket
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> (fails)
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x: undefined;
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cannot use before initialization
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```
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