2d4f3e2ac9
The layer is now redundant, since everything left in "pkgs" is in the "racket-pkgs" category.
41 lines
1.3 KiB
Racket
41 lines
1.3 KiB
Racket
#lang scribble/doc
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@(require scribble/manual scribble/eval "guide-utils.rkt"
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(for-label racket/undefined
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racket/shared))
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@title[#:tag "void+undefined"]{Void and Undefined}
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Some procedures or expression forms have no need for a result
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value. For example, the @racket[display] procedure is called only for
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the side-effect of writing output. In such cases the result value is
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normally a special constant that prints as @|void-const|. When the
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result of an expression is simply @|void-const|, the @tech{REPL} does not
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print anything.
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The @racket[void] procedure takes any number of arguments and returns
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@|void-const|. (That is, the identifier @racketidfont{void} is bound
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to a procedure that returns @|void-const|, instead of being bound
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directly to @|void-const|.)
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@examples[
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(void)
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(void 1 2 3)
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(list (void))
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]
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The @racket[undefined] constant, which prints as @|undefined-const|, is
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sometimes used as the result of a reference whose value is not yet
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available. In previous versions of Racket (before version 6.1),
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referencing a local binding too early produced @|undefined-const|;
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too-early references now raise an exception, instead.
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@margin-note{The @racket[undefined] result can still be produced
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in some cases by the @racket[shared] form.}
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@def+int[
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(define (fails)
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(define x x)
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x)
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(fails)
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]
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