44 lines
1.4 KiB
Racket
44 lines
1.4 KiB
Racket
#reader(lib "docreader.ss" "scribble")
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@require[(lib "manual.ss" "scribble")]
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@require[(lib "eval.ss" "scribble")]
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@require["guide-utils.ss"]
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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 @scheme[display] procedure is called only for
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the side-effect of writing output. In such cases the reslt 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 REPL does not
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print anything.
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The @scheme[void] procedure takes any number of arguments and returns
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@|void-const|. (That is, the identifier @schemeidfont{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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A constant that prints as @undefined-const is used as the result of a
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reference to a local binding when the binding is not yet
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initialized. Such early references are not possible for bindings that
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corerspond to procedure arguments, @scheme[let] bindings, or
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@scheme[let*] bindings; early reference requires a recursive binding
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context, such as @scheme[letrec] or local @scheme[define]s in a
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procedure body. Also, early references to top-level and module-level
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bindings raise an exception, instead of producing
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@|undefined-const|. For these reasons, @undefined-const rarely
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appears.
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@def+int[
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(define (strange)
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(define x x)
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x)
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(strange)
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]
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