►The Racket Reference
▼3 Syntactic Forms
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when
unless
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3.16 Guarded Evaluation: when and unless

+按条件作用：when 与 unless in Racket 指南 introduces when and unless.

语法

(when test-expr body ...+)

Evaluates test-expr. If the result is #f, then the result of the when expression is #<void>. Otherwise, the bodys are evaluated, and the last body is in tail position with respect to the when form.

Examples:
> (when (positive? -5)
    (display "hi"))
> (when (positive? 5)
    (display "hi")
    (display " there"))

hi there

语法

(unless test-expr body ...+)

Equivalent to (when (not test-expr) body ...+).

Examples:
> (unless (positive? 5)
    (display "hi"))
> (unless (positive? -5)
    (display "hi")
    (display " there"))

hi there

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