2.16 Guarded Evaluation: when and unless
Effects If...: when and unless in The Racket Guide introduces when and unless.
syntax
(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.
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syntax
(unless test-expr body ...+)
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