On this page:
when
unless

2.16 Guarded Evaluation: when and unless

+Effects If...: when and unless in The Racket Guide introduces when and unless.

(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

(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