3.11 Guarded Evaluation: when and unless

Effects If...: when and unless in A Guide to PLT Scheme introduces when and unless.

(when test-expr expr ...)

Evaluates the text-expr. If the result is any value other than #f, the exprs are evaluated, and the results are ignored. No expr 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 expr ...)

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

Examples:

> (unless (positive? 5)       (display "hi"))

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

hi there