2.16 Guarded Evaluation: when and unless

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

(when test-expr expr ...)

Evaluates the text-expr. If the result is #f, then the result of the when expression is #<void>. Otherwise, the exprs are evaluated, and the last 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