assert

(assert test-form [(place...) [datum args...]])

Evaluates test-form and signals an error when it is false; returns nil when it is true. The optional datum and arguments work like error's control string and arguments and replace the default "The assertion ... failed." message. This is the lite version of Common Lisp's assert: there is no restart system, so the places list is accepted but ignored (no interactive re-store loop).

A failing assertion aborts execution, so it is shown statically:

(let ((x 0)) (assert (> x 0) (x) "x must be positive, got ~a" x))
; error: x must be positive, got 0