assert

(assert test) / (assert test msg)

nil when the test is truthy, else an AssertionError, which an Exception catch does not take. Its message is Assert failed: <form>, or Assert failed: <msg>\n<form> with a message, the failed form printed readably like the oracle. The message sits in the else branch, so it evaluates only on failure -- lazily, like the oracle. Like and/or, assert has no function value.

assert reads *assert* where it expands, like the oracle's macro: after a top-level (set! *assert* false) (or nil) every later assert is nil, its test and message unevaluated, until a top-level set! back to true. A binding of *assert* around an assert changes nothing.

nil