unwind-protect

(unwind-protect protected cleanup...)

Evaluates the protected form and returns its value, running the cleanup forms on every exit from it: a normal return, an error unwind, and a return/return-from non-local exit. The cleanup values are discarded. A cleanup form that itself signals replaces the pending exit (the newer error wins, as in Common Lisp).

unwind-protect is supported on the interpreter and the JVM backend; the WASM compilers reject it (a WASM error is an uncatchable trap, so there is no unwind path on which to run cleanups). The with-* macros (with-open-file, with-output-to-string, with-input-from-string, and the usocket:with-* family) expand over unwind-protect on the interpreter and the JVM, so they release their handle on every exit there; on WASM they keep the close-on-normal-exit shape.

The cleanup also runs when the protected form exits early via return:

Because an uncaught error aborts the program, the error path is shown statically:

> (unwind-protect (error "boom") (print :cleaned))
:cleaned
Error: boom