warn

(warn datum args...)

Prints a WARNING: message to the standard error stream and returns nil; execution continues. The same condition designators as error are accepted: a control string, literal or computed (same directives as format, with the arguments after it as its format arguments), a quoted condition-type symbol with initargs (the class's :report -- inherited from an ancestor when the class defines none -- becomes the message, format applied to :format-control/:format-arguments for a simple-warning subtype that reports nothing else, and the Condition (type initargs...) was signalled. shape when the class reports nothing at all), or a condition object. A handler-bind handler on warning runs at the signal point before the message is printed, and can call muffle-warning to abort the output (warn then returns nil silently); handler-case catches errors and signal, not warn. Like error, warn has a function value: the interpreter keeps the full designator protocol through #'warn, the compiled backends forward the datum only. The message goes to standard error on every backend, including the WASM --component output (the WASI 0.3 adapter wires fd 2 to wasi:cli/stderr).

Because the message goes to standard error (not standard output) it is shown here statically rather than as a runnable example:

(warn "unexpected value: ~a" x)

The call prints WARNING: unexpected value: 42 (for x = 42) to standard error and evaluates to nil.

Redirecting the report: *error-output*

The destination is the current value of *error-output*, read at call time. Its default value is the designator for the process standard error -- unlike *standard-output*'s t, which names standard output -- so an unredirected warn reaches stderr, and (format *error-output* ...) does too. Binding the variable captures the report, the usual Common Lisp way to test warnings:

The binding is dynamic, so it applies inside called functions as well, and the variable is restored on the way out. It works the same on all four backends.