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 literal control string (same directives as format), a quoted condition-type symbol with initargs (a define-condition :report becomes the message; the Condition (type initargs...) was signalled. shape otherwise), or a condition object. Lite: the warning cannot be handled or muffled (no muffle-warning restart; handler-case catches errors and signal, not warn). Like error, warn is a macro with no function value, so #'warn is unsupported. 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.