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Function WARN


warn datum &rest arguments => nil

Arguments and Values:

datum, arguments---designators for a condition of default type simple-warning.


Signals a condition of type warning. If the condition is not handled, reports the condition to error output.

The precise mechanism for warning is as follows:

The warning condition is signaled

While the warning condition is being signaled, the muffle-warning restart is established for use by a handler. If invoked, this restart bypasses further action by warn, which in turn causes warn to immediately return nil.

If no handler for the warning condition is found

If no handlers for the warning condition are found, or if all such handlers decline, then the condition is reported to error output by warn in an implementation-dependent format.

nil is returned

The value returned by warn if it returns is nil.


  (defun foo (x)
    (let ((result (* x 2)))
      (if (not (typep result 'fixnum))
          (warn "You're using very big numbers."))
=>  FOO
  (foo 3)
=>  6
  (foo most-positive-fixnum)
>>  Warning: You're using very big numbers.
=>  4294967294
  (setq *break-on-signals* t)
=>  T
  (foo most-positive-fixnum)
>>  Break: Caveat emptor.
>>  To continue, type :CONTINUE followed by an option number.
>>   1: Return from Break.
>>   2: Abort to Lisp Toplevel.
>>  Debug> :continue 1
>>  Warning: You're using very big numbers.
=>  4294967294

Side Effects:

A warning is issued. The debugger might be entered.

Affected By:

Existing handler bindings.

*break-on-signals*, *error-output*.

Exceptional Situations:

If datum is a condition and if the condition is not of type warning, or arguments is non-nil, an error of type type-error is signaled.

If datum is a condition type, the result of (apply #'make-condition datum arguments) must be of type warning or an error of type type-error is signaled.

See Also:

*break-on-signals*, muffle-warning, signal

Notes: None.

The following X3J13 cleanup issue, not part of the specification, applies to this section:

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