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warn [Function]

warn datum {&rest arguments} => nil

Arguments and Values::

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

Description::

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.

Examples::

  (defun foo (x)
    (let ((result (* x 2)))
      (if (not (typep result 'fixnum))
          (warn "You're using very big numbers."))
      result))
=>  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.