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37.5.1 Warning Basics

Every warning has a textual message, which explains the problem for the user, and a severity level which is a symbol. Here are the possible severity levels, in order of decreasing severity, and their meanings:

A problem that will seriously impair Emacs operation soon if you do not attend to it promptly.
A report of data or circumstances that are inherently wrong.
A report of data or circumstances that are not inherently wrong, but raise suspicion of a possible problem.
A report of information that may be useful if you are debugging.

When your program encounters invalid input data, it can either signal a Lisp error by calling error or signal or report a warning with severity :error. Signaling a Lisp error is the easiest thing to do, but it means the program cannot continue processing. If you want to take the trouble to implement a way to continue processing despite the bad data, then reporting a warning of severity :error is the right way to inform the user of the problem. For instance, the Emacs Lisp byte compiler can report an error that way and continue compiling other functions. (If the program signals a Lisp error and then handles it with condition-case, the user won't see the error message; it could show the message to the user by reporting it as a warning.)

Each warning has a warning type to classify it. The type is a list of symbols. The first symbol should be the custom group that you use for the program's user options. For example, byte compiler warnings use the warning type (bytecomp). You can also subcategorize the warnings, if you wish, by using more symbols in the list.

— Function: display-warning type message &optional level buffer-name

This function reports a warning, using message as the message and type as the warning type. level should be the severity level, with :warning being the default.

buffer-name, if non-nil, specifies the name of the buffer for logging the warning. By default, it is *Warnings*.

— Function: lwarn type level message &rest args

This function reports a warning using the value of (format message args...) as the message in the *Warnings* buffer. In other respects it is equivalent to display-warning.

— Function: warn message &rest args

This function reports a warning using the value of (format message args...) as the message, (emacs) as the type, and :warning as the severity level. It exists for compatibility only; we recommend not using it, because you should specify a specific warning type.