Signaling an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see Processing of Errors). Here we describe how to signal an error.
Most errors are signaled automatically within Lisp primitives
which you call for other purposes, such as if you try to take the
CAR of an integer or move forward a character at the end of the
buffer. You can also signal errors explicitly with the functions
Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See Quitting.
Every error specifies an error message, one way or another. The message should state what is wrong (“File does not exist”), not how things ought to be (“File must exist”). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation.
errorformat-string &rest args
This function signals an error with an error message constructed by
format-message (see Formatting Strings) to
format-string and args.
These examples show typical uses of
(error "That is an error -- try something else") error→ That is an error -- try something else
(error "Invalid name `%s'" "A%%B") error→ Invalid name ‘A%%B’
error works by calling
signal with two arguments: the
error, and a list containing the string returned by
The text-quoting-style variable controls what quotes are generated; See Keys in Documentation. A call using a format like "Missing `%s'" with grave accents and apostrophes typically generates a message like "Missing ‘foo’" with matching curved quotes. In contrast, a call using a format like "Missing '%s'" with only apostrophes typically generates a message like "Missing ’foo’" with only closing curved quotes, an unusual style in English.
Warning: If you want to use your own string as an error message
verbatim, don’t just write
(error string). If string
string contains ‘%’, ‘`’, or ‘'’ it may be
reformatted, with undesirable results. Instead, use
This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error.
The argument error-symbol must be an error symbol—a symbol
define-error. This is how Emacs Lisp classifies different
sorts of errors. See Error Symbols, for a description of error symbols,
error conditions and condition names.
If the error is not handled, the two arguments are used in printing
the error message. Normally, this error message is provided by the
error-message property of error-symbol. If data is
nil, this is followed by a colon and a comma separated list
of the unevaluated elements of data. For
error message is the CAR of data (that must be a string).
file-error are handled specially.
The number and significance of the objects in data depends on
error-symbol. For example, with a
there should be two objects in the list: a predicate that describes the type
that was expected, and the object that failed to fit that type.
Both error-symbol and data are available to any error
handlers that handle the error:
condition-case binds a local
variable to a list of the form
data) (see Handling Errors).
signal never returns.
(signal 'wrong-number-of-arguments '(x y)) error→ Wrong number of arguments: x, y
(signal 'no-such-error '("My unknown error condition")) error→ peculiar error: "My unknown error condition"
user-errorformat-string &rest args
This function behaves exactly like
error, except that it uses
the error symbol
user-error rather than
error. As the
name suggests, this is intended to report errors on the part of the
user, rather than errors in the code itself. For example,
if you try to use the command
Info-history-back (l) to
move back beyond the start of your Info browsing history, Emacs
user-error. Such errors do not cause entry to the
debugger, even when debug-on-error is non-
See Error Debugging.
Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.