Signaling Errors

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 error and signal.

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.

— Function: error format-string &rest args

This function signals an error with an error message constructed by applying format-message (see Formatting Strings) to format-string and args.
These examples show typical uses of error:
          (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 symbol error, and a list containing the string returned by format-message.
Typically grave accent and apostrophe in the format translate to matching curved quotes, e.g., "Missing `%s'" might result in "Missing ‘foo’". See Text Quoting Style, for how to influence or inhibit this translation.
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 (error "%s"string).

— Function: signal error-symbol data

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 defined with 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 non-nil, this is followed by a colon and a comma separated list of the unevaluated elements of data. For error, the error message is the car of data (that must be a string). Subcategories of file-error are handled specially.
The number and significance of the objects in data depends on error-symbol. For example, with a wrong-type-argument error, 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 (error-symbol.data) (see Handling Errors).
The function 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"

— Function: user-error format-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 signals a user-error. Such errors do not cause entry to the debugger, even when debug-on-error is non-nil. See Error Debugging.

11.7.3.1 How to Signal an Error