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GNU Emacs Lisp Reference Manual

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10.5.3.1 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 section 21.10 Quitting.

The error 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 (see section 4.6 Conversion of Characters and 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 "You have committed %d errors" 10)
     error--> You have committed 10 errors

error works by calling signal with two arguments: the error symbol error, and a list containing the string returned by format.

Warning: If you want to use your own string as an error message verbatim, don't just write (error string). If string contains `%', it will be interpreted as a format specifier, 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 bearing a property error-conditions whose value is a list of condition names. This is how Emacs Lisp classifies different sorts of errors.

The number and significance of the objects in data depends on error-symbol. For example, with a wrong-type-arg 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. See section 10.5.3.4 Error Symbols and Condition Names, for a description of error symbols.

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 section 10.5.3.3 Writing Code to Handle Errors). If the error is not handled, these two values are used in printing the error message.

The function signal never returns (though in older Emacs versions it could sometimes return).

 
(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"

Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.


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