10.6.3.4 Error Symbols and Condition Names

When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.

These narrow classifications are grouped into a hierarchy of wider classes called error conditions, identified by condition names. The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name error which takes in all kinds of errors (but not quit). Thus, each error has one or more condition names: error, the error symbol if that is distinct from error, and perhaps some intermediate classifications.

Function: define-error name message &optional parent

In order for a symbol to be an error symbol, it must be defined with define-error which takes a parent condition (defaults to error). This parent defines the conditions that this kind of error belongs to. The transitive set of parents always includes the error symbol itself, and the symbol error. Because quitting is not considered an error, the set of parents of quit is just (quit).

In addition to its parents, the error symbol has a message which is a string to be printed when that error is signaled but not handled. If that message is not valid, the error message ‘peculiar error’ is used. See Definition of signal.

Internally, the set of parents is stored in the error-conditions property of the error symbol and the message is stored in the error-message property of the error symbol.

Here is how we define a new error symbol, new-error:

(define-error 'new-error "A new error" 'my-own-errors)

This error has several condition names: new-error, the narrowest classification; my-own-errors, which we imagine is a wider classification; and all the conditions of my-own-errors which should include error, which is the widest of all.

The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs.

Naturally, Emacs will never signal new-error on its own; only an explicit call to signal (see Definition of signal) in your code can do this:

(signal 'new-error '(x y))
     error→ A new error: x, y

This error can be handled through any of its condition names. This example handles new-error and any other errors in the class my-own-errors:

(condition-case foo
    (bar nil t)
  (my-own-errors nil))

The significant way that errors are classified is by their condition names—the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. It would be cumbersome to give signal a list of condition names rather than one error symbol.

By contrast, using only error symbols without condition names would seriously decrease the power of condition-case. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification.

See Standard Errors, for a list of the main error symbols and their conditions.