Next: Processing of Errors Up: Errors
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
Quitting, which happens when the user types `C-g', is not considered
an error, but it handled almost like an error. Note: Quitting.
- Function: error FORMAT-STRING &rest ARGS
This function signals an error with an error message constructed by
applying `format' (Note: String Conversion.) to FORMAT-STRING
Typical uses of `error' is shown in the following examples:
(error "You have committed an error.
Try something else.")
error--> You have committed 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
If you want to use a user-supplied 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 different sorts of errors are
The number and significance of the objects in DATA depends on
ERROR-SYMBOL. For example, with a `wrong-type-arg' error, there
are two objects in the list: a predicate which describes the type
that was expected, and the object which failed to fit that type.
Note: Error Names, for a description of error symbols.
Both ERROR-SYMBOL and DATA are available to any error handlers
which handle the error: a list `(ERROR-SYMBOL . DATA)' is
constructed to become the value of the local variable bound in the
`condition-case' form (Note: Handling Errors.). If the error is
not handled, both of them 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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