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13 Assertions and Errors

This section describes two macros that test assertions, i.e., conditions which must be true if the program is operating correctly. Assertions never add to the behavior of a Lisp program; they simply make “sanity checks” to make sure everything is as it should be.

If the optimization property speed has been set to 3, and safety is less than 3, then the byte-compiler will optimize away the following assertions. Because assertions might be optimized away, it is a bad idea for them to include side-effects.

— Special Form: assert test-form [show-args string args...]

This form verifies that test-form is true (i.e., evaluates to a non-nil value). If so, it returns nil. If the test is not satisfied, assert signals an error.

A default error message will be supplied which includes test-form. You can specify a different error message by including a string argument plus optional extra arguments. Those arguments are simply passed to error to signal the error.

If the optional second argument show-args is t instead of nil, then the error message (with or without string) will also include all non-constant arguments of the top-level form. For example:

          (assert (> x 10) t "x is too small: %d")

This usage of show-args is an extension to Common Lisp. In true Common Lisp, the second argument gives a list of places which can be setf'd by the user before continuing from the error. Since Emacs Lisp does not support continuable errors, it makes no sense to specify places.

— Special Form: check-type form type [string]

This form verifies that form evaluates to a value of type type. If so, it returns nil. If not, check-type signals a wrong-type-argument error. The default error message lists the erroneous value along with type and form themselves. If string is specified, it is included in the error message in place of type. For example:

          (check-type x (integer 1 *) "a positive integer")

See Type Predicates, for a description of the type specifiers that may be used for type.

Note that in Common Lisp, the first argument to check-type must be a place suitable for use by setf, because check-type signals a continuable error that allows the user to modify place.

The following error-related macro is also defined:

— Special Form: ignore-errors forms...

This executes forms exactly like a progn, except that errors are ignored during the forms. More precisely, if an error is signaled then ignore-errors immediately aborts execution of the forms and returns nil. If the forms complete successfully, ignore-errors returns the result of the last form.