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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.

Macro: 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 a change 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.

Macro: check-type place type &optional string

This form verifies that place evaluates to a value of type type. If so, it returns nil. If not, check-type signals a continuable wrong-type-argument error. The default error message lists the erroneous value along with type and place 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 section Type Predicates, for a description of the type specifiers that may be used for type.

Note that as in Common Lisp, the first argument to check-type should be a place suitable for use by setf, because check-type signals a continuable error that allows the user to modify place, most simply by returning a value from the debugger.

The following error-related macro is also defined:

Macro: ignore-errors forms…

This executes forms exactly like a progn, except that errors are ignored during the forms. More precisely, if an error is signalled 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.

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