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