Common Lisp the Language, 2nd Edition
These
facilities are designed to make it convenient for the user
to insert error checks into code.
[Macro]
check-type place typespec [string]
[This supersedes the description of check-type given in section 24.2.-GLS]
A check-type form signals an error of type type-error if the contents of place are not of the desired type.
If a condition is signaled, handlers of this condition can use the functions type-error-datum and type-error-expected-type to access the contents of place and the typespec, respectively.
This function can return only if the store-value restart is invoked, either explicitly from a handler or implicitly as one of the options offered by the debugger. The restart is associated with the signaled condition as if by use of with-condition-restarts.
If store-value is called, check-type will store the new value that is the argument to store-value (or that is prompted for interactively by the debugger) in place and start over, checking the type of the new value and signaling another error if it is still not the desired type. Subforms of place may be evaluated multiple times because of the implicit loop generated. check-type returns nil.
The place must be a generalized variable reference acceptable to setf. The typespec must be a type specifier; it is not evaluated. The string should be an English description of the type, starting with an indefinite article (``a'' or ``an''); it is evaluated. If the string is not supplied, it is computed automatically from the typespec. (The optional string argument is allowed because some applications of check-type may require a more specific description of what is wanted than can be generated automatically from the type specifier.)
The error message will mention the place, its contents, and the desired type.
Lisp> (setq aardvarks '(sam harry fred))
=> (SAM HARRY FRED)
Lisp> (check-type aardvarks (array * (3)))
Error: The value of AARDVARKS, (SAM HARRY FRED),
is not a 3-long array.
To continue, type :CONTINUE followed by an option number:
1: Specify a value to use instead.
2: Return to Lisp Toplevel.
Debug> :continue 1
Use Value: #(sam fred harry)
=> NIL
Lisp> aardvarks
=> #<ARRAY-3 13571>
Lisp> (map 'list #'identity aardvarks)
=> (SAM FRED HARRY)
Lisp> (setq aacount 'foo)
=> FOO
Lisp> (check-type aacount (integer 0 *) "a non-negative integer")
Error: The value of AACOUNT, FOO, is not a non-negative integer.
To continue, type :CONTINUE followed by an option number:
1: Specify a value to use instead.
2: Return to Lisp Toplevel.
Debug> :continue 2
Lisp>
[Macro]
assert test-form [({place}*) [datum {argument}*]]
[This supersedes the description of assert given in section 24.2.-GLS]
An assert form signals an error if the value of the test-form is nil. Continuing from this error using the continue restart will allow the user to alter the values of some variables, and assert will then start over, evaluating the test-form again. (The restart is associated with the signaled condition as if by use of with-condition-restarts.) assert returns nil.
The test-form may be any form. Each place (there may be any number of them, or none) must be a generalized variable reference acceptable to setf. These should be variables on which test-form depends, whose values may sensibly be changed by the user in attempting to correct the error. Subforms of each place are evaluated only if an error is signaled, and may be re-evaluated if the error is re-signaled (after continuing without actually fixing the problem).
The datum and arguments are evaluated only if an error is to be signaled, and re-evaluated if the error is to be signaled again.
If datum is a condition, then that condition is used directly. In this case, it is an error to specify any arguments.
If datum is a condition type (a class or class name), then the condition used is effectively the result of (apply #'make-condition datum (list argument)).
If datum is a string, then the condition used is effectively the result of
(make-condition 'simple-error
:format-string datum
:format-arguments (list argument))
If datum is omitted, then a condition of type simple-error is constructed using the test-form as data. For example, the following might be used:
(make-condition 'simple-error :format-string "The assertion ~S failed." :format-arguments '(test-form))
Note that the test-form itself, and not its value, is used as the format argument.
Here is an example of the use of assert:
(setq x (make-array '(3 5) :initial-element 3))
(setq y (make-array '(3 5) :initial-element 7))
(defun matrix-multiply (a b)
(let ((*print-array* nil))
(assert (and (= (array-rank a) (array-rank b) 2)
(= (array-dimension a 1)
(array-dimension b 0)))
(a b)
"Cannot multiply ~S by ~S." a b)
(really-matrix-multiply a b)))
(matrix-multiply x y)
Error: Cannot multiply #<ARRAY-3-5 12345> by #<ARRAY-3-5 12364>.
To continue, type :CONTINUE followed by an option number:
1: Specify new values.
2: Return to Lisp Toplevel.
Debug> :continue 1
Value for A: x
Value for B: (make-array '(5 3) :initial-element 6)
=> #2A(¯(54 54 54 54 54)
(54 54 54 54 54)
(54 54 54 54 54)
(54 54 54 54 54)
(54 54 54 54 54))
