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Common Lisp the Language, 2nd Edition
Next: Exhaustive Case
Analysis Up: Program
Interface to Previous: Signaling Conditions
These facilities are designed to make it convenient for the user to
insert error checks into code.
[Macro]
check-typeplacetypespec[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.
Implementation note: An implementation may choose to
generate a somewhat differently worded error message if it recognizes
that place is of a particular form, such as one of the
arguments to the function that called check-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>Compatibility note: In Zetalisp, the equivalent
facility is called check-arg-type.
[Macro]
asserttest-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-conditiondatum(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.
Implementation note: The debugger need not include the test-form in the error message, and any places should not be included in the message, but they should be made available for the user’s perusal. If the user gives the ``continue’’ command, an opportunity should be presented to alter the values of any or all of the references. The details of this depend on the implementation’s style of user interface, of course.
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))
Next: Exhaustive Case
Analysis Up: Program
Interface to Previous: Signaling Conditions
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