Create a polynomial object from a number using change-class
I have written a polynomial class along the lines described in SICP 2.5.3 (except using defclass). I would like to be able to seamlessly add and multiply polynomials and regular numbers but I can't make change-class accept a number.
I tried to simplify the problem by changing class from an integer to a float:
(change-class 4 'float)
but that gave me the error:
There 开发者_如何学Pythonis no applicable method for the generic function
#<STANDARD-GENERIC-FUNCTION CHANGE-CLASS (7)>
when called with arguments
(4 #<BUILT-IN-CLASS FLOAT>).
[Condition of type SIMPLE-ERROR]
I get an error of the same form from (fyi):
(change-class 4 'polynomial)
I'm going to go ahead and implement a manual conversion but I would prefer to use the built-in clos facilities.
As Xach points out I could use coerce or float to change 4 to a float. That was intended as a simpler example of what I was trying to do and to remove any possibility of my update-instance-for-different-class being wrong.
Here is the longer version of what I tried that didn't work:
(defclass polynomial ()
((var :accessor var :initarg :var :initform 'x)
(terms :accessor terms :initarg :terms :initform (make-empty-term-list))))
(defmethod update-instance-for-different-class :before ((old number)
(new polynomial)
&key)
(setf (slot-value new 'terms) (adjoin-term (make-term old 0)
(make-empty-term-list))))
(change-class 4 'polynomial)
I still get an error like the example above:
There is no applicable method for the generic function
#<STANDARD-GENERIC-FUNCTION CHANGE-CLASS (7)>
when called with arguments
(4 #<STANDARD-CLASS POLYNOMIAL>).
[Condition of type SIMPLE-ERROR]
You can use COERCE or FLOAT to do that:
(float 4) => 4.0
(coerce 4 'float) => 4.0
You can't use CHANGE-CLASS with numbers. Numbers are not instances of CLOS classes. CHANGE-CLASS is also thought to destructively modify an instance to a new class, without changing the identity of the original instance.
I would either:
add methods to CHANGE-CLASS that does what you want
or write a function CHANGE, that implements various custom coercion rules and calls CHANGE-CLASS for any change from one CLOS class to another CLOS class.
精彩评论