Create a polynomial object from a number using change-class

Question

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 is 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]

Answer 1

You can use COERCE or FLOAT to do that:

(float 4) => 4.0
(coerce 4 'float) => 4.0

Answer 2

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.