SICP Section 2.4 Multiple Representations for Abstract Data

Exercise 2.75. Implement the constructor make-from-mag-ang in message-passing style. This procedure should be analogous to the make-from-real-imag procedure given above.

Answer:

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1. (define (make-from-mag-ang r a)  
2.   (define (dispatch op)  
3.     (cond ((eq? op 'real-part)  
4.            (* r (cos a)))  
5.           ((eq? op 'imag-part)  
6.            (* r (sin a)))  
7.           ((eq? op 'magnitude)  
8.            r)  
9.           ((eq? op 'angle)  
10.            a)  
11.           (else  
12.            (error "Unknown op -- MAKE-FROM-MAG-ANG" op))))            
13.   dispatch)  

(define (make-from-mag-ang r a) (define (dispatch op) (cond ((eq? op 'real-part) (* r (cos a))) ((eq? op 'imag-part) (* r (sin a))) ((eq? op 'magnitude) r) ((eq? op 'angle) a) (else (error "Unknown op -- MAKE-FROM-MAG-ANG" op)))) dispatch)
Exercise 2.76. As a large system with generic operations evolves, new types of data objects or new operations may be needed. For each of the three strategies -- generic operations with explicit dispatch, data-directed style, and message-passing-style -- describe the changes that must be made to a system in order to add new types or new operations. Which organization would be most appropriate for a system in which new types must often be added? Which would be most appropriate for a system in which new operations must often be added?

Answer:

• Generic operations with explicit dispatch: explicit dispatch is relatively easier to code. However adding new types would require that all existing functions be changed appropriately. For this reason makes explicit dispatch is less suited for a system in which new types must often be added
• Data-directed style: adding new types would not require extensive changes at the top level. However new packages would need to be added behind the scenes to facilitate operation.
• Message-passing-style: this requires the addition of a suitable type of procedure/object which can accept the messages as necessary. The top level does not need to change.