2.74¶

; Insatiable Enterprises, Inc., is a highly decentralized conglomerate company 
; consisting of a large number of independent divisions located all over the world. 
; The company’s computer facilities have just been interconnected by means of a
; clever network-interfacing scheme that makes the entire network appear to any user
; to be a single computer. Insatiable’s president, in her first attempt to exploit 
; the ability of the network to extract administrative information from division 
; files, is dismayed to discover that, although all the division files have been 
; implemented as data structures in Scheme, the particular data structure used 
; varies from division to division. A meeting of division managers is hastily called 
; to search for a strategy to integrate the files that will satisfy headquarters’ 
; needs while preserving the existing autonomy of the divisions.

; Show how such a strategy can be implemented with data- directed programming. As an
; example, suppose that each division’s personnel records consist of a single file, 
; which contains a set of records keyed on employees’ names. The structure of the set
; varies from division to division. Furthermore, each employee’s record is itself a 
; set (structured differently from division to division) that contains information 
; keyed under identifiers such as address and salary. In particular:

; (a) Implement for headquarters a `get-record` procedure that retrieves a specified 
;     employee’s record from a specified personnel file. The procedure should be 
;     applicable to any division’s file. Explain how the individual divisions’ files 
;     should be structured. In particular, what type information must be supplied?

; Ans:
; Headquarter should have the following structure with division name and file
(define (make-hq-file division file)
    (cons division file))
(define (get-division file) (car file))
(define (get-file file) (cdr file))

; `get-record` method
(define (get-record name file)
    ((get 'get-record (get-division file)) 
        name (get-file file)))

; (b) Implement for headquarters a `get-salary` procedure that returns the salary
;     information from a given employee’s record from any division’s personnel file. 
;     How should the record be structured in order to make this operation work?

(define (make-hq-record division record)
    (cons division record))
(define (get-record-division record) (car record))
(define (get-record-hq record) (cdr record))

(define (get-salary file)
    ((get 'get-salary (get-division file))
        (get-record-hq file)))

; (c) Implement for headquarters a `find-employee-record` procedure. This should 
;     search all the divisions’ files for the record of a given employee and return
;     the record. Assume that this procedure takes as arguments an employee’s name 
;     and a list of all the divisions’ files.
(define (find-employee-record name files)
    (cond ((null? files) '())
          ((not (null? (get-record name (car files)))) 
            (get-record name (car files)))
          (else (find-employee-record name (cdr files)))))

; (d) When Insatiable takes over a new company, what changes must be made in order 
;     to incorporate the new personnel information into the central system?

; Ans:
; The new division's methods should be installed like pacakge
(define (install-new-company)
    (put 'new-company 'get-record new-company-get-record)
    (put 'new-company 'get-salary new-company-get-salary)
    'done)