ex_2_74.clj

(ns sicp.chapter-2.part-4.ex-2-74)

; Exercise 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:
; 1. 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?

(defn make-generic-employee-file
  [division employee-file]
  (cons division employee-file))

(defn division
  [generic-employee-file]
  (first generic-employee-file))

(defn employee-file
  [generic-employee-file]
  (rest generic-employee-file))

(defn get-record
  [employee-name generic-employee-file]
  ((get (division generic-employee-file) :get-record)
   employee-name
   (employee-file generic-employee-file)))

; 2. 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?

(defn employee-record
  [generic-employee-record]
  (println "Debug" generic-employee-record))

(defn get-salary
  [generic-employee-record]
  ((get (division generic-employee-record) :get-salary)
   (employee-record generic-employee-record)))

; 3. 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.

(defn find-employee-record
  [employee-name generic-files]
  (if (empty? generic-files)
    (throw (Exception. "Record not found"))
    (let [record (get-record employee-name (first generic-files))]
      (if record
        record
        (find-employee-record employee-name (rest generic-files))))))

; When Insatiable takes over a new company, what changes must be made in order to incorporate the
; new personnel information into the central system?
; Each new unit must set its own getters in the table.