sicp每日一题[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:

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?
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?
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.
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?

a. 这道题里，部门名称就相当于前面所说的类型，获取部门信息就是操作，我们根据这两个信息从提前建好的二维表里提取雇员所属部门的文件。每个部门文件都应该有一个 get-division 的过程，它根据雇员姓名获取对应的部门文件。

(define (get-record get-division employee-name) 
   ((get 'division get-division) employee-name)) 

b. 这道题跟上一题很像，只是这次是从某个雇员的记录中获取工资信息。

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

c. 这道题就是依次检查所有部门，根据雇员名称获取记录。

(define (find-employee-record employee-name division-list) 
  (if (null? division-list) 
      #f 
      (or (get-record (car division-list) employee-name) 
          (find-employee-record employee-name (cdr division-list))))) 

d. 每次接管一个新公司，都要把上述提到的过程添加到二维表里，使用的时候根据部门类型和操作来获取对应公司的过程。