SICP Exercise 2.74

Question

Insatiable Enterprises, Inc. is a highly decentralised 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?
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?
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.
4. When Insatiable takes over a new company, what changes must be made in order to incorporate the new personnel information into the central system?

Answer

1. It doesn’t matter how the files are structured exactly, as long as they contain the salary information and employee name. This is because of the data-directed approach which allows us to handle any kind of file structure. The actual function, then, is very simple:

      (define (get-record employee file)
        ((get 'get-record file) employee))
   

   Of course, the function with the 'get-record identifier must be stored somewhere and the employee file must also come with some sort of identifier, so the actual function can be looked up.

2. Similar to above:

      (define (get-salary employee file)
        ((get 'get-salary file) employee))
   

   Again, the actual record structure doesn’t matter, as long as there exists some selector to retrieve the salary.

3. A simple recursive function:

      ((define (find-employee-record employee files)
         (cond ((null? files) '())
               ((not (null? (get-record employee (car files))))
                (car files))
               (else (find-employee-record employee (cdr files))))))
   

4. No changes must be made to the actual file structure. However, selectors have to be written to select the needed data (if they don’t exist yet). Then, they must be registered, e.g. using put, in a package, and installed, using tags to add an identifier.