CS 1102 Homework 6: Macros

Assignment Goals

• To give you practice with macros

The Assignment

1. Implementing Objects Through Macros

   Despite their syntactic differences, functional and object-oriented programs are more similar than you probably think they are. What do objects do? They group together data and functions into one piece of data, and you access methods by sending messages to objects. Whereas in functional programs, we might write

   ;; A dillo is a (make-dillo number boolean)
   (define-struct dillo (length dead?))
   
   ;; longer-than? : dillo number -> boolean
   ;; is dillo longer than given length
   (define (longer-than? adillo len)
     (> (dillo-length adillo) len))
   
   ;; run-over : dillo -> dillo
   ;; return dead dillo one unit longer than given dillo
   (define (run-over adillo)
     (make-dillo (+ (dillo-length adillo) 1) true))
   

   We could also have written this in object-style in Scheme using functions to support messages:

   (define make-dillo-obj
     (lambda (length dead?)
       (lambda (message)
         (cond [(symbol=? message 'longer-than?) 
                (lambda (len) (> length len))]
               [(symbol=? message 'run-over) 
                (lambda () (make-dillo-obj (+ length 1) true))]))))
   
   (define d1 (make-dillo-obj 5 false))
   ((d1 'longer-than?) 6)
   ((d1 'longer-than?) 5)
   (define d2 ((d1 'run-over)))
   ((d2 'longer-than?) 5)
   

   While you may believe that this example has the spirit of objects, it certainly doesn't look very convincing. Your job is write macros that provide a better syntax for defining object-oriented classes in Scheme. Your macros should support the following alternative syntax:

   (define dillo-class
     (class (initvars length dead?)
            (method longer-than? (len) (> length len))
            (method run-over () (dillo-class (+ length 1) true))))
   
   (define d3 (dillo-class 5 false))
   (send d3 longer-than? 6)
   (send d3 longer-than? 5)
   (define d4 (send d3 run-over))
   (send d4 longer-than? 5)
   

   To do this, you should implement two macros, one for class and one for send. Your macros should allow someone to define a class with any number (including 0) of initvars and any number (including 0) of methods. Your macros should be such that running the two versions of the dillos code shown above (the make-dillo-obj version and the dillo-class version) with their sample interactions should produce the same answers.

2. Macros for Checking Access Policies

   A software company wants to develop a program to restrict the updates that its employees can make to code, documentation, and status reports. A set of company-defined access rules determine which employees can update which types of files. For example, the company might specify that

   • programmers can read and write code and documentation
   • testers can read code and write documentation
   • managers can read and write reports
   • managers can read documentation
   • ceos can read and write all files

   The company proposes the following language for writing down policies (this example captures the above set of 5 rules).

      (define check-policy
        (policy-checker
         (programmer (read write) (code documentation))
         (tester (read) (code))
         (tester (write) (documentation))
         (manager (read write) (reports))
         (manager (read) (documentation))
         (ceo (read write) (code documentation reports))))
   

   Each policy specifies a program that can be used to check whether a particular job can perform an access to a certain kind of file. For example, the following interaction uses the policy defined above:

      > (check-policy 'programmer 'write 'code)
      true
      > (check-policy 'programmer 'write 'reports)
      false
   

   Write a macro for policy-checker such that this example and interaction works as shown. Your macro should be able to support any policy of this general format (in other words, it should allow different numbers of roles, rules, kinds of files, and kinds of accesses). Note that you are not writing a macro for check-policy: in the example above, check-policy is the name given to the program produced by the macro, so you can use it to check access permissions as shown in the sample interaction.

What to Turn In

Turn in a single file hwk6.ss or hwk6.scm containing your answers. Make sure that both students' names are in a comment at the top of the file.