CIS 352   —   SPRING 2008

Homework 8

Coverage

Logistics

This homework is due in the bin in CST 3-212 by noon on Friday, April 4.

You may work singly or in pairs on this assignment.

N.B. When you are done, please email your code for this homework to Paul () so he can run some tests on it.

EXERCISES

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1. (30 points) EOPL Exercise 3.40. (You can skip the odd and even stuff at the end.)

   If you look in hw08.scm, you will see a start at solving the problem. In particular,

   • The define construct is already added as a new sort of program: a-defn, with fields id and exp.
   • The read-eval-print code as been modified so that every time you fire up read-eval-print, it creates a fresh initial environment (called cur-env). The code you add should use cur-env, not init-env.

     You should delete the indicated line (a placeholder so that hw08.scm will load) and replace it with your code.

   Beyond making changes to read-eval-print, you will also need to write a procedure that checks whether a variable already has a binding in the current environment. Try modelling this procedure after apply-env or apply-env-ref.

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2. (30 points) EOPL Exercise 3.43. (Be sure to answer the questions in the text of the exercise.)

   At the very end of hw08.scm there are some sample programs I used in testing my answer to this question. So, for example, evaluating (run tst1) in Scheme will run tst1. Of course, you should add some of your own tests.

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3. (30 points)

   Consider the following expression in our defined language, evaluated in the interpreter's initial environment:

     define x = 5
     define y = 20
     define z = +(x,y)
     let f = proc(w) +(w,+(x,z))
      in 
         let g = proc(y,x) begin set z = +(z,x); (f z) end
             z = 30
          in 
             begin
               set z = +(z,y);
               (g 1 z)
             end

   1. What is the value of this expression if lexical scoping is used? Support your answer by drawing the environments in effect when (g 1 z) is evaluated and (subsequently) when (f x) is evaluated.

   2. Same question as a, but with dynamic scoping this time.

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