CIS 352   —   SPRING 2009

Homework 4

Coverage

• Essentials of Programming Languages (EOPL): Sections 3.1 and 3.2.

Logistics

• You may work singly or in pairs on this assignment.

• Be sure you mark (via comments) each change you make to the LET interpreter files.
  (See the Homework Policy for more details.)

What to Turn in:

• A copy of the source code of each file modified.

• DO NOT turn in the source code from files you did not modify.

• A run of your code that verifies that it passes all the tests in your test harness.

How to turn in your work:

1. If you have more than one file to submit, bundle them into a zip or tar archive.

2. Click on this electronic-submission link, and use your NetID and password for authorization.
   Important: Make sure you select CIS 352 as the course for which you're submitting.

PROBLEMS

For the following, build on the extensions you did for Lab 5.

1. (33 points)   EOPL Exercise 3.8.

2. (34 points)   EOPL Exercise 3.12.

3. (33 points)   EOPL Exercise 3.15.

Notes

• For Exercise 3.12, add the following to the-grammar in lang.scm:

  (expression
   ("cond" (arbno expression "==>" expression) "end")
   cond-exp)

  A cond-exp has two components,
  1. a list of tests (the expressions on the left-hand-sides of the ==>'s) and
  2. a list of results (the expressions on the right-hand-sides of the ==>'s).

  The code you add to value-of for this exercise will look like:

  (cond-exp (tests results)
    (if (null? tests)
        (eopl:error "value-of: no clause of cond held")
        (let ((test1 (car tests)) 
              (result1 (car results)))
          ...)
    ))

  If test1 fails to evaluate to true, then you will have to do a recursion that looks like:

  (value-of (cond-exp (cdr tests) (cdr results)) env)

  Here are some tests for your implementation of cond:

  ;; Homework 4, problem 2 tests
  (cond-1 "cond zero?(0)  ==> 1   zero?(17) ==> 2 end" 1)
  (cond-2 "cond zero?(17) ==> 1   zero?(0)  ==> 2 end" 2)
  (cond-3 "cond zero?(1)  ==> 1   zero?(2)  ==> 2    zero?(0) ==> 3 end" 3)
  (cond-4 "cond zero?(17) ==> 1   zero?(17) ==> 2 end" error)
  (cond-5 "cond end" error)

• For Exercise 3.15, recall that the body's of lets can have a sequence of expressions, the value of the last one is returned. For example, try evaluating:

  (let ((val1 (+ 9 23))
        (val2 (- 100 3)))
    (display val1)
    (display "\n")
    (display val2)
    (display "\n")
    'all-done)

• Error reports from run-all can occasionally be misleading. If you get an error report that does not make sense, try running that case via run. For example, I had the test

   (div-2 "/(4,2)" 2)

  and got the report

  test: div-5
  /(4,2)
  correct outcome: 2
  actual outcome:  2
  incorrect
  

  When I ran (run "/(4,2)") what came back was 2 and of course what should have come back was #(struct:num-val 2). When I added the call to num-val to the div-case, the interpreter then passed all tests.