CIS 352   —   SPRING 2009

Homework 5

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. (25 points)   EOPL Exercise 3.9

2. (25 points)   EOPL Exercise 3.10

3. (25 points)   EOPL Exercise 3.16

4. (25 points)   EOPL Exercise 3.17

Notes

• Handy trick: Suppose you have an environment, env, and a list of expressions, args =

  (e1 e2 … ek)

  and you want to compute a list of values of the form:

  ((value-of e1 env) (value-of e2 env) … (value-of ek env))

  Then try:

  (map (lambda (e) (value-of e env)) args)

• To handle list values, I have updated

  ⇓ Use these updated files for this homework!!! ⇓

  • data-structures.scm
    • The definition of expval now includes emptylist-val and pair-val cases.
    • expval->car and expval->cdr respectively extract the car and cdr fields from a pair-val.
  • environments.scm
    • The definition of init-env now includes emptylist which is set to (emptylist-val).
    • A definition of extend-env* is now included — this will be very handy for one of the problems.
  • top.scm
    • sloppy->expval (a procedure used by run-all) now handles the list-values.

  ⇑ Use these updated files for this homework!!! ⇑

• Use drscheme-init.scm,   interp.scm,   lang.scm,   and tests.scm from the previous homework.

• You will have to add productions to the-grammar for: car, cdr, null?, cons, list, and let*. You will also have to add cases for each of these in value-of Also, for the revision of let, you will have to change its production in the-grammar and its case in value-of.

  • For list:
    • Below is the list production for the-grammar. (See page 389 in EOPL for an explaination of what separated-list does.)

      (expression ;; Hw 5, exercise 3.10
       ("list" "(" (separated-list expression ",") ")")
       list-exp)
      

    • Here is the beginning of the list-exp case for value-of:

      ;; Homework 5, exercise 3.10
      (list-exp (args)  
        … )
      

  • For the revised let:
    • Below is the let production for the-grammar, which is similar to the production for code in Homework 4.

      (expression ;; Addition for Homework 5
       ("let" (arbno identifier "=" expression) "in" expression)
       let-exp)
      

    • Here is the beginning of the let-exp case for value-of:

      ;; HW 5, Exercise 3.17
      (let-exp (vars exps body)
        … )
      

  • You are on your own for the rest.

• Be sure to include the following tests. (Yes, you need more tests than these.)

  (list-1 "list()" () )
  
  (list-2 "list(1,2,3,4)" (1 2 3 4) )
  
  (list-3 "list(1,list(2,3,list(4)),5)" (1 (2 3 (4)) 5) )
  
  (multi-let-1 "let in 17" 17 )
  
  (multi-let-2 "let x = 30 in let x = -(x,1) y = -(x,2) in -(x,y)" 1 )
  
  (multi-let*-1 "let* in 17" 17 )
  
  (multi-let*-2 "let x = 30 in let* x = -(x,1) y = -(x,2) in -(x,y)" 2 ) 
  ;; My first version of multi-let*-2 had a typo