CS 550
Spring 2015
Programming Languages
Thursdays 18:30-21:20
Korman Center 104C

Geoffrey Mainland
University Crossings 106
Teaching Assistant:
Mark Boady
Warning! This material is for an old version of the course.

In this assignment, you will modify the metacircular interpreter we saw in class. Successfully completing this assignment requires reading and understanding a medium-sized program written by someone else. If you do not have a good understanding of how the interpreter works, please review the material covered in lecture and in the book. Diving straight in to the homework without this understanding is going to make your task much more difficult.

We have provided you with a shell for your solution here. Please extract this tarball in your ~/cs550/git directory and immediately commit the resulting hw2 directory. You can do this as follows:

$ cd ~/cs550/git
$ wget 'https://www.cs.drexel.edu/~mainland/courses/cs550-201435/homework/hw2.tar.gz'
$ tar xf hw2.tar.gz
$ git add hw2
$ git commit -m "Initial check-in for homework 2."

All your changes should be made to the file mceval.rkt. Be sure to commit your work to the repository.

You can compile your code into a running interpreter by typing make in ~/cs550/git/hw2. If make does not complete successfully, it means your code does not compile. Code that does not compile will receive a zero.

We have included several test for your convenience. Passing all provided tests does not guarantee full credit, but failing tests does guarantee less than full credit. You can run the tests by typing make run-tests in ~/cs550/git/hw2.

This assignment is worth 100 points. There are 116 possible points.

Note: Problem 1 is due separately from problems 2–7. No late days may be used for Problem 1.

Working with the interpreter

There are then two ways you can test your evaluator evaluator:

  1. Type make and run the resulting binary, named mceval. The mceval program will repeatedly read in a Scheme expression and pass it to your interpreter for evaluation.

  2. From DrRacket, call the top-mceval function with an expression, like this:

 (top-mceval '(+ 2 3))

I used the second approach. I also made judicious use of display and newline to debug my implementation.

Hints for solving the problems

There are three ways to extend the interpreter:

  1. Add a primitive.
  2. Add a definition to the global environment.
  3. Add a special form.

You should only add a special form when it is absolutely necessary. Most of the time, the standard Scheme evaluation rules are exactly what you want. Solving a problem by adding a definition rather than a new special form is also much easier and avoids cluttering up your eval function.

Use display and newline to print out intermediate expressions! This is extremely helpful when debugging.

Part I

Problem 1: Code Reading Questions (25 points total)

Submit the solutions to this problem only in a file named problem1.txt in the hw2 subdirectory of your git repository.

Problem 1.1: Environment representation (5 points)

What representation does the metacircular evaluator use for environments?

Please be specific. An English description will suffice; however, your answer will be stronger if you also provide examples.

Problem 1.2: Defining the primitives (5 points)

What top-level define contains the list of primitives supported by the metacircular interpreter? Please name the variable.

Problem 1.3: Understanding primitives (5 points)

This is Exercise 4.14 from SICP.

Eva Lu Ator and Louis Reasoner are each experimenting with the metacircular evaluator. Eva types in the definition of map, and runs some test programs that use it. They work fine. Louis, in contrast, has installed the system version of map as a primitive for the metacircular evaluator. When he tries it, things go terribly wrong. Explain why Louis’s map fails even though Eva’s works.

Problem 1.4: Understanding eval (5 points)

This is Exercise 4.2a from SICP.

Louis Reasoner plans to reorder the cond clauses in eval so that the clause for procedure applications appears before the clause for assignments. He argues that this will make the interpreter more efficient: Since programs usually contain more applications than assignments, definitions, and so on, his modified eval will usually check fewer clauses than the original eval before identifying the type of an expression.

What is wrong with Louis’s plan? (Hint: What will Louis’s evaluator do with the expression (define x 3)?)

Problem 1.5: Extending the environment (5 points)

The function setup-environment is used to create the initial global environment used by the metacircular interpreter. For later problems, it will be convenient to add your own definitions to the initial global environment. The most convenient way to do this is to call eval-definition with the appropriate arguments from within the function setup-environment. If you were to add a definition in this manner, what arguments would you pass to eval-definition to add the following top-level define to the initial global environment? You may give your answer in the form of a Scheme expression.

(define (not x) (if x false true))

Part II

Problem 2: Adding Primitives (10 points total)

Add the following primitives: +, *, -, /, <, <=, =, >=, >, and error. 1 point each.

The error primitive should take no arguments and abort the interpreter with the message “Metacircular Interpreter Aborted” (without the quotes).

Problem 3: Implementing and and or (20 points total)

Add support for and and or to your interpreter (10 points each). Be sure your implementation adheres to the Scheme language standard (see here) in terms of how the arguments to and and or are evaluated and in terms of what value is returned.

You will probably want to use the last-exp?, first-exp, and rest-exps helper functions.

Remember that the metacircular interpreter cannot interpret #t and #f; use true and false instead.

Part III

Problem 4: Implementing let (20 points total)

This is Exercise 4.6 from SICP.

Let expressions are derived expressions, because

(let ((<var1> <exp1>) ... (<varn> <expn>))

is equivalent to

((lambda (<var1> ... <varn>)

Implement a syntactic transformation let->combination that reduces evaluating let expressions to evaluating combinations of the type shown above, and add the appropriate clause to eval to handle let expressions.

Hint: Use display to print out the result of let->combination to make sure you get it right! I did not get it right the first time…

Problem 5: Implementing force and delay (20 points total)

Add support for force and delay to your interpreter, where delay‘ed expressions are only evaluated once when force‘d.

For full credit, you must support call-by-need evaluation, which only evaluates delay‘ed expressions once.

If your implementation is call-by-name but otherwise correct, you will receive 10 points. You do not need to submit both a call-by-name and a call-by-need implementation for full credit; just the call-by-need version will do.

It is highly recommended that you implement the call-by-name version first.

We recommend you use memoization to implement call-by-need force and delay.

The implementations of both the call-by-name and call-by-need versions of force and delay were discussed in lecture. The easiest path to success will follow that implementation.

Your solution must demonstrate that you understand the underlying mechanisms for implementing lazy evaluation. Therefore, you should not use Racket’s force and delay or equivalent syntax macros in your solution.

Problem 6: Implementing streams (20 points total)

Add support for the following stream functions to your interpreter:

  1. stream-cons (10 points)
  2. empty-stream (2 points)
  3. stream-empty? (2 points)
  4. stream-first (2 points)
  5. stream-rest (4 points)

You should be able to complete this problem with either the call-by-name or call-by-need implementation of force and delay. That is, you can receive full credit for this problem even if you did not receive full credit for Problem 5.

Your streams should be strict in the head of the stream and lazy in the tail. Note that Racket streams are lazy in both the head and the tail.

Your implementation must use your force and delay from Problem 5. You may not use Racket’s force and delay or equivalent syntax macros in your solution.

Problem 7: Homework Statistics (1 point total)

How long did it take you to complete problems 2–6? Please tell us in a comment at the top of mceval.rkt. You must tell us how long each problem took you to receive the point.