Interpreter Project

Part 4: Special Forms

begin, if, cond

Part 4 was due at 1:00 pm Monday, April 29th
All parts due: NOON on Sunday, May 12th

Quick link: Summary page

Wrapping up Part 3: Test and handin37 and update37

If you are still working on Part 3, please let me know if you need help. It is important that you don't fall behind.

To get the code for Part 4 and some wrap-up tests from Part 3, you will first need to run the command update37. Once you have done so, you will have created a new directory (cs37/labs/i-4) which has the new i-tests and interpreter pieces.

To join your old code and tests (from cs37/labs/i-3) with the new code and tests in cs37/labs/i-4, follow these steps:

  1. Change to the i-4 directory: cd ~/cs37/labs/i-4
  2. Run the merge script to join your old files and the new files together ./merge
Assuming you successfully merged your old and new files together, you should have a a new interpreter.rkt and i-tests.rkt file in your cs37/labs/i-4 directory.

The wrap-up tests for Part 3 that are provided for you simulate the following interaction with the interpreter. Your interpreter should successfully run the tests in i-tests.rkt, but it is strongly recommended that you type some or all of the code in by hand to verify that the interactions are working properly. In particular, i-tests.rkt can not test exit and exit! properly, so you must do this by hand.

> (repl)
INTERPRETER> (define x (cons 1 (cons 2 null)))
x
INTERPRETER> x
(1 2)
INTERPRETER> (null? x)
#f
INTERPRETER> (null? (cdr (cdr x)))
#t
INTERPRETER> (= (car x) (- (car (cdr x)) 1))
#t
INTERPRETER> (define y (cons (car x) x))
y
INTERPRETER> y
(1 1 2)
INTERPRETER> (set! x 5)
x
INTERPRETER> (set! y (+ x 5))
y
INTERPRETER> y
10
INTERPRETER> (* x y)
50
INTERPRETER> exit
INTERPRETER done.
> (repl)
INTERPRETER> (set! x 10)
x
INTERPRETER> x
10
INTERPRETER> exit!
INTERPRETER done.
> (repl)
INTERPRETER> (set! x 10)
set! cannot set undefined identifier:  x

If your code passes all the tests, you should turn in your completed version of Part 3 using handin37.

Introduction to Part 4: Special Forms

Recall that Racket's rule of evaluation states that all the arguments of a procedure must be evaluated before procedure application. We have seen in class that this rule cannot work for a set of "special forms" that includes define and set!, which you have already implemented in your interpreter.

Special forms are "special" because your interpreter must handle each of these forms differently. For example, in a define expression, such as (define x 5), the first argument, x, is never evaluated. An if expression, such as (if (< x y) x y), always evaluates its first argument, but will only evaluate one of its remaining two. Furthermore, since define, if, cond, etc. are not primitives, they have no binding in the environment. Therefore, with special forms, the operator is never evaluated.

On the other hand, all the primitive procedure applications (and, as we will see in Part 5, also lambda-procedure applications) are handled the same way by the your interpreter: The operator and all of the operands are evaluated, then the evaluated operator is applied to the evaluated operands. For example, in the expression (= x y), the operator =, and the operands x and y are all evaluated.

Special Form: begin

Extend your interpreter so that it can handle the special form begin. Though we have not used the begin special form, it is a relatively straightforward one to understand. The syntax of a begin expression is as follows:

(begin exp1 exp2 ... expn)

The begin expression evaluates each of the expressions (exp1 through expn) in order and returns the value of the last expression. (See the Racket documentation for more information on begin if you do not understand how it works.) If there are no expressions except the keyword begin, you should return void. (For now, your interpreter will print out "#<void>"; later, you will modify your interpreter so that it prints out nothing if the result is void.) For example:

INTERPRETER> (begin (define x 5) (+ x 3))
8
INTERPRETER> (begin (display "x + 3 = ") 
                    (display (+ x 3))
                    (newline)
                    'done) 
x + 3 = 8
done
INTERPRETER> (begin) ;this returns void
#<void>
INTERPRETER> (begin (set! x (* x 2)) x)
10
INTERPRETER> x
10
To get the interpreter to return void, use the Racket primitive function void which you call by simply writing (void). You may also wish to add void to the list of primitive functions your interpreter supports.

To implement begin in your interpreter, you should continue using the same style of abstraction we have been using. Please reference the summary sheet so that the functions you write have the same names as those described.

  1. define a tester procedure, in this case, begin?,
  2. write any necessary selectors, in this case, begin-expressions
  3. and write a controller, in this case, eval-begin

Special Form: if

Next, we will allow our interpreter to handle the special form if. The syntax of an if expression is:

(if test-expression then-expression else-expression)
To evaluate an if expression, first the test-expression is evaluated. If the result is true, the then-expression is evaluated and returned as the result of the if expression. Otherwise, the else-expression is evaluated and returned. Important note: In Racket, anything other than #f is considered true. For example:

INTERPRETER> (if (cons 1 2) 'true 'false)
true
INTERPRETER> (if 'not-false #t #f)
#t
INTERPRETER> (if (< 5 4) 'less 'not-less)
not-less
Implement all of the necessary procedures to handle if in your interpreter.

Your interpreter should now pass the following tests:

INTERPRETER> (define lst (cons 1 (cons 2 null)))
lst
INTERPRETER> (if (< (car lst) (car (cdr lst))) (car lst) (car (cdr lst)))
1
INTERPRETER> (* 5 (if (= (+ 3 3) (- 8 2)) 10 20))
50
In Racket, it is not possible to write an if expression that has a then-expression but has no else-expression. However, in our version of Racket, we will allow this to happen. Modify your implementation so that it can handle if expressions of this form. You should return #<void> (the result of calling (void)) if the test-expression evaluates as #f and there is no else-expression. For example:
INTERPRETER> (define x 5)
x
INTERPRETER> (if (= x 6) (+ x 1))  ;this returns void
#<void>
INTERPRETER> (if (= x 5) (+ x 1))
6
Important note: Of course you are allowed to use Racket's if to help define if in your interpreter! (The same is true for cond below.)

Reference the summary sheet for the names of the functions you should implement.

Special Form: cond

For the last portion of Part 4, we will add cond to our interpreter. Recall that the syntax of a cond is:

(cond 
   (test1 exp1 exp2 ... expn) 
   (test2 exp1 exp2 ... expn) 
   ...
   (testn exp1 exp2 ... expn))
To evaluate a cond expression, your interpreter should work as follows:
  1. Evaluate the expression test1.
  2. If test1 evaluates to be anything that is not #f, then any remaining expressions exp1 .. expn are evaluated from left to right, and the value of the last expression is returned as the result of the entire cond.
    1. Notice that cond is a lot like begin. In fact, you may want to reuse your eval-begin procedure to help you when writing your solution to eval-cond... just be careful you pass to eval-begin an expression that matches the format expected by eval-begin
    2. Also, note that if a test condition evaluates to be true but there are no other expressions following the test expression, the return value is the evaluated test expression (which may not be #t).
  3. If test1 evaluates to be #f, then the remaining test expressions are evaluated in order until one is found to be true, or there are no more test expressions.
  4. If no true expression is found, then the result is void.
  5. The symbol else appearing in place of a test expression should be considered to be #t. If an else appears in a clause which is not the last clause, produce an appropriate error message. (Try out cond in DrRacket to see what error message it provides and mimic its behavior.)

Here are some examples:

INTERPRETER> (cond 
              ((= 3 5) "this is false" "so we skip this one")
              ((= 3 3) "this is true" "so we return this message")
              ((= 5 5) "this is also true" 
                       "but since an earlier test was true" 
                       "this never gets returned"))
"so we return this message"
INTERPRETER> (cond 
              (1  "since 1 is not #f, this evaluates to be true")
              (else "and once again, we never get here"))
"since 1 is not #f, this evaluates to be true"
INTERPRETER> (cond
              ((< (+ 1 1) 0) 'nope)
              ((< 1 0) 'still-no)
              ((= 4 5) 'no-matches))  ;this returns void
#<void>
INTERPRETER> (cond 
              ((< (+ 1 1) 0) 'nope)
              ((< 1 0) 'still-no)
              (else 'matched-else))
matched-else
INTERPRETER> (define x 5)
x
INTERPRETER> (cond
              ((set! x (+ x 1)) x)
              (else "Note: be sure x is 6, not 7"))
6
INTERPRETER> x
6
INTERPRETER> (cond
              ((+ 1 1)))
2
INTERPRETER> (cond)  ; returns void
#<void>
Implement all of the procedures necessary to handle cond in your interpreter. (See the summary sheet.) Be careful with first-cond-exp and rest-of-cond-exps and when using eval-begin in your implementation of eval-cond: the keywords "cond" and "begin" at the start of these expressions can cause you some difficulty, so be sure you check those functions if you seem to be having unexplained difficulties!


You should have completed this by 1:00 pm Monday, April 29th.
Continue to part 5.