CS217 Day 17 Friday, May 5, 2000

Parameter Passing II

• Call-by-reference -- see Day 16

• Call-by-name/need -- lazy evaluation

  Expressions are passed unevaluated, together with the environment of the caller, and evaluated when used. Implemented by as "thunk-target"s.

  Here is a program that, under call-by-name, will sum the values of a an expression, passed as an argument.

  letrec sum(e, i, a, b) =
           if greater(a, b)
           then 0
           else begin
                  set i = a;
                  +(e, (sum e i add1(a) b))
  in let i = 0
     in sum(+(i, i), i, 1, 10)
  

  Under call-by-value or call-by-reference, the value is 0.
  But under call-by-name, the value is 385.

Loops -- an "extra"

• Arrays have been implemented in interp3-7clean.scm. I would be nice to have loop expressions to process arrays.

• Assignment 6 has been augmented to have you implement while-do and do-while loops, and exercise them by writing quicksort.

• How can we interpret while-do-exp expressions?

  (while-do-exp (test exp)
    (eval-while-do test exp env))
  

  where -- here is a stab at it --

  (define eval-while-do
    (lambda (test exp env)
      (if (true-value? (eval-expression test env))
  	(begin
  	  (eval-expression exp env)
  	  (eval-while-do test exp env))
  	1)))
  

• What about do-while-exp expressions?

  (do-while-exp (exp test)
    (eval-do-while exp test env))
  

  where -- here is a stab at it --

  (define eval-do-while
    (lambda (exp test env)
      (begin
        (eval-expression exp env)
        (if (true-value? (eval-expression test env))
  	  (begin
  	    (eval-expression exp env)
  	    (eval-while-do test exp env))
  	  1))))
  

• An example -- probably needs debugging, to test while-do:

  let sum = proc(data, a, b)
              let s = 0
                  i = a
              in begin
                   while less?(i, add1(b))
                   do begin
                        set s = +(s, arrayref(data, i));
                        set i = add1(i)
                      end;
                   s
                 end
  in let a = array(3)
     in begin
          arrayset(a, 0, 1);
          arrayset(a, 1, 4);
          arrayset(a, 2, 9);
          (sum a 0 2)
        end
  

We will be looking at Scheme.java, a call-by-value interpreter in Java that parses and evaluates list syntax. We'll want to be able to read and understand the code. Try compiling and running it --

javac Scheme.java
java Scheme

We are scheduled to do continuation passing interpreters next.
We don't have time to get too far with them.
What about moving on to Object-Oriented Languages?