CS 451 --- Fall 2002 --- Assignment Two


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	If our behavior is strict, we do not need fun!
				  -- Zippy the Pinhead
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		       LAZY SCHEME INTERPRETER


Starting with the scheme-in-scheme code in the class notes for
9-Sep-2002, change MY-EVAL to LAZY-EVAL, which is for a lazy dialect
of Scheme otherwise identical to the previously interpreted language.
Also make any other necessary changes to the interpreter, eg in the
apply function and anywhere else you see fit.

You'll also want to define FORCE-TO-DEPTH which takes a "lazy" value
of the sort output by LAZY-EVAL, along with a number, and converts the
given value to a "regular" value down to some depth.  Eg

   (force-to-depth (lazy-eval '(letrec ((a (cons a a))) a) '())
		   3)

   ;; would return this:
   ;;  (((x . x) x . x) (x . x) x . x)
   ;; where the "x" represents whatever you have for that particular
   ;; unforced (i.e. as yet uncalculated) value.

(See lazy.scm for more examples, especially the last one.  Note that
force-to-depth given a depth of zero would just be the identity
function, ie would just return its first argument.  The "depth" refers
to the maximum number of CAR's and CDR's necessary to fetch something
out.)

You will almost certainly want to use DELAY, FORCE and PROMISE? in
your lazy interpreter.

The code in lazy.scm (in the sep-9 notes) should work with your
modified interpreter.

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Due noon Thurs Sep 19, 2002.

Please comment reasonably.  Points for readability and conciseness.

Hand your code in by running the script
 ~bap/bin/hw-handin your-cs451-f2002-hw2.scm
on a CS machine.  It will generate a time stamp which you should save.