Com S 342 --- Principles of Programming Languages

		   EXERCISE 02: SCHEME CODING STYLE
                 (File $Date: 2005/01/25 06:30:28 $)

The purpose of this exercise is for you to learn basic good 
programming style in Scheme.

As with all exercises, this is to be done individually.  It is due
the day this topic is planned to be discussed in class (see the
syllabus: http://www.cs.iastate.edu/~cs342/syllabus.shtml).

All of the programs in this exercise run correctly.  If not, please
contact the course staff for help.

1. [Poor Style I]

J. E. is a programmer with excellent productivity but has the belief
that short programs are always better.  She believes comments are only
for people who don't understand programming languages, and that long
variable names are likewise superfluous.  So, when asked to write a
short program to do something, she fired up Notepad and slammed out
the following code:

  ;l337 J girlz yo yo! m4d sk1llz hEr
  
  (define sq (lambda (a) (* a a))) (define sqeqsq? (lambda (a b)
  (cond
  (
  (null? (cdr a)) #f)
  ((eq? (+ (sq (car a)) (sq (car (cdr a)))) (sq b)) #t
  )
  (else (sqeqsq? (cdr a) b))
  ))
    )
  
Imagine you are a person in charge of updating J's code (or the staff
member charged with grading it) and need to figure out what J was up
to.  Answer these questions without running your Scheme interpreter
and then use the interpreter to verify your answer (if possible).
  
a. What is the type of "sqeqsq?"?  That is, of what type must a and b
   be and what type of value does "sqeqsq?" output?  Be as specific
   as you can (preferably using the type notation for the course,
   available from the course resources page or directly from
   http://www.cs.iastate.edu/~cs342/docs/typedscm_toc.html .) Hint:
   (sqeqsq? '(1 3 4 7) 5) is a good place to start figuring this out.

b. What does "sqeqsq?" do?

c. What could J. E. have done stylistically to indicate better to you
   what she was doing (assuming we could convince her to vary from
   his "short programs are always better" belief)?
    
2. [Poor Style II]
  
Jacobathan Esthequopolis II believes that you can never have too many
comments.  So, when asked to write a short program to do something, he
fired up DrScheme and crafted the following code:
  
  ;;; Function to put not between all the words in an input list.
  (define even-not-between-words-in-the-list
    (lambda (a-sentence-one-would-hope)
      ;; Call subfunction.
      (put-not-in-right-place a-sentence-one-would-hope 0)))
  
  ;;; Function to decrement index before inserting 'not
  (define put-not-in-right-place
    (lambda (a-sentence-that-is-indexed const)
      (cond
        ;; Check if input sentence is null
        ((null? a-sentence-that-is-indexed) '())
        ((odd? const) (cons (car a-sentence-that-is-indexed)
                            (cons 'not (put-not-in-right-place
                                        (cdr a-sentence-that-is-indexed)
                                        (+ 1 const)))))
        ;; Conses not between two words in the sentence
        (else (cons (car a-sentence-that-is-indexed)
                    (put-not-in-right-place
                      (cdr a-sentence-that-is-indexed)
                      (+ 1 const))))))) ; As per above but different

a. What is the type of even-not-between-words-in-the-list?  Be as
   specific as you can. Be as specific as you can (preferably using the
   type notation for the course).  Hint:
   (even-not-between-words-in-the-list '(I am silly and wordy)) is a good
   place to start.
   
b. What does even-not-between-words-in-the-list do?

c. What could Jacobathan have done stylistically to indicate better to
   you what he was doing?


WHAT TO HAND IN

You should have at the beginning of class, written answers to the
above questions (or written out questions and problems you encountered
for each part).  Make sure your name is on these. Attach the
printouts, if any, requested above.

ADDITIONAL THOUGHTS

What happens if you change the name "const" to "cons" in the
function put-not-in-right-place?  Why?