CS 257 (Spring 2007) Homework 4*

1. Write a function cond->if which takes a cond expression and transforms it into a set of nested if expressions. For example,

   > (cond->if '(cond ((> x y) (- x y)) ((< x y) (- y x)) (else 0)))
   (if (> x y) (- x y) (if (< x y) (- y x) 0))
   >
   

   You may assume that every cond expression has an else part.

2. E. Coli we hardly knew ye.

3. The definition of a function, foo, is a <tail definition> if it has the following form:

   (define foo (lambda (<symbol>*) <tail wrt foo>))

   where <symbol>* is zero or more symbols and an expression is <tail wrt foo> if it is not an application of foo:

   <contains no foo applications>

   or

   if it is an application of foo to zero or more expressions which contain no applications of foo:

   (foo <contains no foo applications>*)

   or

   if all applications of foo are contained in expressions which are (themselves) tail recursive with respect to foo and which occur in any of the following contexts:
   • (if <contains no foo applications> <tail wrt foo> <tail wrt foo>)
   • (and <contains no foo applications>* <tail wrt foo>)
   • (or <contains no foo applications>* <tail wrt foo>)

   where <contains no foo applications> is an expression which contains no applications of foo, and <contains no foo applications>* is zero or more expressions which contain no applications of foo.

   Write a function, tail-definition?, which takes an expression, expr, as an argument, and returns #t if the expression is a <tail definition> and #f otherwise. For example:

   > (tail-definition? '(define fact (lambda (x acc) (if (= x 0) acc (fact (- x 1) (* x acc))))))
   #t
   > (tail-definition? '(define fact (lambda (x) (if (= x 0) 1 (* x (fact (- x 1)))))))
   #f
   > (tail-definition? '(define member? (lambda (item ls) (and (not (null? ls)) (or (eq? item (car ls)) (member? item (cdr ls)))))))
   #t
   >
   

   Hint: Divide and conquer. Write helper-functions, tail-wrt? and no-applications-of?. Both take a function name, fname, and an expression, expr, as arguments. tail-wrt? returns #t if expr is tail with respect to fname and #f otherwise. no-applications-of? returns #t if there are no applications of fname in expr, and #f otherwise. For example,

   When You Should Hand It In

   The above should be handed in Mon. Mar 19.

   * This webpage is located at http://cs.unm.edu/~williams/cs257/homework4s07.html