(Difficult) Use call-with-current-continuation (call/cc) to implement the following structured nonlocal control transfers in Scheme. (This requires knowledge of material in Chapter 11.) You will probably want to consult a Scheme manual for documentation not only on call/cc, but on define-syntax and dynamic-wind as well.

(a) Multilevel returns. Model your syntax after the catch and throw of Common Lisp.

(b) True iterators. In a style reminiscent of Exercise 6.34, let an iterator be a function which when call/cc-ed will return either a null list or a pair consisting of an element and an iterator. As in that previous exercise, your implementation should support expressions like (for-iter (lambda (e) (display e) (newline)) (uptoby 10 50 3))

Where the implementation of uptoby in Exercise 6.34 required the use of delay and force, however, you should provide an iterator macro (a Scheme special form) and a yield function that allows uptoby to look like an ordinary tail-recursive function with an embedded yield:

Data From Exercsie 6.34:

Use lazy evaluation (delay and force) to implement iterator objects in Scheme. More specifically, let an iterator be either the null list or a pair consisting of an element and a promise which when forced will return an iterator. Give code for an uptoby function that returns an iterator, and a for-iter function that accepts as arguments a one-argument function and an iterator. These should allow you to evaluate such expressions as (for-iter (lambda (e) (display e) (newline)) (uptoby 10 50 3))

Note that unlike the standard Scheme for-each, for-iter should not require the existence of a list containing the elements over which to iterate; the intrinsic space required for (for-iter f (uptoby 1 n 1)) should be only O(1), rather than O(n).

Transcribed Image Text:

(define uptoby (iterator (low high step) (letrec ((helper (lambda (next) (if (> next high) '() (begin (yield next) (helper (+ next step))))))) ; else clause (helper low))))