Programming Language Pragmatics, Fourth Edition
Programming Language Pragmatics, Fourth Edition
4th Edition
ISBN: 9780124104099
Author: Michael L. Scott
Publisher: Elsevier Science
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Chapter 6, Problem 35E

Explanation of Solution

(a). Multilevel returns using common Lisp:

  • The multilevel returns uses catch and throw statement to implement the problem.
  • The following shows the program instruction to use of multilevel returns.
  • The program incorporate different catch rules based on both syntax rule and dynamic wind.
  • While defining the catches () the stack initially leads to empty.
  • Then the syntax rule function is defined and it checks catch from wrapper class, the output become the old-catches.
  • The dynamic wind uses call-with-current-continuation wrapper.
  • Then throw is used to the print the errors.

(define catches '()) ; catch stack initially empty (define-syntax catch (syntax-rules () ((catch tag form ...)

(let* ((old-catches catches) (wrapper (lambda (c) (set! catches (cons (cons tag c) old-catches)) form ...))) ; real work is done here

(dynamic-wind (lambda ()'()) (lambda () (call-with-current-continuation wrapper)) (lambda () (set! catches old-catches)))))))

(define throw (lambda (tag v) (let ((pop-catches (assq tag catches))) (if pop-catches ((cdr pop-catches) v)

(begin (display "no matching catch!") (newline) ('())))))) ; fatal error

  • The above code can be used in the following manner.

    (define index (lambda (k l) (letrec ((helper (lambda (l i) (cond

     ((null? l) '()) ((equal? (car l) k) (throw 'found i)) (#t (helper (cdr l) (+ 1 i))))))) (helper l 0))))

(catch 'found (index "blind" '("three" "blind" "mice"))) =) 1

Explanation of Solution

(b). Implementing the uptoby function with macro iterator and yield function:

  • The Exercise 6.35 uses lazy evaluation to implement the iterator objects.
  • Implement this to with macro iterator and yield function based on tail recursive leads to improve the performance of program.
  • Tail recursion is a special kind of recursion function, in which the last operation of a function is a recursive call.
  • This recursion may optimized away by executing the call in current stack space and it will return the result, rather than storing it in a new stack frame.
  • The following program explains the lazy evaluation with yield() and macro function with tail recursive manner.

    (define yield '()) ; initial implementation is empty(define-syntax iterator (syntax-rules ()((iterator args stmt ...)(lambda args

    (lambda (cont)(let ((old_yield yield))(dynamic-wind(lambda ()

    (set! yield ; redefine yield for(lambda (v) ; dynamically nested code(let ((return-thunk(lambda (p) (cont (cons v p)))))

    (set! cont (call-with-current-continuationreturn-thunk))))))(lambda () stmt ...)(lambda () (set! yield old_yield)))))))))

    (define for-iter (lambda (f iter) (let* ((p (call-with-current-continuation iter)) (more (pair? p))) (if more

    (let ((v (car p)) (c (cdr p))) (f v) (for-iter f c))'()))))

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