Notes on call/cc.

A break debugging facility is the simplest use of first-class continuations.

> (define top-k 'ignored)
> (call/cc
    (lambda (k)
      (set! top-k k)))
> (define continue
    (lambda ()
      "no break to continue"))
> (define break
    (lambda (v)
      (call/cc
	(lambda (k)
	  (set! continue k)
	  (top-k v)))))
> (+ (break 'break-one) (break 'break-two))
break-two
> (continue 3)
break-one
> (continue 2)
5

Other uses of first-class continuations:

- breadth-first search

- coroutines (EOPL section 9.4, not responsible for)

- multi-tasking


EXCEPTIONS

Java exception syntax:

throw new-exception

try {
  /* try block */
} catch (ExceptionType e1) {
  /* catch statement 1 */
} catch (ExceptionType e2) {
  /* cath statement 2 */
...
} finally {
  /* finalization code */
}


To see how this may be implemented in Scheme, we first need DYNAMIC-WIND to
implement FLUID-LET properly.


(DYNAMIC-WIND prelude-thunk body-thunk postlude-thunk)

usage examples: robots, files

implementation in section 9.5: not responsible for, can be simpler


(fluid-let ((var exp)) body) ==>

  (let ((var^ exp))
    (let ((swap (lambda ()
		  (let ((temp var))
		    (set! var var^)
		    (set! var^ temp)))))
      (dynamic-wind swap (lambda () body) swap)))

Usage examples: binding I/O parameters (actually, Chez Scheme uses
parameters instead), Java-style FINALLY clause.


(define make-parameter ; doesn't have Chez Scheme's guards
  (lambda (v)
    (lambda args
      (if (null? args)
	v
	(set! v (car args))))))

Exercise: define (PARAMIZE param value thunk)


(define-record bad-input-exn (input))

(try
  (lambda () (raise (make-bad-input-exn "junk")))
  bad-input-exn?
  (lambda (input)
    (printf "Bad input: ~s~n" input)))

(define raise
  (lambda (v)
    (error "Unhandled exception:" v)))

(define try
  (lambda (body-thunk guard-proc handler-proc)
    (call/cc
      (lambda (k)
	(fluid-let ((raise (let ((outer-raise raise))
			     (lambda (v)
			       (if (guard-proc v)
				 (k (handler-proc v))
				 (outer-raise v))))))
	  (body-thunk))))))