[Bese-devel] Bug in UCW Examples: Add some numbers?

[Marco Baringer]

"Marco Baringer" <mb at bese.it> writes:

> the only way to fix is to use recursion (which gets 'undone' when
> stepping back to a previous continuation):
>
> (defaction start ((s sum))
>   (let ((how-many (call 'read-a-number
>                         :label "How many numbers should we read?")))
>     (labels ((sum-numbers (how-many sum)
>                 (if (zerop how-many)
>                     ;; all done
>                     (call 'info-message
>                           :message (format nil "The sum is: ~D." sum))
>                     ;; more to go
>                     (sum-numbers (1- how-many)
>                                  (+ sum (call 'read-a-number))))))
>       (sum-numbers how-many 0))))

this is another way to fix the problem. we're using ucw's backtracking
mechanism to work around the fact that continuations don't undo side
effects.

(defaction start ((s sum))
  (loop
     for how-many = (call 'read-a-number :label "How many numbers should we read?")
     initially (backtrack (ucw::context.current-frame *context*) (make-place how-many))
     do (loop
           for count below how-many
           initially (backtrack (ucw::context.current-frame *context*)
                                (make-place count))
           sum (call 'read-a-number) into total
           finally (call 'info-message
                         :message (format nil "The sum is: ~D." total)))))

now that i think about it may be worth the effort, since we've already
got the neccessary infrastructure, to have an action automatically
backtrack all local variables. though this will help in a lot of casse
it will still leave weird edge cases sucha s this one:

(let ((a (cons 1 2)))
  (setf (car a) 'foo)
  ;; the modification to the car of a is not backtracked
  )

the only way to fix this would be to use deep copying of all
backtracked places, this sounds like a _horrible_ idea.

-- 
-Marco
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