[gambit-list] Continuations bug in v4.6.2?
Patrick Li
patrickli.2001 at gmail.com
Mon Dec 9 23:59:27 EST 2013
Ah ha! Thank you Marc for that explanation. It was something so simple all
along! I spent a considerable number of hours staring at these few lines
today. Sorry for leaping to conclusions regarding the integrity of your
code!
-Patrick
On Mon, Dec 9, 2013 at 8:29 PM, Marc Feeley <feeley at iro.umontreal.ca> wrote:
>
> On Dec 9, 2013, at 11:01 PM, Patrick Li <patrickli.2001 at gmail.com> wrote:
>
> > However, this leaves me feeling deeply disturbed. I feel that
> >
> > (f (g x))
> >
> > should *always* be absolutely equivalent to:
> >
> > (let [(temp (g x))]
> > (f temp))
> >
> > My example indicates that this is not necessarily the case. Are there
> other situations where this is not true? I suspect that I don't quite
> understand the interaction between tail-calls and continuations properly.
>
> When evaluation is left-to-right (which is not required by the standard,
> but is common) the expression (f (g x)) is evaluated as though it was
>
> (let ((t1 f)) ;; get value of f
> (let ((t2 g)) ;; get value of g
> (let ((t3 x)) ;; get value of x
> (let ((t4 (t2 t3))) ;; call (g x)
> (t1 t4))))) ;; call (f (g x))
>
> The expression (let ((temp (g x))) (f temp)) forces a different order,
> namely
>
> (let ((t1 g)) ;; get value of g
> (let ((t2 x)) ;; get value of x
> (let ((t3 (t1 t2))) ;; call (g x)
> (let ((t4 f)) ;; get value of f
> (t4 t3))))) ;; call (f temp)
>
> In your code, this makes a difference because f (which is "return-point"
> in your code) is being mutated during the evaluation of the call to g.
>
> In your "version 1", the function originally bound to return-point will be
> called (i.e. the continuation of the function make-jp). In your "version
> 2", the function bound to return-point by the call to (block jp) will be
> called (i.e. the continuation of the call (y 0)).
>
> Marc
>
>
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