[plt-scheme] tree->generator from t-y-scheme question

From: Kim Gybels (kgybels at vub.ac.be)
Date: Wed Dec 8 12:28:49 EST 2004

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> Folks,
> I'm looking at the tree traversal with continuations example in Teach
> Yourself Scheme in Fixnum Days. I think I have an almost complete grasp
> on it but I'm missing one bit of info.
> In generate-leaves:
> ...
> (else 
>     (call/cc 
>     (lambda (rest-of-tree) 
>         (set! generate-leaves 
>             (lambda () 
>                 (rest-of-tree 'resume))) 
>         (caller tree)))))) 
> Is it correct to say that rest-of-tree is passed 'resume because
> generate-leaves takes no parameters?
Yes, 'resume is just a dummy value, because a continuation always takes one
argument, but it is not needed here.

> What is the rest-of-tree continuation in this scenario, i.e. where's the
> "hole"?
The call/cc expression itself is the "hole". When the
rest-of-tree-continuation is called, this "hole" is filled with value
it takes. Nothing is done with the value of the call/cc statement,
therefor the dummy-value.

> ...
> ((generate-leaves 
>     (lambda () 
>         (let loop ((tree tree)) 
> ...
> Are we starting at the "let" here and assigning the remainder of the tree
> to tree?
Yes, that is correct.

The first time the generator is called, generate-leaves is a normal
function, which uses the caller to jump out of its loop and return a
leaf. But after this generate-leaves is now a continuation that
"continues" the traversal.

So in short: with caller you jump out of the loop, and with
generate-leave you jump back in to the loop.

I hope this makes sense :)

I think the example would be more clear if the 2 continuations where
called: return and continue. 

>     Thanks, Joel
> -- 
> OpenPoker: The Linux of poker software
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