On 27/06/2009, at 7:14 AM, Erick Tryzelaar wrote:
>>
>> http://okmij.org/ftp/papers/LL3-collections-enumerators.txt
>>
>> I wonder if we could draw any inspiration from it.
>
>
> I found a partial ocaml implementation of this:
Oleg partially misses the point I think. C++ style iterators
are hard to write because they have to maintain complex
position state, but the whole point is that they can then
be used by master code, as opposed to the slave (callback)
code required by a fold. In the fold, the client code has to
maintain state in an object. In master code some of the
state is maintained using the stack, and therefore is synchronised
properly with control flow.
With generators, C++ style iterators are easy to write, since they
use the stack too: a generator is master code.
The question then reduces to whether master or slave code
is better. For simple jobs, a slave is better, Oleg identifies some
of the reasons (resource leakage, encapsulation etc).
However for a complex job, masters are better because part of
the work is done by the control-flow/stack integration the
language provides.
In C++ however there is an additional argument for iterators:
client code is polyadic (via use of templates). Template clients
can be written independent of the container, since iterators
hide the container type.
In Ocaml the inability to do this is a major bug-bear. You have
to use Hashtbl.iter or List.iter or Set.iter or whatever. In C++
you just call
container.begin()
Of course this is only "textually polyadic".
--
john skaller
skal...@users.sourceforge.net
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