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[Caml-list] Python's yield, Lisp's call-cc or C's setjmp/longjmp in OCaml
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Date: 2003-12-18 (00:15)
From: Nuutti Kotivuori <naked+caml@n...>
Subject: Re: [Caml-list] Python's yield, Lisp's call-cc or C's setjmp/longjmp in OCaml
I've lumped together a bunch of answers to not spam the list too much.

Oleg Trott wrote:
> call/cc is Scheme, Common Lisp has "throw" instead, and ML has
> "raise".

Thanks for the reply, but I was aware that OCaml has exceptions.

Seth J. Fogarty wrote:
> Um.... does lisp have call/cc, I thought that was only scheme? But
> in any case, look at exceptions. They provide a very similar control
> flow arrangement to continuations (although they are weaker).


Ville-Pertti Keinonen wrote:
> There are many different things you could be referring to, some of
> which OCaml does have (exceptions), some of which it doesn't
> (coroutines, first-class continuations, generators etc.).

Well, indeed it was the three latter examples you gave that I was
looking for.

> First-class, capturable continuations are one of the things I often
> wish OCaml had, but implementing them efficiently would require
> significant changes to the execution model.
> SML/NJ has efficient first-class continuations, so it's clearly
> possible, even in the presence of native compilation and exceptions.

That's nice to know! Yes, I'd wish for capturable continuations as
well - but living without them is certainly possible.

Brian Hurt wrote:
> By my measurements, Ocaml's exceptions are faster than C's
> setjmp/longjmp.  Ocaml doesn't provide first-class continuations,
> but most of the things people actually do with call-cc can be done
> in other ways in Ocaml.  I don't know what Python's yield
> instruction does.

Ah yes, I should have been more specific. I was looking for things
that are possible with first-class continuations - but also something
weaker, as Python's yield.

> What do you want to do?

Well, what I was looking for is a way to suspend the execution of
something, and come back to it later. Exceptions provide me a way from
getting out of odd places, but not a way back in those odd places.

Python's yield is just a limited form of suspending executing
something, and continuing it later.

-- Naked

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