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Date: Wed, 09 Dec 2009 04:22:43 +0100
From: Guillaume Yziquel <guillaume.yziquel@citycable.ch>
Reply-To: guillaume.yziquel@citycable.ch
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To: OCaml List <caml-list@inria.fr>,
	=?UTF-8?B?RGFuaWVsIELDvG56bGk=?= <daniel.buenzli@erratique.ch>
Subject: Re: Recursion on React.events.
References: <4B1F0E3A.3040907@citycable.ch>
In-Reply-To: <4B1F0E3A.3040907@citycable.ch>
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X-Spam: no; 0.00; guillaume:01 guillaume:01 recursion:01 recursive:01 val:01 recursive:01 endline:01 stdin:01 recursion:01 rec:01 rec:01 tail:01 tail:01 behaves:02 breaks:02 

Guillaume Yziquel a =C3=A9crit :
> Hello.
>=20
> I've been dabbling with recursive definition of React events.
>=20
> Suppose I have a function called schedule of type
>=20
>     val schedule : Calendar.t -> unit React.event
>=20
> which throws out an event at the specified time or date.
>=20
> I've been trying to create a regular_schedule function that throws=20
> events at a regular pace. Daniel B=C3=BCnzli's module is great, but som=
etimes=20
> a bit rough to get by, specifically on examples such as this one.
>=20
> So I came up with a recursive definition of such a React.event:
>=20
>> let rec regular_schedule start_time period =3D
>>   React.E.switch React.E.never begin React.E.map
>>     begin fun () -> regular_schedule (Calendar.add (Calendar.now ())=20
>> period) period end
>>     begin schedule start_time end
>>   end
>=20
> First question:
>=20
> It almost works, in the sense that if you insert a print_endline after=20
> the fun (), well, it indeeds prints stuff on stdin at the specified=20
> pace. However, somehow, the event as a whole behaves as React.E.never.=20
> So it doesn't work. I guess it has to do with the way React.switch work=
s=20
> synchronously, but I really do not get it.

OK. The following piece of code works out, but it seems to me that using=20
a React.E.select in this position is a rather ugly hack.

let rec regular_schedule start period =3D
   let waiting_for =3D schedule start in
   React.E.switch React.E.never begin React.E.map
     begin fun () -> React.E.select [waiting_for; regular_schedule
       (Calendar.add (Calendar.now ()) period) period]
     end waiting_for
   end


> Second question:
>=20
> Is there somehow a notion of 'tail recursion' for such constructs?

I gave some thought to it, and it seems quite plausible that what is=20
done in the first place is analoguous to tail recursion. And that using=20
React.E.select breaks the tail recursion. Am I offtrack?

> All the best,

--=20
      Guillaume Yziquel
http://yziquel.homelinux.org/