Re: [Rd] PROTECT and OCaml GC.

From: Guillaume Yziquel <guillaume.yziquel_at_citycable.ch>
Date: Sat, 09 Jan 2010 01:56:25 +0100

Simon Urbanek a écrit :
>
> If you have suggestions for extending the API, feel free to post them
> with exact explanations how in general that extensions could be useful
> (general is the key word here - I think so far it was rather to hack
> around your way of implementing it). [And FWIW tryEval *is* part of the
> API].
>
> Cheers,
> Simon

Hi.

Concerning tryEval, I had a look at context.c, and it says:

> /*
> This is a simple interface for evaluating R expressions
> from C with a guarantee that one will return to the
> point in the code from which the call was made (if it does
> return at all).
> This uses R_TopleveExec to do this. It is important
> in applications that embed R or wish to make general
> callbacks to R with error handling.
>
> It is currently hidden with a data structure definition
> and C routine visible only here. The R_tryEval() is the
> only visible aspect. This can be lifted into the header
> files if necessary. (DTL)
>
> R_tryEval is in Rinternals.h (so public), but not in the API.
> */

Now, here's a feature I'd like to implement with the API:

In OCaml, there's a library called Lwt. It's a library implementing lightweight, or green, threads. The idea is the following:

Suppose I have to evaluate f (). I do:

let lwt_f = Lwt.return (f ()).

Now, I want to evaluate g (f ()). I write:

let lwt_g = lwt_f >>= (function x -> Lwt.return (g x)).

This might seem overly complicated, and it somehow is. The interesting point is this ">>=" operator. Because it allows to compose two (or more) computations, and it does context commutations.

So this implements threads in a cooperative way, all running in one single real thread.

What I want to do is to be able to launch some R code, and have it multithread with other OCaml code, within one single real thread. Therefore I have to implement the commutation context somewhere in the evaluation mechanism of R itself.

The API doesn't support that, I guess.

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

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