[Rd] PROTECT and OCaml GC.

Guillaume Yziquel guillaume.yziquel at citycable.ch
Sat Jan 9 01:56:25 CET 2010

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


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

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