[m-dev.] Exposing stack frames

[Fergus Henderson]

On 09-May-2001, Ralph Becket <rbeck at microsoft.com> wrote:
> It occurred to me that if we had some way of exposing stack frames
> and so forth via some (possibly unsafe) interface, we might be able
> to implement stuff like coroutining, exceptions and native GC purely
> in Mercury.
> 
> Is this a sensible thing to ask for?

Yes.  Not as a standard Mercury library, but as an implementation-specific
interface, definitely.

I've been arguing for this kind of thing (Mercury level interfaces to
the low-level RTTI-related stuff) for a while now, in particular in
connection with the implementation of deep_copy (a major part of the
native GC) and the debugger.

We've discussed this at some of the previous Mercury meetings, although
I don't know whether it made it into the minutes.  I think the concensus
was that this would be a good idea, but that it would be quite a bit of
effort to implement.

There's actually already some support for low-level RTTI in std_util.m:

 | 	% The rest of the interface is for use by
 | 	% implementors only.
 | 
 | :- type functor_tag_info
 |         --->    functor_integer(int)
 |         ;       functor_float(float)
 |         ;       functor_string(string)
 |         ;       functor_enum(int)
 |         ;       functor_local(int, int)
 |         ;       functor_remote(int, int, list(univ))
 |         ;       functor_unshared(int, list(univ))
 |         ;       functor_notag(univ)
 |         ;       functor_equiv(univ).
 | 
 |         % get_functor_info is a variant of deconstruct for use by the compiler,
 |         % specifically prog_rep.m and static_term.m. It differs from
 |         % deconstruct in two main ways. First, instead of returning the
 |         % function symbol, it returns implementation information about
 |         % its tag. Second, it succeeds for just the kinds of terms needed
 |         % to represent procedure bodies for ordinary procedures. For the time
 |         % being, these are procedures that do not involve higher order code
 |         % or tabling.
 | :- pred get_functor_info(univ::in, functor_tag_info::out) is semidet.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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