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Date: Tue, 20 Aug 2013 11:16:31 +0200
From: Goswin von Brederlow <goswin-v-b@web.de>
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Subject: Re: [Caml-list] Obj.magic, Toploop, HOL Light & pretty-printing
 exceptions

On Mon, Aug 19, 2013 at 10:18:58PM -0500, Bill Richter wrote:
> Goswin, thanks for the reply!  I'm not sure if I'm using Obj.magic
> (I'll study this question), and I couldn't tell if your prohibition of
> Obj.magic extends to Toploop.getvalue, which I definitely am using:
> 
> >    let exec = ignore o Toploop.execute_phrase false Format.std_formatter
> >      o !Toploop.parse_toplevel_phrase o Lexing.from_string;;
> 
> Thanks for explaining why `eval' crashes the GC because of advanced
> pointers by 2. Perhaps I can draw you into what I think is the real
> discussion.  How does one complete the standard Scheme-type exercise
> of building an ocaml meta-interpreter inside ocaml?  You can do this
> easily in Scheme because of the quote feature.  What I and my teachers
> Freek, Mark and Vince are doing is, I think, trying to emulate the
> Scheme quote feature with Toploop & Obj.magic.  

Am I right in that the executing itself works fine. So your only remaining
problem is returning the result?

I think that just doesn't work like you want. A function can only ever
return exactly one type (or throw an exception). But the return type
of eval would have to depend on the input. So I think you can't return
the result. You can only pretty print it.

But I might be wrong. I never used Toploop. Maybe it has something to
return the result including some type encoding. Something like (but
more complex):

type result = Int of int | String of string

MfG
	Goswin