Thanks for the clarification. Again, I only have polymorphic variants,
no functions, no floats, no lazy values, etc. 

The reason why I need to go through some Obj conversion is essentially
the same reason safe marshalling needs to go through that conversion:
storing typed information on an untyped medium (or, in this case,
polymorphically typed information on a non-polymorphically typed medium)
-- along with a type certificate. It just happens that this
less-richly-typed medium is an exception.

Cheers,
 David

P.S.:
 I have tried alternative solutions but the best I can come up with
involves something like 30% slowdown on my testsuite. Which is a bit too
much for something which is supposed to be speed-critical.

On Fri, 2008-02-29 at 10:52 +0900, Jacques Garrigue wrote:
> From: David Teller <David.Teller@univ-orleans.fr>
> 
> > Interesting. Can I assume that, if my type is boxed (in this case, a
> > polymorphic variant), I can successfully convert it to Obj.t and back ?
> 
> Yes : polymorphic variants are either ints or normal blocks, so this
> should work. This is also true for normal variants. Actually I don't
> know of any non-uniform representation problem outside of floats.
> 
> Note however that this problem of non-uniform representation is no the
> only danger when using Obj.magic imprudently. I recall another problem
> with functional values. I'm afraid only Xavier could explain that one.
> 
> If you have only (polymorphic) variants, and if you keep the types
> monomorphic (i.e. always add complete type annotations to Obj.magic or
> Obj.obj), things should work properly in the current implementation.
> 
> Of course you should limit that kind of uses to things like
> persistant storage or C interfaces, where you have to go through an
> untyped world anyway, and avoid it at all costs in plain ml programs.
> 
> Jacques Garrigue
> 
-- 
David Teller
 Security of Distributed Systems
  http://www.univ-orleans.fr/lifo/Members/David.Teller
 Angry researcher: French Universities need reforms, but the LRU act brings liquidations.