Re: [Caml-list] Presumed bug in OCaml's garbage collector or in its Weak module...

["François Bobot" <francois.bobot@cea.fr>]

On 18/06/2016 18:54, Leo White wrote:
>> Shallow copies don't seem very safe in the presence of out-of-OCaml-heap C pointers [*].
>> Perhaps Weak.get_copy should raise if it encounters a Custom_tag?
>
> Shallow copies aren't even type-safe in the proper sense of the word,
> since copying a value of abstract type breaks abstraction. This is why
> Obj.dup is in the Obj module. Really Weak.get_copy shouldn't exist, or
> should at least be part of Obj not Weak.

That's interesting, perhaps I should not have given access to get_copy in Ephemeron.

>
> I suspect Weak.get_copy only really exists for implementing the weak
> hash tables, and even there using it means that you could give Weak.Make
> a valid argument whose equal function used `==` and it would give
> incorrect results.

The weak hashtable of ephemerons doesn't use get_copy, specifically because of that. The weak set of 
Weak are mostly used for hashconsing where the equal function given to Make must not be (==). But 
for the other use-case, it could be indeed a problem.


However it would be interesting to keep the nice property that `get_copy` give us for the 
implementation of weak structure, without using unsafe features. The nice property is that a value 
is not kept alive just because it collides with a value that is often accessed. Perhaps a kind of 
transient roots, which are considered as root only at the end of the each part of mark phase, would 
allow to mark the value as few time as possible. But the API would not be pretty...

-- 
François