Re: [Ocaml-lib-devel] ExtList: using _setcdr instead of Obj.magic

[Nicolas C. <war...@fr...>]

> > It is called , it should actually be called when call - complexity is
lower
> > than O(n) since the overhead is not big.
>
> Which is never in list functions, that I know about.

I was talking about complexity in term of numbers of calls to _setcdr.
BTW, you're right, I'm not sure there is functions with only one call to
_setcdr

> I've yet to hear this in a case where it turned out to be so.

This will need a very major changes in the entire Ocaml sources since it
will involve modifying the current memory representation, and I'm pretty
sure it won't happen before few years.

> And I
> notice the changes are sneaking out of ExtList- I note that Enum has an
> Obj.magic call which is effectively _setcdr.  On my to-do list was to take
> a long hard look at Enum.force to see if I could move that _setcdr back
> into ExtList where it belongs.

From my point of view, the setcdr is a trick which should of course not be
put in the interface extList.mli ( we agreed on that before ). And so if you
want to use it in Enum, you have to rewrite it. This is a performance trick,
since we could always build the list in the other way and then rev it, and a
safe trick since we could instead use a non tail-rec call.
As a performance trick , it should be inlined.
As a safe trick, it shouldn't be shown to the user, and a _setcdr function
shouldn't even exists :)

Nicolas Cannasse