Re: [Caml-list] How much optimized is the 'a option type ?

[Yaron Minsky <yminsky@janestreet.com>]

I also agree with Gabriel that an option-specific optimization is not
clearly the right move.

But I wonder if a more general optimization that provided the
possibility of minting "fast-path" variants.  i.e., one could have an
annotation that marked a given branch of a variant as the
"no-indirection" one, i.e., the one that doesn't lead to the
allocation of an extra block:

type ('a,'b) result =
     | Ok of 'a [@@no_indirection]
     | Error of 'b

would lead to a type where [Ok x == x].  Some cleverness is required
then for the representation of the [Error] branch.  In particular,
you'd need some dynamic test you could run to see if you were using a
value that was not the fast-path one.

The thing that I don't know if there's a solution for is the nesting
problem.  i.e., can you effectively distinguish:

  Ok (Ok (Error x))

from

  Error x

since they would have the same physical representation.  I'm not sure
if some variant of the counting trick used for options would work here
or not.  But if you could get this, it would make it possible to avoid
a large number of dirty Obj.magic hacks that people need to do to
build efficient datastructures in practice.  The fact that the stdlib
needs to use Obj.magic to get the necessary performance is, I think, a
sign that something important is missing from the language.  I'm not
sure if this is quite it, to be clear.

y


On Fri, Jan 17, 2014 at 8:46 AM, Nicolas Braud-Santoni
<nicolas.braudsantoni@gmail.com> wrote:
> On 17/01/2014 12:23, Jonathan Kimmitt wrote:
>> In my humble opinion the main purpose of Some _ | None is to avoid the
>> requirement for a nil pointer in OCaml. If an external function wants to
>> return nil in order to indicate, for example that a certain resource is not
>> available, it can return None instead and this prevents dereferencing a nil
>> pointer in OCaml because the None cannot be dereferenced.
> Yes.
> This doesn't forbid the compiler from representing 'a option values as
> pointers.
> Indeed, the type system already enforces that the None case is handled
> and the representation of None and Some _ do not matter.
>
> That said, I agree with Gabriel Scherer : adding optimizations specific
> to 'a option might refrain people wanting to switch to more appropriate
> datatypes.
> However, would is be possible to “optimize away” all types of the form
> “type 'a t = X of 'a | A | B | ...” (with at most one non-constant
> constructor) ?
> Would it be worth doing ?
>
>
> Kind regards,
> Nicolas
>