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Re: Proposal for study: Add a categorical Initial type to ocaml
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Date: -- (:)
From: Jacques Garrigue <Jacques.Garrigue@i...>
Subject: Option types and O'Labl merger
From: chet@watson.ibm.com
> Of course, it is problematic that "None" and "Some None" are
> indistinguishable.  But is it a reason to not have such a facility (as
> a storage-cost-free "option" type constructor)?
> 
> I'd like to believe that the answer is "no" -- that the efficiency
> values of such a type-constructor outweigh the semantic difficulties.

In fact the semantic problem is not that bad. When I implemented
O'Labl, I had also the idea that it would be nice to have option types
without space (and dereference) cost, particularly since options are
used to implement optional arguments. I even went as far as
implementing it once, but dropped it since didn't make a big
difference in efficiency, and might become a pain to maintain.

The idea is just to represent Some^n(v) (the Some constructor applied
n times to v) as v when v is not an option, and Some^n(None) as n. To
be more precise, since you want to distinguish it from (n : int), you
represent it as a pointer in an unused memory area. Word addresses
starting at 0 would be a possible approach. Semantically this was
correct since ML polymorphism is not strong enough to allow the
dynamic creation of Some^n(None) for an arbitrary n. That is, the
maximum n is a function of the program, and not of its input.
Remark however that since you have polymorphic recursion in O'Labl
now, this is no longer true, meaning that you would need a runtime
check to make sure that you don't create wrong values.

By the way, and this also an answer to another mail I saw recently,
O'Caml and O'Labl are going to merge soon. That is, O'Labl features
will be merged into the official O'Caml release (beware though that
the syntax may be slightly changed to fit everybody's taste).
If you really think that having such costless options would be useful,
it is the time to make pressure to have them included in the merger.

Regards,

	Jacques

> >>>>> "WC" == William Chesters <williamc@dai.ed.ac.uk> writes:
> 
>     WC> The problem is that if you have a value of the type `int
>     WC> option option', you have to be able to distinguish between
>     WC> `None' and `Some None'!  If both the enumeration-indirections
>     WC> are elided, you can't; if only one of them is, you have
>     WC> inconsistency which would presumably have to be resolved using
>     WC> whole-program optimisation or something.
> 

------------------------------------------------------
Jacques Garrigue, visiting INRIA from Kyoto University
		          Jacques.Garrigue at inria.fr