Re: [Caml-list] Arrays and private types

[Gabriel Scherer <gabriel.scherer@gmail.com>]

> Thanks Gabriel, very nice solution. If I go this way, I guess there is
> no way to access array elements using the usual a.(i) syntax (where i
> = M.key i)... [...]
> Is this a problem I can solve using a camlp4 decorator ?

I don't think you need -- nor want to use -- a camlp4 extension. a.(i)
is desugared into (Array.get a i) at a purely syntactical level in
OCaml, so you could overload its behavior by changing the Array module
in the typing environment.

With my example you could write, for example:
module A1 = ArrayMake(struct end)
let () =
  let module Array = A1 in
  let k = A1.key in
  assert (A1.make 3 true).(k 2);;

You could even define the ArrayMake functor so that it returns a
structure with an Array submodule. You would then write, using 3.12
"local open" syntax:

module A1 = ArrayMake(struct end)
let () =
  let open A1 in
  assert (Array.make 3 true).(k 2)

That said, I don't think that the slight readability benefit of
writing a.(i) instead of (get a i) will outweigh the confusion among
your readers that don't understand what you're doing with this weird
Array stuff.

On Thu, Mar 15, 2012 at 12:04 PM, Pietro Abate
<Pietro.Abate@pps.jussieu.fr> wrote:
> On 15/03/12 00:00, Gabriel Scherer wrote:
>> Here is a proposal:
>>   https://gitorious.org/gasche-snippets/private-array-keys-type/blobs/master/private_array_key_types.ml
>>
>> It works by using a functor to generate "fresh" private types for
>> keys. Note that the arrays themselves are still polymorphic (no
>> IntArray FloatArray etc.). The user still has to use the discipline to
>> produce a new application of ArrayMake each time she wants to use a
>> different kind of array: if she only does `module A = ArrayMake(struct
>> end)` and then use `A` for everything, there will be no additional
>> safety guarantee.
>
> Thanks Gabriel, very nice solution. If I go this way, I guess there is
> no way to access array elements using the usual a.(i) syntax (where i
> = M.key i)... (I've noticed your cleaver use of private on the array
> type to avoid using the normal array syntax on your private arrays).
>
> Is this a problem I can solve using a camlp4 decorator ?
>
> This seems a bit complicated as the a.(i) syntax will be context
> dependent, that is, the same syntax used with two different array types
> should be translated to different get/set calls... and at the camlp4
> level I don't have access to type information... Ideally, instead of
> changing all my array access calls, I'd like just to change the type
> of my indexes such that all my generic ints will be replaced by
> M.key of the appropriate type...
>
> p
>
> ps: please do not Cc me. I'm subscribed to the list.
>
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