Re: [Caml-list] Subtyping (or something like it)

[Reed Wilson <cedilla@gmail.com>]

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That looks pretty good! One of the things I tried was to use the private
type declaration. However, I failed to take the coercion into account, so I
thought they couldn't actually be used as bigarrays.
I'd still like to avoid casting to bigarrays, but unless a miracle comes
along, I think this is how I'll do it.
Thanks!

On Thu, Nov 16, 2017 at 4:49 PM, Yawar Amin <yawar.amin@gmail.com> wrote:

> Hi Reed, this is maybe not exactly what you specified, but a `private`
> type abbreviation ( https://caml.inria.fr/pub/do
> cs/manual-ocaml/extn.html#s-private-types-abbrev ) should work. You'll
> have to cast your safe array type to a normal bigarray to get the indexing
> and other operations, e.g.:
>
> (my_array : ro t :> (int, int8_unsigned_elt, c_layout) Array1.t).{idx}
>
> Also, see http://camltastic.blogspot.ca/2008/05/phantom-types.html for a
> really cool way to mix phantom types and subtyping.
>
> Cheers,
>
> Yawar
>
> On Thu, Nov 16, 2017 at 4:46 PM, Reed Wilson <cedilla@gmail.com> wrote:
>
>> A while ago, I made a module which basically extended bigarrays with a
>> few more functions. However, I wanted to have a distinction between
>> read-only and read-write values. The equivalence with bigarrays was
>> irrelevant, so my interface just looked like this:
>>
>> type ro
>> type rw
>> type 'a t
>> val read_only : 'a t -> ro t
>> val get : 'a t -> int -> int
>> val put : rw t -> int -> int -> unit
>> ...
>>
>> Basically, any function that wrote to the type would have to take a "rw
>> t", but anything else would take " 'a t". From what I know about C, "ro"
>> behaves like the "const" qualifier - not water-proof but it catches some of
>> my common mistakes.
>>
>> Now, however, I want to re-export the equivalence between 'a t and
>> bigarrays (mostly for the .{} operator), but also have the
>> read-only/read-write distinction. The problem is that if I write:
>> type 'a t = (int, int8_unsigned_elt, c_layout) Array1.t
>>
>> then OCaml will see that "ro t" and "rw t" are the same and freely let me
>> use "ro t" values in, for example, the "put" function above.
>>
>> To summarize, is there a way to make two types internally represented by
>> bigarrays, the first of which:
>> * can use the bigarray functions (specifically .{} )
>> * can use all of my new functions
>> and the second type
>> * can't use bigarray functions
>> * can only use a subset of my new functions
>>
>> I don't want to have two different versions of each function that I have
>> to keep track of.
>>
>> Thanks,
>> Reed Wilson
>>
>> --
>> ç
>>
>
>


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ç

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