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

[Yawar Amin <yawar.amin@gmail.com>]

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Hi Reed, this is maybe not exactly what you specified, but a `private` type
abbreviation (
https://caml.inria.fr/pub/docs/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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