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From: Lukasz Stafiniak <lukstafi@gmail.com>
Date: Fri, 1 Feb 2013 21:39:32 +0100
Message-ID: <CAJMfKEWYhH8LRC_RFvai1qqA2k-=mx4w+jAohXovtvzi66gN0Q@mail.gmail.com>
To: Ashish Agarwal <agarwal1975@gmail.com>
Cc: Caml List <caml-list@inria.fr>
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Subject: Re: [Caml-list] deep coercion does not work for some stdlib types

On Fri, Feb 1, 2013 at 9:27 PM, Ashish Agarwal <agarwal1975@gmail.com> wrote:
>
> But for arrays it doesn't work:
>
> # let a = Array.of_list l;;
> val a : N.t array = [|1; 2; 3|]
>
> # (a :> int array);;
> Error: Type N.t array is not a subtype of int array
>
> Is this because the array type does not have a variance annotation? If so,
> why doesn't it?

You must have heard about the "mistake" of Go having covariant arrays.
In OCaml, arrays are invariant because they are mutable. After you
have coerced "let b = (a :> X.t array)", you can modify "a.(0) <- y"
with a value "y" that is not "X.t".

It would be great to have a separate type "tuple", supported by
Pervasives, that differs from "array" only in that it is immutable and
covariant.