Re: [Caml-list] Question on Variant Types

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 6/29/06, Seth J. Fogarty <sfogarty@gmail.com> wrote:
> If you have time, I have one more question I can't seem to solve. Which
> quite possibly has as simple an answer as the other two. You've been very
> helpful so far, and I don't want to impose, so feel free to let me know if
> you've not the time.
>
> type foo = [`A of int | `B | 'D of foo]
> type bar = [`A of int | `C of foo * bar | 'D of bar]
>
> let rec occurs i x =
>    match x with
>    |`A j -> i = j
>    |`C (foo, bar) -> (occurs i foo) or (occurs i bar)
>    |_ -> false
>
> I would like occurs to work on bars and foos. But as it is, occurs won't
> work on either, because it assigns the `C variant the type "`C of 'b * 'b".
> Even if I spell out `D and `B, I cannot get it to accept.

As long as foo and bar are two subtypes of a common type, you can
still solve the problem by defining that type and using subtyping:

type foobar = [`A of int | `B | `C of foobar * foobar | `D of foobar]
let occurs_foo i x = occurs i (x : foo :> foobar)
let occurs_bar i x = occurs i (x : bar :> foobar)

Jacques Garrigue