Re: [Caml-list] Mutually recursive closures?

[Jonathan Protzenko <jonathan.protzenko@gmail.com>]

You can use equirecursive types, which can be enabled through the 
-rectypes command-line switch. With that option, your example above 
type-checks. However, these are not enabled by default for a variety of 
reasons, the most important one being it makes it much easier to shoot 
yourself in the foot.

Cheers,

jonathan

On Sat 10 Sep 2011 01:14:46 AM CEST, Anthony Tavener wrote:
> I was considering returning a couple of closures to help organize my 
> UI code, essentially representing current UI mode by one of these 
> closures. But then I run into a problem because the types are infinite 
> (returns a function, which returns a function, ...)
>
> A simplified example:
>
> # let rec a () = printf "state a\n"; b
>        and b () = printf "state b\n"; a
>
> Error: This expression has type unit -> unit -> 'a
>        but an expression was expected of type 'a
>
>
> Is there a way I can do this? To express (or 'hide') the cyclic nature 
> of the type resolution?
>
> I've considered using continuations, but that seems heavy-weight for 
> what I'm looking to do. And as far as I can tell I'd need to leverage 
> Oleg's delimcc (which I'd love to start using and wrap my head around 
> -- but for a task worthy of it!).
>
> I can use a variant to represent states/modes and have a dispatcher 
> which runs the right code... but this introduces what feels like an 
> unnecessary layer of distraction. Returning the closure of the "next 
> state" seems straightforward, but introduces cycles into the typing. :(
>
> I'm hoping I'm missing something simple. Thank-you for any assistance!
>
>  -Tony
>