Re: [Caml-list] Mutually recursive closures?

[Anthony Tavener <anthony.tavener@gmail.com>]

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Thanks Jonathan! I've seen -rectypes mentioned over the years and always
glossed over it thinking "Ah, I'll never need that!" :P

Understandable that it's a good default to have disabled. I'll experiment
first and if I like the results I'll try to limit compiling with -rectypes
to the smallest bit of code using it.


On Fri, Sep 9, 2011 at 5:31 PM, Jonathan Protzenko <
jonathan.protzenko@gmail.com> wrote:

> 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
>>
>>

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