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Date: Fri, 9 Sep 2011 17:14:46 -0600
Message-ID: <CAN=ouMSM0SaKnKs3WCFk=yH5yLdWoToi3YtW_i5hTT300CpaKw@mail.gmail.com>
From: Anthony Tavener <anthony.tavener@gmail.com>
To: caml-list@inria.fr
Content-Type: multipart/alternative; boundary=00151740271ae2be5704ac8a57fd
Subject: [Caml-list] Mutually recursive closures?


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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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I was considering returning a couple of closures to help organize my UI cod=
e, essentially representing current UI mode by one of these closures. But t=
hen I run into a problem because the types are infinite (returns a function=
, which returns a function, ...)<div>
<br></div><div>A simplified example:</div><div><div><br></div><div># let re=
c a () =3D printf &quot;state a\n&quot;; b</div><div>=A0 =A0 =A0 =A0and b (=
) =3D printf &quot;state b\n&quot;; a</div></div><div><br></div><div><div>E=
rror: This expression has type unit -&gt; unit -&gt; &#39;a</div>
<div>=A0 =A0 =A0 =A0but an expression was expected of type &#39;a</div></di=
v><div><br></div><div><br></div><div>Is there a way I can do this? To expre=
ss (or &#39;hide&#39;) the cyclic nature of the type resolution?</div><div>=
<br>
</div><div>I&#39;ve considered using continuations, but that seems heavy-we=
ight for what I&#39;m looking to do. And as far as I can tell I&#39;d need =
to leverage Oleg&#39;s delimcc (which I&#39;d love to start using and wrap =
my head around -- but for a task worthy of it!).</div>
<div><br></div><div>I can use a variant to represent states/modes and have =
a dispatcher which runs the right code... but this introduces what feels li=
ke an unnecessary layer of distraction. Returning the closure of the &quot;=
next state&quot; seems straightforward, but introduces cycles into the typi=
ng. :(</div>
<div><br></div><div>I&#39;m hoping I&#39;m missing something simple. Thank-=
you for any assistance!</div><div><br></div><div>=A0-Tony</div><div><br></d=
iv>

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