comonads, io

[Nicolas.Oury]

Hello,

If I understand what you're saying, it seems that an intuitive view of OI
would be :
passing a token allowing to do IO operations.

We pass the token to the main function.
After an operation we give the token to next.

It's unsafe to crete a new token.

>From this, I think the safety become intuitive.

Cheers,
N.

On Tue, 31 Dec 2002, Ashley Yakeley wrote:

> In article <oqznqrnz32.fsf@premise.demon.co.uk>,
>  peter@premise.demon.co.uk (Peter G. Hancock) wrote:
>
> > Frustratingly, I can't seem to grasp the intuition.  I'm aware that
> > the state-monad s->(s,a) has a dual state-in-context
> > comonad (s,s->a). How does the side-effect manifest itself?
> > The (ineffective) hints the authors give revolve around the idea that
> > side-effects "derive" from the context of a program.  Any other hints?
> ...
> > I can't make the slightest sense of Kieburtz's OI co-monad, with
> > commands like coGetChar :: OI Handle -> Char.
>
> I've been wondering this myself. I always think of an object of type "IO
> a" as "an imperative action that returns an a", but I have no similar
> understanding for "OI a". But I think OI programs have the same
> restrictions against unsafety as IO programs.
>
>   class Comonad w where
>     (=>>) :: w a -> (w a -> b) -> w b
>     (.>>) :: w a -> b -> w b
>     coeval :: w a -> a
>
>   instance Comonad OI where etc.
>
> The first thing I notice is that you can't create objects of type "OI
> a". I think your main program would be:
>
>   main :: OI () -> ()
>
> Equivalent to "unsafePerformIO" would be:
>
>   unsafeOI :: OI ()
>
>   unsafeMakeOI :: a -> OI a
>   unsafeMakeOI a = unsafeOI .>> a
>
> The other thing I notice is that functions of type "(Comonad w) => w a
> -> b" seem to be equivalent to functions of type "(Comonad w) => w () ->
> a -> b". For instance:
>
>   coGetChar :: OI Handle -> Char
>   coGetChar' :: OI () -> Handle -> Char
>
>   coGetChar' oi h = coGetChar (oi .>> h)
>
>   coGetChar oih = coGetChar' (oih .>> ()) (coeval oih)
>
> You certainly can't pull anything like this with monads. This would
> suggest comonads don't need to by type-constructors at all. But I'm not
> sure if it's correct. Opinions?
>
> --
> Ashley Yakeley, Seattle WA
>
>
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