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 <[EMAIL PROTECTED]>, > [EMAIL PROTECTED] (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 > > > _______________________________________________ > Haskell-Cafe mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
