On Thu, 2009-02-05 at 13:01 -0800, David Leimbach wrote: > > > On Thu, Feb 5, 2009 at 12:27 PM, Jonathan Cast > <jonathancc...@fastmail.fm> wrote: > > On Thu, 2009-02-05 at 12:21 -0800, David Leimbach wrote: > > > > > > On Thu, Feb 5, 2009 at 11:25 AM, Andrew Wagner > > <wagner.and...@gmail.com> wrote: > > I think the point of the Monad is > that it > > works as a container of stuff, that > still > > allows mathematically pure things to > happen, > > while possibly having some opaque > "other > > stuff" going on. > > > This at least sounds, very wrong, even if it's not. > Monads > > are not impure. IO is, but it's only _one_ instance > of Monad. > > All others, as far as I know, are pure. It's just > that the > > bind operation allows you to hide the stuff you > don't want to > > have to worry about, that should happen every time > you compose > > two monadic actions.
> > Well all I can tell you is that I can have (IO Int) in a > function as a > > return, and the function is not idempotent in terms of the > "stuff" > > inside IO being the same. > > > Sure it's the same. > > > cmp /bin/cat /bin/cat > > cp /bin/cat ~ > > cmp /bin/cat ~/cat > > > > Pretty much the same, anyway. > So if IO represents a program that when executed interacts with the > world's state, is it safe to say that when I return (State Int Int), > that I'm returning a "State program"? I won't object to it. Othe people might, though. > That'd make sense as it really does look like we force the State to be > evaluated with runState, evalState or execState. > The only difference with IO then is that to get IO programs to run, > you have to do it inside another IO program. Meh. Combining IO sub-programs into larger programs doesn't really `get them to run'. Better to say that an IO value is meaningful only to the computer, and not mathematically (denotationally) useful. jcc _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe