On Thu, 2009-02-05 at 15:52 -0800, David Leimbach wrote: > > > On Thu, Feb 5, 2009 at 2:38 PM, Jonathan Cast > <jonathancc...@fastmail.fm> wrote: > > 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. > > All Haskell programs start as
> main :: IO () > though... so they all get evaluated in the context of another IO () > don't they? Well... Haskell compilers and runhaskell-style interpreters (not regular Hugs/ghci!) take the value of Main.main as `the program'. But that feels (to me --- I could be wrong) like an aspect of a particular hosted environment. REPLs can handle programs that aren't wrapped up in IO at all; and there's no reason why IO has to be the type of IO-performning-things in REPLs, either. You could just as well write a REPL that took, say, tangible values [http://haskell.org/haskellwiki/TV] as input instead, and displayed them. So it's more a matter of Haskell implementations can be given an IO value to run than that combining IO values together somehow runs them. jcc _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
