[Haskell-cafe] Monad explanation

[David Leimbach]

On Thu, Feb 5, 2009 at 2:38 PM, Jonathan Cast <jonathanccast at 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
> > <jonathanccast at 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.andrew at 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
>

All Haskell programs start as

main :: IO ()

though... so they all get evaluated in the context of another IO () don't
they?
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