[Haskell-cafe] Monad explanation
jonathanccast at fastmail.fm
Thu Feb 5 19:11:26 EST 2009
On Thu, 2009-02-05 at 15:52 -0800, David Leimbach wrote:
> 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
> > >
> > >
> > > 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
> > "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_
> > 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.
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