[Haskell-cafe] Monad explanation
jonathanccast at fastmail.fm
Thu Feb 5 17:38:07 EST 2009
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
> > allows mathematically pure things to
> > while possibly having some opaque
> > stuff" going on.
> > This at least sounds, very wrong, even if it's not.
> > 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
> > 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.
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