> The first approximation for a state monad is an entity that wraps a state.
> You can only change the state that's inside the monad, by passing the
> monad
> a function that changes the state and then let the monad invoke your
> function and change the state accordingly.
>
> If you think of it that way, the state itself is never visible outside the
> monad (or it doesn't have to be). That's how you guarantee that nobody can
> mess with it outside of the monad's control.
>
> A monad is really an extension to something like this:
>
> atomically (f,g) = { // simplified STM monad
> lockandstarttransaction
> var v = readoldvalue
> v = f (v)
> v = g (v)
> setnewvalue (v)
> unlockandendtransaction
> }
>
> As you see, the actual value never escapes the "monad"...
>
> The mathematics behind it, the scala syntax (for comprehension) and the
> actual function signatures are a little more complicated, to allow
> composition, but this is the general idea...
>
> scalaz has a state monad, here's a good post on that:
>
http://blog.tmorris.net/the-state-monad-for-scala-users/>
> here's an (complex) example of using the scalaz.State monad:
>
https://github.com/scalaz/scalaz/blob/master/example/src/main/scala/scalaz/example/ExampleState.scala>
> good luck!
> Razvan
>
>
> -----Original Message-----
> From: Dennis Haupt
> Sent: Thursday, November 11, 2010 8:04 AM
> To: Razvan Cojocaru
> Cc:
[hidden email] ;
[hidden email]
> Subject: Re: [scala-user] functional/immutable programming question
>
> i can imagine programming everything using only immutable
> objects/structs/values as long as there is one *mutable* entry point for
> the
> current complete world which i then navigate through.
>
> for example, in tetris, there's a board. i can easily do:
> val newBoard = oldBoard.withUserMove(move)
> to execute a single move and get the resulting board.
>
> but to make the game run forever, i need at least ONE global mutable state
> do be able to do this:
>
> var myMostRecentWorld = new Board
> while (true) {
> myMostRecentWorld =
> myMostRecentWorld.withUserMove(getMoveFromSomeWhere)
> }
>
> if i try to avoid even that mutable state, i can only come up with
> *really*
> weird stuff like
>
> def advanceOneStep(world:Board) {
> advanceOneStep(world.withUserMove(getMoveFromSomeWhere)
> }
>
> which will eventually lead to a stackoverflow or outofmemory
>
> how does the state/io-monad work? like my "var currentWorld"?
>
>
> -------- Original-Nachricht --------
> > Datum: Thu, 11 Nov 2010 07:27:34 -0500
> > Von: Razvan Cojocaru <
[hidden email]>
> > An: Dennis Haupt <
[hidden email]>
> > CC: Robert Wills <
[hidden email]>, "
[hidden email]"
> > <
[hidden email]>
> > Betreff: Re: [scala-user] functional/immutable programming question
>
> > It's like the IO monad but a little different ;)
> >
> > Thanks,
> > Razvan
> >
> > On 2010-11-11, at 5:00 AM, "Dennis Haupt" <
[hidden email]> wrote:
> >
> > > what is a state monad?
> > >
> > > -------- Original-Nachricht --------
> > >> Datum: Thu, 11 Nov 2010 09:53:56 +0000
> > >> Von: Robert Wills <
[hidden email]>
> > >> An: Dennis Haupt <
[hidden email]>
> > >> CC:
[hidden email]
> > >> Betreff: Re: [scala-user] functional/immutable programming question
> > >
> > >> In a pure language the 'something' holding the state would be a state
> > >> or writer monad.
> > >>
> > >> Of course if this were a proper client-server environment you would
> > >> need to have some impurity or else your program would never be able
> to
> > >> handle non-determinism.
> > >>
> > >> -Rob
> > >>
> > >> On Thu, Nov 11, 2010 at 9:45 AM, Dennis Haupt <
[hidden email]> wrote:
> > >>> in pure functional programming, there are no side effects. this
> means,
> > >> the only "effect" a method has is its returned value, right? it
> doesn't
> > hold
> > >> a mutable state.
> > >>>
> > >>> if so, how do i implement something that HAS to hold a state? what
> if
> > >> let's say 10 clients try to add elements to the same list instance?
> > there has
> > >> to be a managing class that holds the list, right?
> > >>>
> > >>>
> > >>> --
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