[Haskell-cafe] Buttons and Clicks - State Monad

[Cale Gibbard]

You might be misunderstanding the purpose of the State Int monad somewhat.

A computation of type State Int a is internally represented by a
function of type Int -> (Int, a). When you call runState, you
effectively apply this pure function to an initial state, and get a
final state and result. You won't be able to do anything with the
State monad that you couldn't already do with such a function, it's
more or less a notational convenience.

In particular, the state of your counter will not be preserved between
calls to runState unless you arrange that the final state returned
from the last call to runState is passed along as the initial state in
the next one. Of course, this effectively defeats the purpose of using
the State monad in the first place.

Since event handlers in wxHaskell must be in the IO monad, there's no
machinery in place to handle forwarding your state along, so the State
monad is not terribly useful here. On the other hand, it's rather easy
to write a function of type State s a -> IORef s -> IO a which takes
the initial state from the IORef and updates the IORef with the new
state.

 - Cale

2009/1/31 guenni68 <redcom at fedoms.com>:
> Hi,
>
> in this piece here http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=1083#a1083
> I'm trying to create a button that, every time when clicked, increases
> a counter by one and does a putStrLn of the counters current value.
>
> I'm trying to write this without any use of IORef but merely using the
> state monad.
>
> Can anybody show me how to do this?
>
> Günther
>
> The UI Code is wxHaskell
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