[Haskell-cafe] Refactoring from State monad to ST monad, for STUArray

[Ryan Ingram]

You can also do something like the following:

newtype StateST st s a = StateST { internalRunStateST :: ReaderT
(STRef st s) (ST st) a }

instance MonadState s (StateST s st) where
    get = ask >>= readSTRef
    put s = ask >>= \ref -> writeSTRef ref s

runStateST :: StateST st s a -> s -> ST st a
runStateST m s = do
    ref <- newSTRef s
    runReaderT (internalRunStateST m) ref

  -- ryan


On Feb 2, 2008 9:05 AM, Derek Elkins <derek.a.elkins at gmail.com> wrote:
> On Sat, 2008-02-02 at 12:33 -0500, Denis Bueno wrote:
> > Is it possible to use the ST monad as a (drop-in) replacement for the
> > State monad in the following situation?  If not, is there a "best
> > practice" for refactoring?
> >
> > I have a bunch of functions that return state actions:
> >
> >     type MyState = ...
> >
> >     foo1 :: T1 -> State MyState a
> >     foo2 :: T2 -> State MyState a
> >     ...
> >     foon :: Tn -> State MyState a
> >
> > And I'd like to refactor this to use the ST monad, mechanically, if
> > possible.  All uses of the MyState inside State are single-threaded.
> >
> > In my application, MyState is a record with 5 or so fields.  One of
> > those fields uses a list to keep track of some information, and I'd
> > like to change that to STUArray, because it changes my bottleneck
> > operations from O(n) to O(1).  This, of course, requires having the ST
> > monad around, in order to achieve the proper time complexity.
> >
> > Is there an easy way to do this?  In the future, should I *start out*
> > with the ST monad if I suspect I'll need to use an imperative data
> > structure for efficiency reasons?  I started out with State because
> > I'm modeling a transition system, so it seemed natural.
> >
> > Any advice is appreciated.
>
> %s/State MyState/MyMonad s/g
>
> type MyState s = ... s ...
>
> type MyMonad s = StateT (MyState s) (ST s)
>
>
>
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