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

[Denis Bueno]

Thanks for all the responses.  I have never used monad transformers
before, but StateT is welcome and really cool.  I didn't even think to
look them up.

I have a follow up question.  I eventually get to a point where I have
a value of type (ST s (Maybe (STUArray s Int Int))), and I need
somehow to get rid of the Maybe, so I can call runSTUArray on it.  The
function containing this value returns a pure type:

> data Solution = Sat (UArray Int Int) | Unsat deriving (Eq)

I've included the function body below, along with a few comments that
hopefully make my problem clear enough.  Let me know if there's any
more detail needed:

> solve :: StdGen -> Cnf -> Solution
> solve rnd cnf =
>    -- To solve, we simply take baby steps toward the solution using solveStep,
>    -- starting with the empty assignment.
>    Sat . runSTUArray $
>    do solution <- -- this block, as you can see,
>                   -- is the (ST s (STUArray s Int Int)) value
>          evalStateT (stepToSolution $ do
>                        initialAssignment <- lift (newArray (1, numVars cnf) 0)
>                        solveStep initialAssignment)
>          SC{cnf=cnf, dm=Map.empty, dl=[], bad=Set.empty, rnd=rnd}
>       case solution of -- `solution' is the (Maybe (STUArray s Int Int)) value
>         Nothing -> error "unsat"
>         Just m -> return m

Using `error' in the Nothing case is exactly what I'd like to avoid.
How should I improve this?


On Feb 2, 2008 2:57 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> 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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> >
>



-- 
                              Denis