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

[Daniel Fischer]

Am Samstag, 2. Februar 2008 23:51 schrieb Daniel Fischer:
> Am Samstag, 2. Februar 2008 23:17 schrieb 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?
>
> Would
>
> solve rnd cnf =
>     case evalStateT ... of
> 	Nothing -> Unsat
> 	Just st -> Sat $ runSTUArray st
>
> work? Might need some explicit 'forall s.' or not typecheck at all, didn't
> test.
>
Obviously not.
Try
solve rnd cnf = runST $ do 
	solution <- evalStateT ...
	case solution of
	   Nothing -> return Unsat
	   Just ar -> unsafeFreeze ar