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