Am Samstag, 28. Februar 2009 13:23 schrieb Phil:
Hi,
The problem is HOW DO I WRAP ANOTHER INDEPENDENT STATE AROUND THIS?
After some googling it looked like the answer may be Monad Transformers. Specifically we could add a StateT transform for our Box Muller state to our VanDerCorput State Monad. Google didn¹t yield a direct answer here so I¹m not even sure if my thinking is correct, people describe the process of using a transform as wrapping one monad in another¹ or threading one monad into another¹. What we want to do is have some internal state controlled by an independent outer state - this sounds about right to me?
If you absolutely don't want to have a state describing both, yes.
So I started playing around with the code, and got the below to compile.
test :: StateT (Bool,Double) (State Int) Double test = do (isStored,normal) <- get let (retNorm,storeNorm) = if isStored then (normal,0) else (n1,n2) where n1 = 2 n2 = 3 put (not isStored, storeNorm) return retNorm
Now this is incomplete and may be even wrong! I¹ll Explain my thinking:
(Bool,Double) is equivalent to myState and storedNormal in the C example The last Double is the return value of the BoxMuller Monad The (State Int) is supposed to represent the VanDerCorput monad but the compiler (GHC 6.10) will only let me specify one parameter with it so I¹ve put the state and left the return type to the gods!!.... As I said this isn¹t quite right any ideas how to specify the type?
You can't, the second argument to StateT must be a Monad, hence a type constructor you can pass an arbitrary type which then produces a new type from that. Fortunately, you don't need to. Say you have type VDCMonad = State Int nextVDC :: VDCMonad Double nextVDC = do n <- get put $! (n+1) return $ calculateVDCFromInt n Then you could have boxMullerVDC :: StateT (Maybe Double) VDCMonad Double boxMullerVDC = StateT $ \s -> case s of Just d -> return (d,Nothing) Nothing -> do d1 <- nextVDC d2 <- nextVDC let (b1,b2) = boxMullerTransform d1 d2 return (b1,Just b2) (I find a state of Maybe a more natural to indicate that *maybe* I have one a in store to use directly, than using (Bool,a)). However, I suspect that you would get better code if you abstracted over the sequence of pseudorandom Doubles and had simply calculation :: Sate [Double] whatever calculation = ??? result = evalState calculation bmVDC bmVDC = boxMuller $ map vanDerCorput [1 .. ] where boxMuller (k:n:more) = u:v:boxMuller more where (u,v) = bmTransform k n
The next few lines get and test the BoxMuller state, this seems to work OK to me, the problem is when I try to look at the STATE OF THE INTERNAL monad. n1 and n2 should evaluate and increment the state of VanDerCorput monad but I can¹t get anything to compile here. 2 and 3 are just dummy values to make the thing compile so I could debug.
My last gripe is how to actually call this from a pure function do I need to use both evalStateT and evalState I can¹t see how to initialize both the inner and outer state ?
result = evalState (evalStateT calculation Nothing) 1
OK I think that¹s more than enough typing, apologies for the war&peace sized post.
Any help muchly muchly appreciated,
Many Thanks,
Phil.
HTH, Daniel