# Stateful nondeterminism

### From HaskellWiki

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## Latest revision as of 08:47, 27 January 2007

From New monads

If you want to do nondeterministic computation with local states for each of your threads and a global state shared by all your threads, use this monad:

newtype SuperState s t a = SuperState { runSuperState :: (s -> t -> (t,[(a,s)])) } instance Monad (SuperState s t) where return a = SuperState $ \s t -> (t,[(a,s)]) (SuperState x) >>= f = SuperState $ \s t -> let (t',stateList) = x s t in foldl (\(newt,sofar) (v,s) -> let (t'',lst) = runSuperState (f v) s newt in (t'',sofar++lst)) (t',[]) stateList instance MonadPlus (SuperState s t) where mzero = mz mplus = mp getGlobal = SuperState $ \s t-> (t,[(t,s)]) getLocal = SuperState $ \s t -> (t,[(s,s)]) putLocal s = SuperState $ \_ t -> (t,[((),s)]) putGlobal t = SuperState $ \s _ -> (t,[((),s)]) mz = SuperState $ \_ t -> (t,[]) mp (SuperState a) (SuperState b) = SuperState $ \s t -> let (t',stateList) = a s t (t'',stateList') = b s t' in (t'',stateList++stateList')