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