Difference between revisions of "Stateful nondeterminism"

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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:
 
<haskell>
 
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')
 
</haskell>
 
 
[[Category:Idioms]]
 
[[Category:Code]]
 

Revision as of 14:26, 6 February 2021