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ClathomasPrime/performIfSuccedes.hs

Created April 16, 2019 22:21
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MonadState code which only "actually" alters state if some subcomputation succedes
Raw
performIfSuccedes.hs
performIfSuccedes :: MonadState s m => m (Maybe a) -> m (Maybe a)
performIfSuccedes m = do
s <- get
res <- m
case res of
Nothing -> put s >> return Nothing
Just a -> return (Just a)
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