Well, without "fail" part: newtype IOMayfail a = IOMayfail (MaybeT IO a) deriving Monad Matthias Görgens wrote on 25.06.2009 17:14:
By the way, how would one write the following with Monad Transformers?
newtype IOMayfail a = IOMayfail (IO (Maybe a))
instance Monad IOMayfail where return = IOMayfail . return . return (>>=) a f = IOMayfail (bind (run a) (run . f)) fail s = trace s (IOMayfail $ return Nothing)
run :: IOMayfail a -> IO (Maybe a) run (IOMayfail a) = a
bind :: IO (Maybe a) -> (a -> IO (Maybe b)) -> IO (Maybe b) bind a f = do r <- a case r of Nothing -> return Nothing Just r' -> f r' Lift :: IO a -> IOMayfail a lift f = IOMayfail (f >>= return . return)
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