[Haskell-cafe] combining monads with IO

[Miguel Mitrofanov]

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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