Re: [Haskell-cafe] Nested Monads Questions
Bulat Ziganshin wrote: Hello Chris, Saturday, August 12, 2006, 4:05:44 AM, you wrote: Nine Base Monads: IO STM ST ST.Lazy GenParser [] Maybe Either (->) Seven MonadTrans: ListT ContT ErrorT ReaderT StateT WriterT RWST i'm not sure, but isn't Id monad also required for completeness? Yes, Identity is required for completeness. And I have added to http://haskell.org/haskellwiki/NewMonads#MonadBase this definition: -- One can recover MonadIO and liftIO from MonadBase class (MonadBase IO m) => MonadIO' m where liftIO' :: IO a -> m a liftIO' = liftBase Of course, the above is unneeded since you can always write liftBase instead of liftIO. >... at least it's included in MonadLib by Iavor S. Diatchki: http://www.csee.ogi.edu/~diatchki/monadLib/monadLib-2.0.tar.gz Hah...I knew someone else had done this. Also, there is 2.0.1 version of monadLib at http://www.cse.ogi.edu/~diatchki/monadLib/ His version is called BaseM, and uses a fundep: -- | Provides means to execute a computation in the base of a tower of monads. class (Monad m, Monad b) => BaseM m b | m -> b where inBase :: b a -> m a instance BaseM IO IOwhere inBase x = x instance BaseM [] []where inBase x = x instance BaseM Maybe Maybe where inBase x = x I am not sure I like the "inBase" name. I think "fromBase" might be a better match to its type. The "inBase" seems more like "toBase" which is backwards. My small test did not need the fundep, and I wonder if there is some creative example that shows either that the fundep is useful or a counter example that shows something very very clever that would otherwise violate the fundep. I *might* be able to imagine a transformer stack that pretends to have different base monads. am i correctly understand that your module is update on Monad transformers lib already included in GHC? Essentially, that is exactly what it is. It completely replaces MonadIO. -- Chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Nested Monads Questions
Hello Chris, Saturday, August 12, 2006, 4:05:44 AM, you wrote: > Nine Base Monads: >IO STM ST ST.Lazy GenParser [] Maybe Either (->) > Seven MonadTrans: >ListT ContT ErrorT ReaderT StateT WriterT RWST i'm not sure, but isn't Id monad also required for completeness? at least it's included in MonadLib by Iavor S. Diatchki: http://www.csee.ogi.edu/~diatchki/monadLib/monadLib-2.0.tar.gz am i correctly understand that your module is update on Monad transformers lib already included in GHC? -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Nested Monads Questions
I put a more comprehensive MonadBase module on the wiki at: http://haskell.org/haskellwiki/NewMonads Nine Base Monads: IO STM ST ST.Lazy GenParser [] Maybe Either (->) Seven MonadTrans: ListT ContT ErrorT ReaderT StateT WriterT RWST ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Nested Monads Questions
I will try any make a simpler explanation: Hi I'm trying to understand Monad Transformers. The code below works as expected but I have the following questions: - why can I use liftIO but not lift in the doSomething function? Replacing liftIO with (lift . lift) does work: doSomething :: MyM Int doSomething = do dataRef <- asks myData logMsg "Writing" lift . lift $ do mv <- atomically $ readTVar dataRef putStrLn mv logMsg "Written" return 1 This is because lift only move you one level though the MonadTrans stack of types. Let's look at what MonadTrans means: class MonadTrans (t::(* -> *) -> * -> *) where lift :: forall (m::* -> *) a. Monad m => m a -> t m a -- Imported from Control.Monad.Trans instance MonadTrans (ReaderT r) -- Imported from Control.Monad.Reader instance Monoid w => MonadTrans (WriterT w) -- Imported from Control.Monad.Writer So the only thing MonadTrans does is provide the 'lift' function. Your type is type MyM a = WriterT [Entry] (ReaderT MyData IO) a To really see what (lift . lift) is doing, consider the most type specific: -- lift :: (MonadTrans t, Monad m) => m a -> t m a liftIOtoReader :: IO a -> (ReaderT MyData) IO a liftIOtoReader = lift liftReaderToWriter :: (ReaderT MyData IO) a -> (WriterT [Entry]) (ReaderT MyData IO) a liftReaderToWriter = lift doSomething :: MyM Int doSomething = do dataRef <- asks myData logMsg "Writing" liftReaderToWriter . liftIOtoReader $ do --lift $ do mv <- atomically $ readTVar dataRef putStrLn mv logMsg "Written" return 1 In liftIOToReader, "m" is "IO" and "t" is (ReaderT MyData) In liftReaderToWriter, "m" is (ReaderT MyData IO) and "t" is (WriterT [Entry]) So how does liftIO work? The effect of the instances of liftIO recursively expand liftIO to (lift . liftIO) to (lift . (lift . liftIO)) to (lift . (lift . (lift . liftIO))) until it reaches the IO monad, where liftIO = id. So it builds the correct number of composed calls to lift when it is compiled. And the same things could be done with STM or (ST s), or any other base monad. Writing that sentence made me realize I could make a liftBase function to be a superset of liftIO, liftSTM, liftST: class (Monad m,Monad b) => MonadBase b m where liftBase :: b a -> m a instance MonadBase IO IO where liftBase = id instance MonadBase (ST s) (ST s) where liftBase = id instance MonadBase STM STM where liftBase = id instance (MonadBase b m,Monoid a) => MonadBase b (WriterT a m) where liftBase = lift . liftBase instance (MonadBase b m) => MonadBase b (ReaderT a m) where liftBase = lift . liftBase And so this now works: type MyM' s a = WriterT [String] (ReaderT Int (ST s)) a testMyM' :: forall s. MyM' s Int testMyM' = do tell ["a"] foo <- lift (ask) tell ["b"++show foo] liftBase (go foo) where go :: Int -> ST s Int go f = do a <- newSTRef f modifySTRef a (+1) readSTRef a main2 = do let q = runST (runReaderT (runWriterT (testMyM')) 17 ) print q *Main> main2 (18,["a","b17"]) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Nested Monads Questions
2006/8/11, Stefan Aeschbacher <[EMAIL PROTECTED]>: HiI'm trying to understand Monad Transformers. The code below works asexpected but I have the following questions: - why can I use liftIO but not lift in the doSomething function?I will first try to explain why it is not possible to use lift. Short version : In the definition of MyMtype MyM a = WriterT [Entry] (ReaderT MyData IO) a WriterT is parameterized with a *fixed* monad type, namely (ReaderT MyData IO). But in order to be able to instantiate MonadTrans and defining lift, this value has to be a parameter. The parameter should then take on different values, depending on which monad to lift. I.e. MyM a = ... should instead look like MyM m a = ... Longer version:Looking at the definition of MonadTrans and lift one sees that lift, given a monadic value, produces a transformed version of this monad. class MonadTrans t where lift :: Monad m => m a -> t m a In the case with the 'doSomething function', we wish to lift an action of type (IO ()) into MyM. So, what is the generell type of 'lift (some IO () action)' , e.g lift (putStr "hello") ? Examining the definition of lift above (or using :t ) , concludes that : lift (putStr "hello") :: MonadTrans t => t IO (). Due to the type of 'doSomething' (doSomething :: MyM Int)the monad transformer 't' should have type MyM, making the result of the lift operation MyM IO (). However, this is where it fails. According to the definition of MyM it can´t be parameterised with more than one type (not with both IO and ()). But, a monad transformer MUST have kind ((* -> *) -> * -> *) in order to be able to create a valid return type for lift. So even if we wished towrite our own instance for MonadTrans MyM, it wouldn't be possible.Compare with the following example which on the other hand does work with lift. type MyM2 m a = WriterT [Entry] m adoSomethingElse :: MyM2 IO IntdoSomethingElse = do lift $ putStrLn "hello" return 2Now, MyM2 has the right kind. And since (WriterT w) ,for any Monoid w, instantiates the MonadTrans class, it is possible to use the lift function to produce a value of type MyM2 IO ().So, why does liftIO work ? Consider the definition of MonadIO :class (Monad m) => MonadIO m where liftIO :: IO a -> m aThe monad that should embed the IO action (m above) has kind (* -> *). This makes an instance for MyM possible. Because MyM is a synonym for a WriterT monad,an instance for this is allready defined in the Controll.Monad.WriterT module:instance (Monoid w, MonadIO m) => MonadIO (WriterT w m) where liftIO = lift . liftIO Actually it also requires that the inner monad, i.e. ReaderT in this case, also instantiates the MonadIO, which luckily it does :) Looking at this, It's not hard to get lost in the jungle of monads :)From my own experience (which isn't long), I think the most effective way of learningis trying to write all definitions and instances by you're own, getting a feeling for what is really going on..Hope that this will be of any help! - why is there no liftSTM function? Don't know about that, but someone else sure does..Regards/Joel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Nested Monads Questions
Hi
I'm trying to understand Monad Transformers. The code below works as
expected but I have the following questions:
- why can I use liftIO but not lift in the doSomething function?
- why is there no liftSTM function?
now to the code:
module Main where
import Control.Monad.Reader
import Control.Monad.Writer
import Control.Concurrent.STM
type MyM a = WriterT [Entry] (ReaderT MyData IO) a
data MyData = MyData {myData::TVar String}
data Entry = Log String deriving (Eq, Show)
logMsg :: String -> MyM ()
logMsg s = tell [Log s]
doSomething :: MyM Int
doSomething = do
dataRef <- asks myData
logMsg "Writing"
liftIO $ do
--lift $ do
mv <- atomically $ readTVar dataRef
putStrLn mv
logMsg "Written"
return 1
main :: IO ()
main = do
i <- atomically $ newTVar "2"
log <- runReaderT (runWriterT doSomething) (MyData i)
print log
regards and thanks
Stefan
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