Composing monads (sort of)
Hi, I have a set of functions: f1 :: DBRecord -> Maybe Int f2 :: Int -> IO Maybe DBRecord f3 :: DBRecord -> Maybe Int The odd numbered functions are field accessors, accessing a field that might hold an identifier for another record. The even numbered functions are record fetch functions that get records from the database. I want to compose these so that I can navigate structures of joined records in the database. How can I concisely compose these functions without having to write a cascade of case statements such as: case f1 rec1 of Nothing -> return Nothing Just id1 -> do rec2 <- f2 id2 return $ case rec2 of Nothing -> return Nothing Just rec2' -> case f3 rec2' of .... I understand that if I was just dealing with Maybe I could use the fact that Maybe is a monad. I am also not sure if composing the IO and the Maybe will get me what I want (some of the functions only return Maybe Int). Cheers Mark PS Heard this on the 'West Wing' and thought it was appropriate in a way: A coach goes up to a player and asks "Are you ignorant or apathetic?". The player replies "I don't know and I don't care".
Hey Mark,
How can I concisely compose these functions without having to write a cascade of case statements such as:
case f1 rec1 of Nothing -> return Nothing Just id1 -> do rec2 <- f2 id2 return $ case rec2 of Nothing -> return Nothing Just rec2' -> case f3 rec2' of .... I understand that if I was just dealing with Maybe I could use the fact that Maybe is a monad. Yes, you can write like this:
id2 <- f1 rec1 rec2 <- f2 id2 rec3 <- f3 rec2 return rec3 or, even shorter: id2 <- f1 rec1 rec2 <- f2 id2 f3 rec2
The cool thing of the Maybe monad is that it combines a result in such a way that it removes the plumbing of constantly checking for Nothing. I can definitely recommand you the following tutorials: http://www.nomaware.com/monads/html/index.html http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.en.html Those two tutorials really helped me. Good luck, Chris
Wait, there are two monads in scene here, IO and Maybe. The right solution is to compose them indeed. One could use the MaybeT monad transformer defined in the 'All about monads' tutorial [1], or we could just define the IOmaybe monad:
import Data.Traversable (mapM)
newtype IOMaybe a = IOM { iom :: IO (Maybe a) }
instance Monad IOMaybe where return = IOM . return . Just c >>= f = IOM$ do mb_v <- iom c mapM (iom.f) mb_v >>= return . join
Now we can define:
t1 = IOM . return . f1 t2 = IOM . f2 t3 = IOM . return . f3 traverse rec1 = t1 rec1 >>= t2 >>= t3
And this scheme lends itself very well to define any kind of traversal. Note that I used the more general version of mapM defined in Data.Traversable in the definition of the (>>=) combinator. A more conventional definition is given the 'All about monads' tutorial. Cheers pepe 1- http://www.nomaware.com/monads/html/index.html On 16/12/2006, at 15:35, Chris Eidhof wrote:
Hey Mark,
How can I concisely compose these functions without having to write a cascade of case statements such as:
case f1 rec1 of Nothing -> return Nothing Just id1 -> do rec2 <- f2 id2 return $ case rec2 of Nothing -> return Nothing Just rec2' -> case f3 rec2' of .... I understand that if I was just dealing with Maybe I could use the fact that Maybe is a monad. Yes, you can write like this:
id2 <- f1 rec1 rec2 <- f2 id2 rec3 <- f3 rec2 return rec3 or, even shorter: id2 <- f1 rec1 rec2 <- f2 id2 f3 rec2
The cool thing of the Maybe monad is that it combines a result in such a way that it removes the plumbing of constantly checking for Nothing. I can definitely recommand you the following tutorials:
http://www.nomaware.com/monads/html/index.html http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.en.html
Those two tutorials really helped me.
Good luck, Chris _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Once I start needing to combine Maybe with other monads, I usually
take a moment to generalize the appropriate Maybe parts to MonadError
e m => m. Then we can just use the (ErrorT e IO) monad.
Nick
On 12/16/06, Pepe Iborra
Wait, there are two monads in scene here, IO and Maybe. The right solution is to compose them indeed. One could use the MaybeT monad transformer defined in the 'All about monads' tutorial [1], or we could just define the IOmaybe monad:
import Data.Traversable (mapM)
newtype IOMaybe a = IOM { iom :: IO (Maybe a) }
instance Monad IOMaybe where return = IOM . return . Just c >>= f = IOM$ do mb_v <- iom c mapM (iom.f) mb_v >>= return . join
Now we can define:
t1 = IOM . return . f1 t2 = IOM . f2 t3 = IOM . return . f3 traverse rec1 = t1 rec1 >>= t2 >>= t3
And this scheme lends itself very well to define any kind of traversal. Note that I used the more general version of mapM defined in Data.Traversable in the definition of the (>>=) combinator. A more conventional definition is given the 'All about monads' tutorial.
Cheers pepe
1- http://www.nomaware.com/monads/html/index.html
On 16/12/2006, at 15:35, Chris Eidhof wrote:
Hey Mark,
How can I concisely compose these functions without having to write a cascade of case statements such as:
case f1 rec1 of Nothing -> return Nothing Just id1 -> do rec2 <- f2 id2 return $ case rec2 of Nothing -> return Nothing Just rec2' -> case f3 rec2' of .... I understand that if I was just dealing with Maybe I could use the fact that Maybe is a monad. Yes, you can write like this:
id2 <- f1 rec1 rec2 <- f2 id2 rec3 <- f3 rec2 return rec3 or, even shorter: id2 <- f1 rec1 rec2 <- f2 id2 f3 rec2
The cool thing of the Maybe monad is that it combines a result in such a way that it removes the plumbing of constantly checking for Nothing. I can definitely recommand you the following tutorials:
http://www.nomaware.com/monads/html/index.html http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.en.html
Those two tutorials really helped me.
Good luck, Chris _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
participants (4)
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Chris Eidhof -
Mark Wassell -
Nicolas Frisby -
Pepe Iborra