On Tue, Feb 22, 2011 at 2:03 PM, Alberto G. Corona wrote:

Recently I had to navigatate trough data structures chained with mutable referenes in th STM monad. The problem is that their values are enveloped in  Either or Maybe results. functional compositions in the Either of Maybe , or list  monads are not possible when the values are  embedded inside effect monads (i.e. STM or IO) . I tried  to find some trick to handle it. to summarize, given:  foo, :  a -> m (Maybe b)  bar :   b -> m (Maybe c)  baz :  c -> m (Maybe d)

These are isomorphic to: foo :: a -> MaybeT m a And so on (from the MaybeT package on hackage). So to compose these three, lift them into MaybeT and then use Kleisli composition: MaybeT . foo >=> MaybeT . bar >=> MaybeT . baz Luke

how to compose foo bar and baz? Or, at least,  Are something out there to handle it in the less painful way?.

I solved the generalized problem  (chaining  any double monadic combination) with a sort of monadic connector that acts as a " double monadic" operator

...

...

...

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==  so that return. return (x :: a) >>>>== foo >>>== bar >>>== baz can be possible. Although I don't know if  it is the best solution. I wonder why nobody has written about it before: class (Monad m, Monad n) => Bimonad m n where  (>>>=)   ::  n a -> (a -> m(n b)) -> m(n b) (>>>>==) :: (Bimonad m n) => m (n a) -> (a -> m(n b)) -> m (n b) (>>>>==) x  f =  x >>= \y -> y >>>=  f x >>>> f = x >>>>== \ _-> f infixl 1 >>>>==, >>>> The instance for handling the Maybe monad under any other monad is very similar to the definition of the "normal" monad: instance (Monad m) => Bimonad m Maybe where    Just x  >>>= f = f x    Nothing >>>= _ = return $ Nothing

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You need the MaybeT and EitherT monad transformers: http://hackage.haskell.org/packages/archive/MaybeT/0.1.2/doc/html/Control-Mo... http://hackage.haskell.org/packages/archive/MaybeT/0.1.2/doc/html/Control-Mo... http://hackage.haskell.org/packages/archive/EitherT/0.0.1/doc/html/Control-M... http://hackage.haskell.org/packages/archive/EitherT/0.0.1/doc/html/Control-M...With MaybeT, you can wrap foo, bar, and baz with a MaybeT constructor, which gives you a new monad that you can compose the usual way. For example: result <- runMaybeT (MaybeT foo >>= MaybeT bar >>= MaybeT baz) case result of Just x -> ... Nothing -> ... - Job On Tue, Feb 22, 2011 at 4:03 PM, Alberto G. Corona wrote:

Recently I had to navigatate trough data structures chained with mutable referenes in th STM monad. The problem is that their values are enveloped in Either or Maybe results.

functional compositions in the Either of Maybe , or list monads are not possible when the values are embedded inside effect monads (i.e. STM or IO) . I tried to find some trick to handle it.

to summarize, given:

foo, : a -> m (Maybe b) bar : b -> m (Maybe c) baz : c -> m (Maybe d)

how to compose foo bar and baz? Or, at least, Are something out there to handle it in the less painful way?.

I solved the generalized problem (chaining any double monadic combination) with a sort of monadic connector that acts as a " double monadic" operator >>>>== so that

return. return (x :: a) >>>>== foo >>>== bar >>>== baz

can be possible. Although I don't know if it is the best solution. I wonder why nobody has written about it before:

class (Monad m, Monad n) => Bimonad m n where (>>>=) :: n a -> (a -> m(n b)) -> m(n b)

(>>>>==) :: (Bimonad m n) => m (n a) -> (a -> m(n b)) -> m (n b) (>>>>==) x f = x >>= \y -> y >>>= f

x >>>> f = x >>>>== \ _-> f

infixl 1 >>>>==, >>>>

The instance for handling the Maybe monad under any other monad is very similar to the definition of the "normal" monad:

instance (Monad m) => Bimonad m Maybe where Just x >>>= f = f x Nothing >>>= _ = return $ Nothing

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Am 22.02.2011 um 22:03 schrieb Alberto G. Corona:

Recently I had to navigatate trough data structures chained with mutable referenes in th STM monad. The problem is that their values are enveloped in Either or Maybe results.

functional compositions in the Either of Maybe , or list monads are not possible when the values are embedded inside effect monads (i.e. STM or IO) . I tried to find some trick to handle it.

to summarize, given:

foo, : a -> m (Maybe b) bar : b -> m (Maybe c) baz : c -> m (Maybe d)

how to compose foo bar and baz? Or, at least, Are something out there to handle it in the less painful way?.

import Control.Monad.Trans.Maybe fooBarBaz = runMaybeT (MaybeT . foo >=> MaybeT . bar >=> MaybeT . baz) (untested)

I solved the generalized problem (chaining any double monadic combination) with a sort of monadic connector that acts as a " double monadic" operator >>>>== so that

return. return (x :: a) >>>>== foo >>>== bar >>>== baz

can be possible. Although I don't know if it is the best solution. I wonder why nobody has written about it before:

class (Monad m, Monad n) => Bimonad m n where (>>>=) :: n a -> (a -> m(n b)) -> m(n b)

(>>>>==) :: (Bimonad m n) => m (n a) -> (a -> m(n b)) -> m (n b) (>>>>==) x f = x >>= \y -> y >>>= f

x >>>> f = x >>>>== \ _-> f

infixl 1 >>>>==, >>>>

The instance for handling the Maybe monad under any other monad is very similar to the definition of the "normal" monad:

instance (Monad m) => Bimonad m Maybe where Just x >>>= f = f x Nothing >>>= _ = return $ Nothing

Ignoring the newtype wrappers, this is the same as the actual monad instance of MaybeT: newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) } instance (Monad m) => Monad (MaybeT m) where fail _ = MaybeT (return Nothing) return = lift . return x >>= f = MaybeT $ do v <- runMaybeT x case v of Nothing -> return Nothing Just y -> runMaybeT (f y)