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
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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