[Haskell-cafe] monadic plumbing
Antoine Latter
aslatter at gmail.com
Tue Feb 22 22:20:00 CET 2011
On Tue, Feb 22, 2011 at 3:03 PM, Alberto G. Corona <agocorona at gmail.com> 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?.
The MaybeT monad transformer should work pretty well for this.
I would use a custom lifting operator:
> liftMaybe :: m (Maybe a) -> MaybeT m a
> liftMaybe = MaybeT
and then:
> resultMaybe <- runMaybeT $ do
> b <- liftMaybe $ foo a
> c <- liftMaybe $ bar b
> liftMaybe $ baz c
Here, 'resultMaybe' will be of type 'Maybe d'.
Antoine
>
> 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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