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)
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