Re: [Haskell-cafe] Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a)

Well, "try" is really doing two things: chaining Maybes, and then adding a monadic context: try :: Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a) try = liftM2 (<|>) (You could weaken the assumption by using (Applicative m) instead) "tries" is similar, only there is an intermediate "threading" step [m x] -> m [x]: tries :: Monad m => [m (Maybe a)] -> m (Maybe a) tries = liftM asum . sequence These are both special cases, they only rely on Maybe being an Alternative: try :: (Monad m, Alternative f) => m (f a) -> m (f a) -> m (f a) tries :: (Monad m, Alternative f) => [m (f a)] -> m (f a) If you *really* want to generalise you can even write this. ([] is also an unnecessary specialisation right?:)) tries :: (Monad m, Alternative f, Traversable t) => t (m (f a)) -> m (f a) "justm" is a bit different, as you rely on Maybe's concrete structure by using 'maybe'. However you can still generalise it if you really want to. The first thing to realise is because you are "binding" with an (a -> _) function you'll need to use the monadic structure of both 'm' and 'Maybe' to unpack-repack properly. The second is the need of n (m x) -> m (n x), which is Data.Traversable:mapM justm :: (Monad m, Monad n, Traversable n) => m (n a) -> (a -> m (n b)) -> m (n b) justm m f = m >>= liftM join . mapM f However if you ask me, these generalisations are completely useless in practice 99 out of a 100 times. Your original functions are way more discoverable and intuitive. Generalising just for the sake of generalising is rarely a good design practice when you write "real" software. Imho these abstractions only make sense when you are designing a library API and you want to make as few assumptions as you can about the user's calling context. On 11 November 2014 22:41, Evan Laforge wrote:

On Tue, Nov 11, 2014 at 9:18 AM, Tom Ellis wrote:

...

On Tue, Nov 11, 2014 at 08:57:48AM -0800, Evan Laforge wrote:

...

try :: Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a) try action alternative = maybe alternative (return . Just) =<< action

Looks like the MonadPlus instance for MaybeT to me

runMaybeT $ MaybeT (print "first" >> return (Just 1)) `mplus` MaybeT (print "second" >> return (Just 2))

Ah, so it looks like it does exist, but requires explicit running and wrapping, e.g. compare to:

try (print "first" >> return (Just 1)) $ print "second" >> return (Just 2)

I guess it's like 'justm' then, which is also just MaybeT, but with less typing.

Thanks! _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe