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

11 Nov 2014


      On Tue, Nov 11, 2014 at 05:18:56PM +0000, 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))
Just 1
I mistranscribed the output.  The output is

    "first"
    Just 1