I wrote:
The Either Monad/Applicative provides multi-level exit from nested complex computations.
Evan Laforge wrote:
Can you give a specific example? I'm trying to think of how this is different from the normal exceptional escape mechanism. I use Either as a Maybe + info all the time, but I think you're talking about something more sophisticated here.
No something very simple. Here's a contrived example that computes the number of days in a month: daysInMonth :: Int -> Int -> Int daysInMonth month year = either id id $ do when (month `elem` [4,6,9,11]) $ Left 30 when (month == 2) $ do when (year `mod` 4 /= 0) $ Left 28 when (year `mod` 400 == 0) $ Left 29 when (year `mod` 100 == 0) $ Left 28 Left 29 return 31 If the answer becomes known somewhere in the middle of the calculation, Left causes the calculation to exit at that point and return the given answer. If the calculation makes it all the way to the end without exiting, the answer is 31. This is exactly the same way that Either works for exception handling - except we don't require an Error instance for the Left type. Regards, Yitz