On Thu, Nov 13, 2014 at 11:07 AM, Andras Slemmer <0slemi0@gmail.com> wrote:
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)
That's different to Evan's original function. Evan's solution short-circuits: it does not execute the second action if the first succeeds. But your one runs both actions unconditionally. For example, the expression try (return $ Just ()) (putStrLn "second action executed" >> return Nothing) outputs "second action executed" with your solution, but not with Evan's. The lesson is, applicative and monadic composition don't always yield the same results. Chris