Joel Burget, Fri 2012-09-14 @ 19:08:29-0700: > I find the Monad instance for Maybe and Either very useful. You can do > things like the following (which technically only uses the Applicative > instance): > > Prelude Control.Applicative> (*3) <$> (+2) <$> Just 1 > Just 9 > Prelude Control.Applicative> (*3) <$> (+2) <$> Nothing > Nothing > Prelude Control.Applicative> (*3) <$> (+2) <$> Left "error" :: Either String > Int > Left "error" > Prelude Control.Applicative> (*3) <$> (+2) <$> Right 1 :: Either String Int > Right 9
Nitpick: You are using the Functor instances of Maybe and Either here, not Applicative. (<$>) == fmap; it just happens to be defined in the Control.Applicative module.
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