[Haskell-cafe] Tutorial: Haskell for the Evil Genius

[Taylor Hedberg]

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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