Tuesday, April 14, 2015

Generalized swap

Generalized swap

I pushed a pull request to Edward Kmett’s either package to implement two functions some guys was complaining not to find: flipEither :: Either e a -> Either a e and flipEitherT :: EitherT e m a -> EitherT a m e.

When implementing the functions, I wondered: “Hey, flipping stuff is a pretty common operation. Don’t we have an abstraction for that yet?”. I haven’t found any.

Meet Swap

I decided to make a little typeclass to see what it’d be.

class Swap s where
  swap :: s a b -> s b a

instance Swap (,) where
  swap (a,b) = (b,a)

instance Swap Either where
  swap = flipEither

-- let’s go wild and fooled
instance Swap Map where
  swap = fromList . fmap swap . toList

If you think that’s handy, I’ll write a little package with default instances to make it live.

Happy hacking folks!

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