→ dualizer ←

Delete half (minus ε) of your Haskell code!

Dualizer allows you to eliminate the dual of all your code. Rather than implementing, say, Comonad directly, you can define it in terms of its dual – Monad:

-- indicates that Functor is its own dual
labelSelfDual ''Functor

-- expands to:
--
--   class Functor f => Coapplicative f where
--     extract :: f a -> a -- the dual of pure
makeDualClass ''Applicative "Coapplicative" [('pure, "extract")]

-- expands to:
--
--   class Coapplicative m => Comonad m where
--     (=>>) :: m b -> (m b -> a) -> m a
makeDualClass ''Monad "Comonad" [('(>>=) , "=>>")]

See Dual.Example for a bit more.