Paper: https://fumieval.github.io/papers/en/2015-Haskell-objects.html
This package provides composable objects and instances.
The primal construct, Object, models object-oriented objects. Object f g represents an object.
newtype Object f g = Object { runObject :: forall x. f x -> g (x, Object f g) }An object interprets a message f a and returns the result a and the next object Object f g, on g.
data Counter a where
Increment :: Counter ()
Print :: Counter Int
counter :: Int -> Object Counter IO
counter n = Object $ \case
Increment -> return ((), counter (n + 1))
Print -> print n >> return (n, counter n)new :: Object f g -> IO (Instance f g) creates an instance of an object.
(.-) :: (MonadIO m, MonadMask m) => Instance f m -> f a -> m a sends a message to an instance. This can be used to handle instances in the typical OOP fashion.
> i <- new (counter 0)
> i .- Increment
> i .- Print
1
> i .- Increment
> i .- Print
2Interestingly, Object (Skeleton t) m and Object t m are isomorphic (Skeleton is an operational monad). cascading lets objects to handle an operational monad.