Paper: https://fumieval.github.io/papers/en/2015-Haskell-objects.html

This package provides composable objects and instances.

Introduction

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
2

Interestingly, Object (Skeleton t) m and Object t m are isomorphic (Skeleton is an operational monad). cascading lets objects to handle an operational monad.