objective

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