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-- From a blog post: http://www.jonmsterling.com/posts/2012-01-12-unifying-monoids-and-monads-with-polymorphic-kinds.html
-------------------- FUNCTIONAL DEPENDENCY VERSION ----------------
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances, FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE UnicodeSyntax #-}
module Main where
import Control.Monad (Monad(..), join)
import Data.Monoid (Monoid(..))
-- First we define the type class Monoidy:
class Monoidy to comp id m | m to → comp id where
munit :: id `to` m
mjoin :: (m `comp` m) `to` m
-- We use functional dependencies to help the typechecker understand that
-- m and ~> uniquely determine comp (times) and id.
--
-- This kind of type class would not have been possible in previous
-- versions of GHC; with the new kind system, however, we can abstract
-- over kinds!2 Now, let’s create types for the additive and
-- multiplicative monoids over the natural numbers:
newtype Sum a = Sum a deriving Show
newtype Product a = Product a deriving Show
instance Num a ⇒ Monoidy (→) (,) () (Sum a) where
munit _ = Sum 0
mjoin (Sum x, Sum y) = Sum $ x + y
instance Num a ⇒ Monoidy (→) (,) () (Product a) where
munit _ = Product 1
mjoin (Product x, Product y) = Product $ x * y
-- It will be slightly more complicated to make a monadic instance with
-- Monoidy. First, we need to define the identity functor, a type for
-- natural transformations, and a type for functor composition:
data Id α = Id { runId :: α } deriving Functor
-- A natural transformation (Λ f g α. (f α) → (g α)) may be encoded in Haskell as follows:
data NT f g = NT { runNT :: ∀ α. f α → g α }
-- Functor composition (Λ f g α. f (g α)) is encoded as follows:
data FC f g α = FC { runFC :: f (g α) }
-- Now, let us define some type T which should be a monad:
data Wrapper a = Wrapper { runWrapper :: a } deriving (Show, Functor)
instance Monoidy NT FC Id Wrapper where
munit = NT $ Wrapper . runId
mjoin = NT $ runWrapper . runFC
-- With these defined, we can use them as follows:
test1 = do { print (mjoin (munit (), Sum 2))
-- Sum 2
; print (mjoin (Product 2, Product 3))
-- Product 6
; print (runNT mjoin $ FC $ Wrapper (Wrapper "hello, world"))
-- Wrapper {runWrapper = "hello, world" }
}
-- We can even provide a special binary operator for the appropriate monoids as follows:
(<+>) :: Monoidy (→) (,) () m ⇒ m → m → m
(<+>) = curry mjoin
test2 = print (Sum 1 <+> Sum 2 <+> Sum 4) -- Sum 7
-- Now, all the extra wrapping that Haskell requires for encoding this is
-- rather cumbersome in actual use. So, we can give traditional Monad and
-- Monoid instances for instances of Monoidy:
instance Monoidy (→) (,) () m ⇒ Monoid m where
mempty = munit ()
mappend = curry mjoin
-- instance (Functor m, Monoidy NT FC Id m) ⇒ Monad m where
instance Monad Wrapper where
return x = runNT munit $ Id x
x >>= f = runNT mjoin $ FC (f `fmap` x)
-- And so the following works:
test3
= do { print (mappend mempty (Sum 2))
-- Sum 2
; print (mappend (Product 2) (Product 3))
-- Product 6
; print (join $ Wrapper $ Wrapper "hello")
-- Wrapper {runWrapper = "hello" }
; print (Wrapper "hello, world" >>= return)
-- Wrapper {runWrapper = "hello, world" }
}
main = test1 >> test2 >> test3
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