Functor.hs

{-# LANGUAGE TypeOperators, EmptyDataDecls, StandaloneDeriving, DeriveFunctor #-}
{-# OPTIONS_GHC -Wall #-}
----------------------------------------------------------------------
-- |
-- Module      :  FunctorCombo.Functor
-- Copyright   :  (c) Conal Elliott 2010
-- License     :  BSD3
-- 
-- Maintainer  :  conal@conal.net
-- Stability   :  experimental
-- 
-- Standard building blocks for functors
----------------------------------------------------------------------

module FunctorCombo.Functor
  (
    Const(..),Void,voidF,Unit,unit,Id(..),unId,inId,inId2,(:+:)(..),eitherF
  , (:*:)(..),(:.)(..),unO,inO,inO2,(~>)
  , pairF, unPairF, inProd, inProd2
  ) where


import Control.Applicative (Applicative(..),Const(..))

import Control.Compose (Id(..),unId,inId,inId2,(:.)(..),unO,inO,inO2,(~>))


-- infixl 9 :.
infixl 7 :*:
infixl 6 :+:


{--------------------------------------------------------------------
    Generic functor constructors
--------------------------------------------------------------------}

-- | Empty/zero type constructor (no inhabitants)
data Void a

voidF :: Void a -> b
voidF = error "voidF: no value of type Void"


-- | Unit type constructor (one inhabitant)
type Unit = Const ()

-- | The unit value
unit :: Unit ()
unit = Const ()

-- From Control.Compose:
-- 
--   newtype Id a = Id a

-- | Product on unary type constructors
data (f :*: g) a = f a :*: g a deriving (Show)

-- | Sum on unary type constructors
data (f :+: g) a = InL (f a) | InR (g a) deriving (Show)

eitherF :: (f a -> b) -> (g a -> b) -> (f :+: g) a -> b
eitherF p _ (InL fa) = p fa
eitherF _ q (InR ga) = q ga

-- From Control.Compose:
-- 
--   newtype (g :. f) a = O (g (f a))


{--------------------------------------------------------------------
    Functor and Applicative instances for generic constructors
--------------------------------------------------------------------}

instance Functor Void where
  fmap _ = error "Void fmap: no void value"  -- so ghc won't complain

-- deriving instance Functor Void
-- 
-- Leads to
-- 
-- ghc: panic! (the 'impossible' happened)
--   (GHC version 6.12.1 for i386-apple-darwin):
-- 	TcPat.checkArgs
-- 
-- See ticket <http://hackage.haskell.org/trac/ghc/ticket/4220>.
-- 
-- TODO: replace explicit definition with deriving, when the compiler fix
-- has been around for a while.


-- instance Functor Id where
--   fmap h (Id a) = Id (h a)

-- deriving instance Functor Id

-- instance (Functor f, Functor g) => Functor (f :+: g) where
--   fmap h (InL fa) = InL (fmap h fa)
--   fmap h (InR ga) = InR (fmap h ga)

-- i.e.,
-- 
--     fmap h . InL  ==  InL . fmap h
--     fmap h . InR  ==  InR . fmap h

deriving instance (Functor f, Functor g) => Functor (f :+: g)

-- instance (Functor f, Functor g) => Functor (f :*: g) where
--   fmap h (fa :*: ga) = fmap h fa :*: fmap h ga

-- Or:
deriving instance (Functor f, Functor g) => Functor (f :*: g)

-- TODO: Verify that the deriving instances are equivalent to the explicit versions.

-- What about Applicative instances?  I think Void could implement (<*>)
-- but not pure.  Hm.  Id and (:*:) are easy, while (:+:) is problematic.

-- instance Applicative Id where
--   pure a = Id a
--   Id f <*> Id x = Id (f x)

-- instance Applicative Id where
--   pure  = Id
--   (<*>) = inId2 ($)

instance (Applicative f, Applicative g) => Applicative (f :*: g) where
  pure a = pure a :*: pure a
  (f :*: g) <*> (a :*: b) = (f <*> a) :*: (g <*> b)

-- instance (Functor g, Functor f) => Functor (g :. f) where
--   fmap = inO.fmap.fmap

-- or

-- deriving instance (Functor g, Functor f) => Functor (g :. f)


{--------------------------------------------------------------------
    Some handy structural manipulators
--------------------------------------------------------------------}

pairF :: (f a, g a) -> (f :*: g) a
pairF (fa , ga) = (fa :*: ga)

-- pairF = uncurry (:*:)

unPairF :: (f :*: g) a -> (f a, g a)
unPairF (fa :*: ga) = (fa , ga)

-- Could also define curryF, uncurryF

inProd :: ((f a , g a) -> (h b , i b)) -> ((f :*: g) a -> (h :*: i) b)
inProd = unPairF ~> pairF


inProd2 :: ((f a , g a) -> (h b , i b) -> (j c , k c))
        -> ((f :*: g) a -> (h :*: i) b -> (j :*: k) c)
inProd2 = unPairF ~> inProd