{-# LANGUAGE 
  EmptyDataDecls, 
  FlexibleContexts, 
  FlexibleInstances, 
  GADTs, 
  MultiParamTypeClasses,
  RankNTypes, 
  ScopedTypeVariables,
  TypeOperators, 
  TypeFamilies,
  TypeSynonymInstances,
  UndecidableInstances  #-}
-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Category.Limit
-- Copyright   :  (c) Sjoerd Visscher 2010
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  sjoerd@w3future.com
-- Stability   :  experimental
-- Portability :  non-portable
-----------------------------------------------------------------------------
module Data.Category.Limit (

  -- * Prelimiairies
  
  -- ** Diagonal Functor
    Diag(..)
  , DiagF
  
  -- ** Cones
  , Cone
  , Cocone
  , coneVertex
  , coconeVertex
  
  -- * Limits
  , LimitFam
  , Limit
  , LimitUniversal
  , limitUniversal
  , limit
  , limitFactorizer
  
  -- * Colimits
  , ColimitFam
  , Colimit
  , ColimitUniversal
  , colimitUniversal
  , colimit
  , colimitFactorizer
  
  -- * Limits of a certain type
  , HasLimits(..)
  , HasColimits(..)
  
  -- ** As a functor
  , LimitFunctor(..)
  , ColimitFunctor(..)
  
  -- ** Limits of type Void
  , HasTerminalObject(..)
  , HasInitialObject(..)
  , Zero
  
  -- ** Limits of type Pair
  , BinaryProduct
  , HasBinaryProducts(..)
  , ProductFunctor(..)
  , (:*:)
  , BinaryCoproduct
  , HasBinaryCoproducts(..)
  , CoproductFunctor(..)
  , (:+:)
  
  -- ** Limits of type Hask
  , ForAll(..)
  , endoHaskLimit
  , Exists(..)
  , endoHaskColimit
  
) where

import Prelude hiding ((.), Functor, product)
import qualified Prelude (Functor)
import qualified Control.Arrow as A ((&&&), (***), (|||), (+++))

import Data.Category
import Data.Category.Functor
import Data.Category.NaturalTransformation
import Data.Category.Product
import Data.Category.Discrete

infixl 3 ***
infixl 3 &&&
infixl 2 +++
infixl 2 |||


-- | The diagonal functor from (index-) category J to (~>).
data Diag :: (* -> * -> *) -> (* -> * -> *) -> * where
  Diag :: Diag j (~>)
  
type instance Dom (Diag j (~>)) = (~>)
type instance Cod (Diag j (~>)) = Nat j (~>)
type instance Diag j (~>) :% a = Const j (~>) a

instance (Category j, Category (~>)) => Functor (Diag j (~>)) where 
  Diag % f = Nat (Const $ src f) (Const $ tgt f) $ const f

-- | The diagonal functor with the same domain and codomain as @f@.
type DiagF f = Diag (Dom f) (Cod f)



-- | A cone from N to F is a natural transformation from the constant functor to N to F.
type Cone   f n = Nat (Dom f) (Cod f) (ConstF f n) f

-- | A co-cone from F to N is a natural transformation from F to the constant functor to N.
type Cocone f n = Nat (Dom f) (Cod f) f (ConstF f n)


-- | The vertex (or apex) of a cone.
coneVertex :: Cone f n -> Obj (Cod f) n
coneVertex (Nat (Const x) _ _) = x

-- | The vertex (or apex) of a co-cone.
coconeVertex :: Cocone f n -> Obj (Cod f) n
coconeVertex (Nat _ (Const x) _) = x



-- | Limits in a category @(~>)@ by means of a diagram of type @j@, which is a functor from @j@ to @(~>)@.
type family LimitFam j (~>) f :: *

type Limit f = LimitFam (Dom f) (Cod f) f

-- | A limit of @f@ is a universal morphism from the diagonal functor to @f@.
type LimitUniversal f = TerminalUniversal f (DiagF f) (Limit f)

-- | @limitUniversal@ is a helper function to create the universal property from the limit and the limit factorizer.
limitUniversal :: (Cod f ~ (~>)) 
  => Cone f (Limit f)
  -> (forall n. Cone f n -> n ~> Limit f)
  -> LimitUniversal f
limitUniversal l lf = TerminalUniversal
  { tuObject           = coneVertex l
  , terminalMorphism   = l
  , terminalFactorizer = const lf
  }

-- | A limit of the diagram @f@ is a cone of @f@.
limit :: LimitUniversal f -> Cone f (Limit f)
limit = terminalMorphism

-- | For any other cone of @f@ with vertex @n@ there exists a unique morphism from @n@ to the limit of @f@.
limitFactorizer :: (Cod f ~ (~>)) => LimitUniversal f -> (forall n. Cone f n -> n ~> Limit f)
limitFactorizer lu c = terminalFactorizer lu (coneVertex c) c



-- | Colimits in a category @(~>)@ by means of a diagram of type @j@, which is a functor from @j@ to @(~>)@.
type family ColimitFam j (~>) f :: *

type Colimit f = ColimitFam (Dom f) (Cod f) f

-- | A colimit of @f@ is a universal morphism from @f@ to the diagonal functor.
type ColimitUniversal f = InitialUniversal f (DiagF f) (Colimit f)

-- | @colimitUniversal@ is a helper function to create the universal property from the colimit and the colimit factorizer.
colimitUniversal :: (Cod f ~ (~>)) 
  => Cocone f (Colimit f)
  -> (forall n. Cocone f n -> Colimit f ~> n)
  -> ColimitUniversal f
colimitUniversal l lf = InitialUniversal
  { iuObject          = coconeVertex l
  , initialMorphism   = l
  , initialFactorizer = const lf
  }

-- | A colimit of the diagram @f@ is a co-cone of @f@.
colimit :: ColimitUniversal f -> Cocone f (Colimit f)
colimit = initialMorphism

-- | For any other co-cone of @f@ with vertex @n@ there exists a unique morphism from the colimit of @f@ to @n@.
colimitFactorizer :: (Cod f ~ (~>)) => ColimitUniversal f -> (forall n. Cocone f n -> Colimit f ~> n)
colimitFactorizer cu c = initialFactorizer cu (coconeVertex c) c



-- | An instance of @HasLimits j (~>)@ says that @(~>)@ has all limits of type @j@.
class (Category j, Category (~>)) => HasLimits j (~>) where
  limitUniv :: Obj (Nat j (~>)) f -> LimitUniversal f

-- | If every diagram of type @j@ has a limit in @(~>)@ there exists a limit functor.
--
--   Applied to a natural transformation it is a generalisation of @(***)@:
--
--   @l@ '***' @r =@ 'LimitFunctor' '%' 'arrowPair' @l r@
data LimitFunctor :: (* -> * -> *) -> (* -> * -> *) -> * where
  LimitFunctor :: HasLimits j (~>) => LimitFunctor j (~>)

type instance Dom (LimitFunctor j (~>)) = Nat j (~>)
type instance Cod (LimitFunctor j (~>)) = (~>)
type instance LimitFunctor j (~>) :% f = LimitFam j (~>) f

instance (Category j, Category (~>)) => Functor (LimitFunctor j (~>)) where
  LimitFunctor % n @ Nat{}  = limitFactorizer (limitUniv (tgt n)) (n . limit (limitUniv (src n)))



-- | An instance of @HasColimits j (~>)@ says that @(~>)@ has all colimits of type @j@.
class (Category j, Category (~>)) => HasColimits j (~>) where
  colimitUniv :: Obj (Nat j (~>)) f -> ColimitUniversal f

-- | If every diagram of type @j@ has a colimit in @(~>)@ there exists a colimit functor.
--
--   Applied to a natural transformation it is a generalisation of @(+++)@:
--
--   @l@ '+++' @r =@ 'ColimitFunctor' '%' 'arrowPair' @l r@
data ColimitFunctor :: (* -> * -> *) -> (* -> * -> *) -> * where
  ColimitFunctor :: HasColimits j (~>) => ColimitFunctor j (~>)
  
type instance Dom (ColimitFunctor j (~>)) = Nat j (~>)
type instance Cod (ColimitFunctor j (~>)) = (~>)
type instance ColimitFunctor j (~>) :% f = ColimitFam j (~>) f

instance (Category j, Category (~>)) => Functor (ColimitFunctor j (~>)) where
  ColimitFunctor % n @ Nat{}  = colimitFactorizer (colimitUniv (src n)) (colimit (colimitUniv (tgt n)) . n)



-- | A terminal object is the limit of the functor from /0/ to (~>).
class Category (~>) => HasTerminalObject (~>) where
  
  type TerminalObject (~>) :: *
  
  terminalObject :: Obj (~>) (TerminalObject (~>))
  
  terminate :: Obj (~>) a -> a ~> TerminalObject (~>)


type instance LimitFam Void (~>) f = TerminalObject (~>)

instance (HasTerminalObject (~>)) => HasLimits Void (~>) where
  
  limitUniv (Nat f _ _) = limitUniversal
    (voidNat (Const terminalObject) f)
    (terminate . coneVertex)


-- | @()@ is the terminal object in @Hask@.
instance HasTerminalObject (->) where
  
  type TerminalObject (->) = ()
  
  terminalObject = id
  
  terminate _ _ = ()

-- | @Unit@ is the terminal category.
instance HasTerminalObject Cat where
  
  type TerminalObject Cat = CatW Unit
  
  terminalObject = CatA Id
  
  terminate (CatA _) = CatA $ Const Z


-- | An initial object is the colimit of the functor from /0/ to (~>).
class Category (~>) => HasInitialObject (~>) where
  
  type InitialObject (~>) :: *
  
  initialObject :: Obj (~>) (InitialObject (~>))

  initialize :: Obj (~>) a -> InitialObject (~>) ~> a


type instance ColimitFam Void (~>) f = InitialObject (~>)

instance HasInitialObject (~>) => HasColimits Void (~>) where
  
  colimitUniv (Nat f _ _) = colimitUniversal
    (voidNat f (Const initialObject))
    (initialize . coconeVertex)


-- | Any empty data type is an initial object in Hask.
data Zero

instance HasInitialObject (->) where
  
  type InitialObject (->) = Zero
  
  initialObject = id
  
  -- With thanks to Conor McBride
  initialize _ x = x `seq` error "we never get this far"

instance HasInitialObject Cat where
  
  type InitialObject Cat = CatW Void
  
  initialObject = CatA Id
  
  initialize (CatA _) = CatA Nil



type family BinaryProduct ((~>) :: * -> * -> *) x y :: *

-- | The product of 2 objects is the limit of the functor from Pair to (~>).
class Category (~>) => HasBinaryProducts (~>) where
  
  proj1 :: Obj (~>) x -> Obj (~>) y -> BinaryProduct (~>) x y ~> x
  proj2 :: Obj (~>) x -> Obj (~>) y -> BinaryProduct (~>) x y ~> y

  (&&&) :: (a ~> x) -> (a ~> y) -> (a ~> BinaryProduct (~>) x y)

  (***) :: (a1 ~> b1) -> (a2 ~> b2) -> (BinaryProduct (~>) a1 a2 ~> BinaryProduct (~>) b1 b2)
  l *** r = (l . proj1 (src l) (src r)) &&& (r . proj2 (src l) (src r)) where


type instance LimitFam Pair (~>) f = BinaryProduct (~>) (f :% Z) (f :% S Z)

instance HasBinaryProducts (~>) => HasLimits Pair (~>) where

  limitUniv (Nat f _ _) = limitUniversal
    (pairNat (Const $ x *** y) f (Com $ proj1 x y) (Com $ proj2 x y))
    (\c -> c ! Z &&& c ! S Z)
    where
      x = f % Z
      y = f % S Z


type instance BinaryProduct (->) x y = (x, y)

instance HasBinaryProducts (->) where
  
  proj1 _ _ = fst
  proj2 _ _ = snd
  
  (&&&) = (A.&&&)
  (***) = (A.***)

type instance BinaryProduct Cat (CatW c1) (CatW c2) = CatW (c1 :**: c2)

instance HasBinaryProducts Cat where
  
  proj1 (CatA _) (CatA _) = CatA Proj1
  proj2 (CatA _) (CatA _) = CatA Proj2
  
  CatA f1 &&& CatA f2 = CatA ((f1 :***: f2) :.: DiagProd)
  CatA f1 *** CatA f2 = CatA (f1 :***: f2)


type instance BinaryProduct (c1 :**: c2) (x1, x2) (y1, y2) = (BinaryProduct c1 x1 y1, BinaryProduct c2 x2 y2)

instance (HasBinaryProducts c1, HasBinaryProducts c2) => HasBinaryProducts (c1 :**: c2) where
  
  proj1 (x1 :**: x2) (y1 :**: y2) = proj1 x1 y1 :**: proj1 x2 y2
  proj2 (x1 :**: x2) (y1 :**: y2) = proj2 x1 y1 :**: proj2 x2 y2
  
  (f1 :**: f2) &&& (g1 :**: g2) = (f1 &&& g1) :**: (f2 &&& g2)
  (f1 :**: f2) *** (g1 :**: g2) = (f1 *** g1) :**: (f2 *** g2)


-- | Binary product as a bifunctor.
data ProductFunctor ((~>) :: * -> * -> *) = ProductFunctor
type instance Dom (ProductFunctor (~>)) = (~>) :**: (~>)
type instance Cod (ProductFunctor (~>)) = (~>)
type instance ProductFunctor (~>) :% (a, b) = BinaryProduct (~>) a b
instance HasBinaryProducts (~>) => Functor (ProductFunctor (~>)) where
  ProductFunctor % (a1 :**: a2) = a1 *** a2

-- | The product of two functors.
data p :*: q where 
  (:*:) :: (Functor p, Functor q, Dom p ~ Dom q, Cod p ~ (~>), Cod q ~ (~>), HasBinaryProducts (~>)) => p -> q -> p :*: q
type instance Dom (p :*: q) = Dom p
type instance Cod (p :*: q) = Cod p
type instance (p :*: q) :% a = BinaryProduct (Cod p) (p :% a) (q :% a)
instance (Category (Dom p), Category (Cod p)) => Functor (p :*: q) where
  (p :*: q) % f = (p % f) *** (q % f)

type instance BinaryProduct (Nat c d) x y = x :*: y

instance (Category c, HasBinaryProducts d) => HasBinaryProducts (Nat c d) where
  
  proj1 (Nat f _ _) (Nat g _ _) = Nat (f :*: g) f $ \z -> proj1 (f % z) (g % z)
  proj2 (Nat f _ _) (Nat g _ _) = Nat (f :*: g) g $ \z -> proj2 (f % z) (g % z)
  
  Nat a f af &&& Nat _ g ag = Nat a (f :*: g) $ \z -> af z &&& ag z
  Nat f1 f2 f *** Nat g1 g2 g = Nat (f1 :*: g1) (f2 :*: g2) $ \z -> f z *** g z
  
  

type family BinaryCoproduct ((~>) :: * -> * -> *) x y :: *

-- | The coproduct of 2 objects is the colimit of the functor from Pair to (~>).
class Category (~>) => HasBinaryCoproducts (~>) where

  inj1 :: Obj (~>) x -> Obj (~>) y -> x ~> BinaryCoproduct (~>) x y
  inj2 :: Obj (~>) x -> Obj (~>) y -> y ~> BinaryCoproduct (~>) x y

  (|||) :: (x ~> a) -> (y ~> a) -> (BinaryCoproduct (~>) x y ~> a)
    
  (+++) :: (a1 ~> b1) -> (a2 ~> b2) -> (BinaryCoproduct (~>) a1 a2 ~> BinaryCoproduct (~>) b1 b2)
  l +++ r = (inj1 (tgt l) (tgt r) . l) ||| (inj2 (tgt l) (tgt r) . r) where
    

type instance ColimitFam Pair (~>) f = BinaryCoproduct (~>) (f :% Z) (f :% S Z)

instance HasBinaryCoproducts (~>) => HasColimits Pair (~>) where
  
  colimitUniv (Nat f _ _) = colimitUniversal
    (pairNat f (Const $ x +++ y) (Com $ inj1 x y) (Com $ inj2 x y))
    (\c -> c ! Z ||| c ! S Z)
    where
      x = f % Z
      y = f % S Z


type instance BinaryCoproduct (->) x y = Either x y

instance HasBinaryCoproducts (->) where
  
  inj1 _ _ = Left
  inj2 _ _ = Right
  
  (|||) = (A.|||)
  (+++) = (A.+++)
  
  
type instance BinaryCoproduct (c1 :**: c2) (x1, x2) (y1, y2) = (BinaryCoproduct c1 x1 y1, BinaryCoproduct c2 x2 y2)

instance (HasBinaryCoproducts c1, HasBinaryCoproducts c2) => HasBinaryCoproducts (c1 :**: c2) where
  
  inj1 (x1 :**: x2) (y1 :**: y2) = inj1 x1 y1 :**: inj1 x2 y2
  inj2 (x1 :**: x2) (y1 :**: y2) = inj2 x1 y1 :**: inj2 x2 y2
  
  (f1 :**: f2) ||| (g1 :**: g2) = (f1 ||| g1) :**: (f2 ||| g2)
  (f1 :**: f2) +++ (g1 :**: g2) = (f1 +++ g1) :**: (f2 +++ g2)


-- | Binary coproduct as a bifunctor.
data CoproductFunctor ((~>) :: * -> * -> *) = CoproductFunctor
type instance Dom (CoproductFunctor (~>)) = (~>) :**: (~>)
type instance Cod (CoproductFunctor (~>)) = (~>)
type instance CoproductFunctor (~>) :% (a, b) = BinaryCoproduct (~>) a b
instance HasBinaryCoproducts (~>) => Functor (CoproductFunctor (~>)) where
  CoproductFunctor % (a1 :**: a2) = a1 +++ a2

-- | The coproduct of two functors.
data p :+: q where 
  (:+:) :: (Functor p, Functor q, Dom p ~ Dom q, Cod p ~ (~>), Cod q ~ (~>), HasBinaryCoproducts (~>)) => p -> q -> p :+: q
type instance Dom (p :+: q) = Dom p
type instance Cod (p :+: q) = Cod p
type instance (p :+: q) :% a = BinaryCoproduct (Cod p) (p :% a) (q :% a)
instance (Category (Dom p), Category (Cod p)) => Functor (p :+: q) where
  (p :+: q) % f = (p % f) +++ (q % f)

type instance BinaryCoproduct (Nat c d) x y = x :+: y

instance (Category c, HasBinaryCoproducts d) => HasBinaryCoproducts (Nat c d) where
  
  inj1 (Nat f _ _) (Nat g _ _) = Nat f (f :+: g) $ \z -> inj1 (f % z) (g % z)
  inj2 (Nat f _ _) (Nat g _ _) = Nat g (f :+: g) $ \z -> inj2 (f % z) (g % z)
  
  Nat f a fa ||| Nat g _ ga = Nat (f :+: g) a $ \z -> fa z ||| ga z
  Nat f1 f2 f +++ Nat g1 g2 g = Nat (f1 :+: g1) (f2 :+: g2) $ \z -> f z +++ g z



newtype ForAll f = ForAll { unForAll :: forall a. f a }

type instance LimitFam (->) (->) (EndoHask f) = ForAll f

endoHaskLimit :: Prelude.Functor f => LimitUniversal (EndoHask f)
endoHaskLimit = limitUniversal
  (Nat (Const id) EndoHask $ \_ -> unForAll)
  (\c n -> ForAll ((c ! id) n)) -- ForAll . (c ! id)


data Exists f = forall a. Exists (f a)

type instance ColimitFam (->) (->) (EndoHask f) = Exists f

endoHaskColimit :: Prelude.Functor f => ColimitUniversal (EndoHask f)
endoHaskColimit = colimitUniversal
  (Nat EndoHask (Const id) $ \_ -> Exists)
  (\c (Exists fa) -> (c ! id) fa) -- (c ! id) . unExists