{-# LANGUAGE CPP, TypeFamilies, Rank2Types, FlexibleContexts, FlexibleInstances, GADTs, StandaloneDeriving, UndecidableInstances #-}

-- explicit dictionary higher-kind instances are defined in
-- - base-4.9
-- - transformers >= 0.5
-- - transformes-compat >= 0.5 when transformers aren't 0.4
#define EXPLICIT_DICT_FUNCTOR_CLASSES (MIN_VERSION_base(4,9,0) || MIN_VERSION_transformers(0,5,0) || (MIN_VERSION_transformers_compat(0,5,0) && !MIN_VERSION_transformers(0,4,0)))

#define HAS_GENERIC (__GLASGOW_HASKELL__ >= 702)
#define HAS_GENERIC1 (__GLASGOW_HASKELL__ >= 706)

-- Polymorphic typeable
#define HAS_POLY_TYPEABLE MIN_VERSION_base(4,7,0)

#ifdef __GLASGOW_HASKELL__
{-# LANGUAGE DeriveDataTypeable #-}
#if __GLASGOW_HASKELL__ >= 800
{-# LANGUAGE ConstrainedClassMethods #-}
#endif
#if HAS_GENERIC
{-# LANGUAGE DeriveGeneric #-}
#endif
#endif



-----------------------------------------------------------------------------
-- |
-- Copyright   :  (C) 2008-2015 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  experimental
-- Portability :  non-portable
--
----------------------------------------------------------------------------
module Data.Functor.Foldable
  (
  -- * Base functors for fixed points
    Base
  , ListF(..)
  -- * Fixed points
  , Fix(..), unfix
  , Mu(..)
  , Nu(..)
  -- * Folding
  , Recursive(..)
  -- ** Combinators
  , gapo
  , gcata
  , zygo
  , gzygo
  , histo
  , ghisto
  , futu
  , chrono
  , gchrono
  -- ** Distributive laws
  , distCata
  , distPara
  , distParaT
  , distZygo
  , distZygoT
  , distHisto
  , distGHisto
  , distFutu
  , distGFutu
  -- * Unfolding
  , Corecursive(..)
  -- ** Combinators
  , gana
  -- ** Distributive laws
  , distAna
  , distApo
  , distGApo
  , distGApoT
  -- * Refolding
  , hylo
  , ghylo
  -- ** Changing representation
  , refix
  -- * Common names
  , fold, gfold
  , unfold, gunfold
  , refold, grefold
  -- * Mendler-style
  , mcata
  , mhisto
  -- * Elgot (co)algebras
  , elgot
  , coelgot
  -- * Zygohistomorphic prepromorphisms
  , zygoHistoPrepro
  ) where

import Control.Applicative
import Control.Comonad
import Control.Comonad.Trans.Class
import Control.Comonad.Trans.Env
import qualified Control.Comonad.Cofree as Cofree
import Control.Comonad.Cofree (Cofree(..))
import Control.Monad (liftM, join)
import Control.Monad.Free (Free(..))
import Control.Monad.Trans.Except (ExceptT(..), runExceptT)
import Data.Functor.Identity
import Control.Arrow
import Data.Function (on)
import Data.Functor.Classes
import Text.Read
import Text.Show
#ifdef __GLASGOW_HASKELL__
import Data.Data hiding (gunfold)
#if HAS_POLY_TYPEABLE
#else
import qualified Data.Data as Data
#endif
#if HAS_GENERIC
import GHC.Generics (Generic)
#endif
#if HAS_GENERIC1
import GHC.Generics (Generic1)
#endif
#endif

import Data.Monoid (Monoid (..))
import Prelude

import qualified Data.Foldable as F
import qualified Data.Traversable as T

import qualified Data.Bifunctor as Bi
import qualified Data.Bifoldable as Bi
import qualified Data.Bitraversable as Bi

type family Base t :: * -> *

class Functor (Base t) => Recursive t where
  project :: t -> Base t t

  cata :: (Base t a -> a) -- ^ a (Base t)-algebra
       -> t               -- ^ fixed point
       -> a               -- ^ result
  cata f = c where c = f . fmap c . project

  para :: (Base t (t, a) -> a) -> t -> a
  para t = p where p x = t . fmap ((,) <*> p) $ project x

  gpara :: (Corecursive t, Comonad w) => (forall b. Base t (w b) -> w (Base t b)) -> (Base t (EnvT t w a) -> a) -> t -> a
  gpara t = gzygo embed t

  -- | Fokkinga's prepromorphism
  prepro
    :: Corecursive t
    => (forall b. Base t b -> Base t b)
    -> (Base t a -> a)
    -> t
    -> a
  prepro e f = c where c = f . fmap (c . cata (embed . e)) . project

  --- | A generalized prepromorphism
  gprepro
    :: (Corecursive t, Comonad w)
    => (forall b. Base t (w b) -> w (Base t b))
    -> (forall c. Base t c -> Base t c)
    -> (Base t (w a) -> a)
    -> t
    -> a
  gprepro k e f = extract . c where c = fmap f . k . fmap (duplicate . c . cata (embed . e)) . project

distPara :: Corecursive t => Base t (t, a) -> (t, Base t a)
distPara = distZygo embed

distParaT :: (Corecursive t, Comonad w) => (forall b. Base t (w b) -> w (Base t b)) -> Base t (EnvT t w a) -> EnvT t w (Base t a)
distParaT t = distZygoT embed t

class Functor (Base t) => Corecursive t where
  embed :: Base t t -> t
  ana
    :: (a -> Base t a) -- ^ a (Base t)-coalgebra
    -> a               -- ^ seed
    -> t               -- ^ resulting fixed point
  ana g = a where a = embed . fmap a . g

  apo :: (a -> Base t (Either t a)) -> a -> t
  apo g = a where a = embed . (fmap (either id a)) . g

  -- | Fokkinga's postpromorphism
  postpro
    :: Recursive t
    => (forall b. Base t b -> Base t b) -- natural transformation
    -> (a -> Base t a)                  -- a (Base t)-coalgebra
    -> a                                -- seed
    -> t
  postpro e g = a where a = embed . fmap (ana (e . project) . a) . g

  -- | A generalized postpromorphism
  gpostpro
    :: (Recursive t, Monad m)
    => (forall b. m (Base t b) -> Base t (m b)) -- distributive law
    -> (forall c. Base t c -> Base t c)         -- natural transformation
    -> (a -> Base t (m a))                      -- a (Base t)-m-coalgebra
    -> a                                        -- seed
    -> t
  gpostpro k e g = a . return where a = embed . fmap (ana (e . project) . a . join) . k . liftM g

hylo :: Functor f => (f b -> b) -> (a -> f a) -> a -> b
hylo f g = h where h = f . fmap h . g

fold :: Recursive t => (Base t a -> a) -> t -> a
fold = cata

unfold :: Corecursive t => (a -> Base t a) -> a -> t
unfold = ana

refold :: Functor f => (f b -> b) -> (a -> f a) -> a -> b
refold = hylo

-- | Base functor of @[]@.
data ListF a b = Nil | Cons a b
  deriving (Eq,Ord,Show,Read,Typeable
#if HAS_GENERIC
          , Generic
#endif
#if HAS_GENERIC1
          , Generic1
#endif
          )

#if EXPLICIT_DICT_FUNCTOR_CLASSES
instance Eq2 ListF where
  liftEq2 _ _ Nil        Nil          = True
  liftEq2 f g (Cons a b) (Cons a' b') = f a a' && g b b'
  liftEq2 _ _ _          _            = False

instance Eq a => Eq1 (ListF a) where
  liftEq = liftEq2 (==)

instance Ord2 ListF where
  liftCompare2 _ _ Nil        Nil          = EQ
  liftCompare2 _ _ Nil        _            = LT
  liftCompare2 _ _ _          Nil          = GT
  liftCompare2 f g (Cons a b) (Cons a' b') = f a a' `mappend` g b b'

instance Ord a => Ord1 (ListF a) where
  liftCompare = liftCompare2 compare

instance Show a => Show1 (ListF a) where
  liftShowsPrec = liftShowsPrec2 showsPrec showList

instance Show2 ListF where
  liftShowsPrec2 _  _ _  _ _ Nil        = showString "Nil"
  liftShowsPrec2 sa _ sb _ d (Cons a b) = showParen (d > 10)
    $ showString "Cons "
    . sa 11 a
    . showString " "
    . sb 11 b

instance Read2 ListF where
  liftReadsPrec2 ra _ rb _ d = readParen (d > 10) $ \s -> nil s ++ cons s
    where
      nil s0 = do
        ("Nil", s1) <- lex s0
        return (Nil, s1)
      cons s0 = do
        ("Cons", s1) <- lex s0
        (a,      s2) <- ra 11 s1
        (b,      s3) <- rb 11 s2
        return (Cons a b, s3)

instance Read a => Read1 (ListF a) where
  liftReadsPrec = liftReadsPrec2 readsPrec readList

#else
instance Eq a   => Eq1   (ListF a) where eq1        = (==)
instance Ord a  => Ord1  (ListF a) where compare1   = compare
instance Show a => Show1 (ListF a) where showsPrec1 = showsPrec
instance Read a => Read1 (ListF a) where readsPrec1 = readsPrec
#endif

-- These instances cannot be auto-derived on with GHC <= 7.6
instance Functor (ListF a) where
  fmap _ Nil        = Nil
  fmap f (Cons a b) = Cons a (f b)

instance F.Foldable (ListF a) where
  foldMap _ Nil        = Data.Monoid.mempty
  foldMap f (Cons _ b) = f b

instance T.Traversable (ListF a) where
  traverse _ Nil        = pure Nil
  traverse f (Cons a b) = Cons a <$> f b

instance Bi.Bifunctor ListF where
  bimap _ _ Nil        = Nil
  bimap f g (Cons a b) = Cons (f a) (g b)

instance Bi.Bifoldable ListF where
  bifoldMap _ _ Nil        = mempty
  bifoldMap f g (Cons a b) = mappend (f a) (g b)

instance Bi.Bitraversable ListF where
  bitraverse _ _ Nil        = pure Nil
  bitraverse f g (Cons a b) = Cons <$> f a <*> g b

type instance Base [a] = ListF a
instance Recursive [a] where
  project (x:xs) = Cons x xs
  project [] = Nil

  para f (x:xs) = f (Cons x (xs, para f xs))
  para f [] = f Nil

instance Corecursive [a] where
  embed (Cons x xs) = x:xs
  embed Nil = []

  apo f a = case f a of
    Cons x (Left xs) -> x : xs
    Cons x (Right b) -> x : apo f b
    Nil -> []

-- | Example boring stub for non-recursive data types
type instance Base (Maybe a) = Const (Maybe a)
instance Recursive (Maybe a) where project = Const
instance Corecursive (Maybe a) where embed = getConst

-- | Example boring stub for non-recursive data types
type instance Base (Either a b) = Const (Either a b)
instance Recursive (Either a b) where project = Const
instance Corecursive (Either a b) where embed = getConst

-- | A generalized catamorphism
gfold, gcata
  :: (Recursive t, Comonad w)
  => (forall b. Base t (w b) -> w (Base t b)) -- ^ a distributive law
  -> (Base t (w a) -> a)                      -- ^ a (Base t)-w-algebra
  -> t                                        -- ^ fixed point
  -> a
gcata k g = g . extract . c where
  c = k . fmap (duplicate . fmap g . c) . project
gfold k g t = gcata k g t

distCata :: Functor f => f (Identity a) -> Identity (f a)
distCata = Identity . fmap runIdentity

-- | A generalized anamorphism
gunfold, gana
  :: (Corecursive t, Monad m)
  => (forall b. m (Base t b) -> Base t (m b)) -- ^ a distributive law
  -> (a -> Base t (m a))                      -- ^ a (Base t)-m-coalgebra
  -> a                                        -- ^ seed
  -> t
gana k f = a . return . f where
  a = embed . fmap (a . liftM f . join) . k
gunfold k f t = gana k f t

distAna :: Functor f => Identity (f a) -> f (Identity a)
distAna = fmap Identity . runIdentity

-- | A generalized hylomorphism
grefold, ghylo
  :: (Comonad w, Functor f, Monad m)
  => (forall c. f (w c) -> w (f c))
  -> (forall d. m (f d) -> f (m d))
  -> (f (w b) -> b)
  -> (a -> f (m a))
  -> a
  -> b
ghylo w m f g = extract . h . return where
  h = fmap f . w . fmap (duplicate . h . join) . m . liftM g
grefold w m f g a = ghylo w m f g a

futu :: Corecursive t => (a -> Base t (Free (Base t) a)) -> a -> t
futu = gana distFutu

distFutu :: Functor f => Free f (f a) -> f (Free f a)
distFutu = distGFutu id

distGFutu :: (Functor f, Functor h) => (forall b. h (f b) -> f (h b)) -> Free h (f a) -> f (Free h a)
distGFutu _ (Pure fa) = Pure <$> fa
distGFutu k (Free as) = Free <$> k (distGFutu k <$> as)

-------------------------------------------------------------------------------
-- Fix
-------------------------------------------------------------------------------

newtype Fix f = Fix (f (Fix f))

unfix :: Fix f -> f (Fix f)
unfix (Fix f) = f

instance Eq1 f => Eq (Fix f) where
  Fix a == Fix b = eq1 a b

instance Ord1 f => Ord (Fix f) where
  compare (Fix a) (Fix b) = compare1 a b

instance Show1 f => Show (Fix f) where
  showsPrec d (Fix a) =
    showParen (d >= 11)
      $ showString "Fix "
      . showsPrec1 11 a

instance Read1 f => Read (Fix f) where
  readPrec = parens $ prec 10 $ do
    Ident "Fix" <- lexP
    Fix <$> step (readS_to_Prec readsPrec1)

#ifdef __GLASGOW_HASKELL__
#if HAS_POLY_TYPEABLE
deriving instance Typeable Fix
deriving instance (Typeable f, Data (f (Fix f))) => Data (Fix f)
#else
instance Typeable1 f => Typeable (Fix f) where
   typeOf t = mkTyConApp fixTyCon [typeOf1 (undefined `asArgsTypeOf` t)]
     where asArgsTypeOf :: f a -> Fix f -> f a
           asArgsTypeOf = const

fixTyCon :: TyCon
#if MIN_VERSION_base(4,4,0)
fixTyCon = mkTyCon3 "recursion-schemes" "Data.Functor.Foldable" "Fix"
#else
fixTyCon = mkTyCon "Data.Functor.Foldable.Fix"
#endif
{-# NOINLINE fixTyCon #-}

instance (Typeable1 f, Data (f (Fix f))) => Data (Fix f) where
  gfoldl f z (Fix a) = z Fix `f` a
  toConstr _ = fixConstr
  gunfold k z c = case constrIndex c of
    1 -> k (z (Fix))
    _ -> error "gunfold"
  dataTypeOf _ = fixDataType

fixConstr :: Constr
fixConstr = mkConstr fixDataType "Fix" [] Prefix

fixDataType :: DataType
fixDataType = mkDataType "Data.Functor.Foldable.Fix" [fixConstr]
#endif
#endif

type instance Base (Fix f) = f
instance Functor f => Recursive (Fix f) where
  project (Fix a) = a
instance Functor f => Corecursive (Fix f) where
  embed = Fix

refix :: (Recursive s, Corecursive t, Base s ~ Base t) => s -> t
refix = cata embed

toFix :: Recursive t => t -> Fix (Base t)
toFix = refix

fromFix :: Corecursive t => Fix (Base t) -> t
fromFix = refix

-------------------------------------------------------------------------------
-- Lambek
-------------------------------------------------------------------------------

-- | Lambek's lemma provides a default definition for 'project' in terms of 'cata' and 'embed'
lambek :: (Recursive t, Corecursive t) => (t -> Base t t)
lambek = cata (fmap embed)

-- | The dual of Lambek's lemma, provides a default definition for 'embed' in terms of 'ana' and 'project'
colambek :: (Recursive t, Corecursive t) => (Base t t -> t)
colambek = ana (fmap project)

newtype Mu f = Mu (forall a. (f a -> a) -> a)
type instance Base (Mu f) = f
instance Functor f => Recursive (Mu f) where
  project = lambek
  cata f (Mu g) = g f
instance Functor f => Corecursive (Mu f) where
  embed m = Mu (\f -> f (fmap (fold f) m))

instance (Functor f, Eq1 f) => Eq (Mu f) where
  (==) = (==) `on` toFix

instance (Functor f, Ord1 f) => Ord (Mu f) where
  compare = compare `on` toFix

instance (Functor f, Show1 f) => Show (Mu f) where
  showsPrec d f = showParen (d > 10) $
    showString "fromFix " . showsPrec 11 (toFix f)

#ifdef __GLASGOW_HASKELL__
instance (Functor f, Read1 f) => Read (Mu f) where
  readPrec = parens $ prec 10 $ do
    Ident "fromFix" <- lexP
    fromFix <$> step readPrec
#endif

data Nu f where Nu :: (a -> f a) -> a -> Nu f
type instance Base (Nu f) = f
instance Functor f => Corecursive (Nu f) where
  embed = colambek
  ana = Nu
instance Functor f => Recursive (Nu f) where
  project (Nu f a) = Nu f <$> f a

instance (Functor f, Eq1 f) => Eq (Nu f) where
  (==) = (==) `on` toFix

instance (Functor f, Ord1 f) => Ord (Nu f) where
  compare = compare `on` toFix

instance (Functor f, Show1 f) => Show (Nu f) where
  showsPrec d f = showParen (d > 10) $
    showString "fromFix " . showsPrec 11 (toFix f)

#ifdef __GLASGOW_HASKELL__
instance (Functor f, Read1 f) => Read (Nu f) where
  readPrec = parens $ prec 10 $ do
    Ident "fromFix" <- lexP
    fromFix <$> step readPrec
#endif

zygo :: Recursive t => (Base t b -> b) -> (Base t (b, a) -> a) -> t -> a
zygo f = gfold (distZygo f)

distZygo
  :: Functor f
  => (f b -> b)             -- An f-algebra
  -> (f (b, a) -> (b, f a)) -- ^ A distributive for semi-mutual recursion
distZygo g m = (g (fmap fst m), fmap snd m)

gzygo
  :: (Recursive t, Comonad w)
  => (Base t b -> b)
  -> (forall c. Base t (w c) -> w (Base t c))
  -> (Base t (EnvT b w a) -> a)
  -> t
  -> a
gzygo f w = gfold (distZygoT f w)

distZygoT
  :: (Functor f, Comonad w)
  => (f b -> b)                        -- An f-w-algebra to use for semi-mutual recursion
  -> (forall c. f (w c) -> w (f c))    -- A base Distributive law
  -> f (EnvT b w a) -> EnvT b w (f a)  -- A new distributive law that adds semi-mutual recursion
distZygoT g k fe = EnvT (g (getEnv <$> fe)) (k (lower <$> fe))
  where getEnv (EnvT e _) = e

gapo :: Corecursive t => (b -> Base t b) -> (a -> Base t (Either b a)) -> a -> t
gapo g = gunfold (distGApo g)

distApo :: Recursive t => Either t (Base t a) -> Base t (Either t a)
distApo = distGApo project

distGApo :: Functor f => (b -> f b) -> Either b (f a) -> f (Either b a)
distGApo f = either (fmap Left . f) (fmap Right)

distGApoT
  :: (Functor f, Functor m)
  => (b -> f b)
  -> (forall c. m (f c) -> f (m c))
  -> ExceptT b m (f a)
  -> f (ExceptT b m a)
distGApoT g k = fmap ExceptT . k . fmap (distGApo g) . runExceptT

-- | Course-of-value iteration
histo :: Recursive t => (Base t (Cofree (Base t) a) -> a) -> t -> a
histo = gcata distHisto

ghisto :: (Recursive t, Functor h) => (forall b. Base t (h b) -> h (Base t b)) -> (Base t (Cofree h a) -> a) -> t -> a
ghisto g = gcata (distGHisto g)

distHisto :: Functor f => f (Cofree f a) -> Cofree f (f a)
distHisto = distGHisto id

distGHisto :: (Functor f, Functor h) => (forall b. f (h b) -> h (f b)) -> f (Cofree h a) -> Cofree h (f a)
distGHisto k = Cofree.unfold (\as -> (extract <$> as, k (Cofree.unwrap <$> as)))

chrono :: Functor f => (f (Cofree f b) -> b) -> (a -> f (Free f a)) -> (a -> b)
chrono = ghylo distHisto distFutu

gchrono :: (Functor f, Functor w, Functor m) =>
           (forall c. f (w c) -> w (f c)) ->
           (forall c. m (f c) -> f (m c)) ->
           (f (Cofree w b) -> b) -> (a -> f (Free m a)) ->
           (a -> b)
gchrono w m = ghylo (distGHisto w) (distGFutu m)

-- | Mendler-style iteration
mcata :: (forall y. (y -> c) -> f y -> c) -> Fix f -> c
mcata psi = psi (mcata psi) . unfix

-- | Mendler-style course-of-value iteration
mhisto :: (forall y. (y -> c) -> (y -> f y) -> f y -> c) -> Fix f -> c
mhisto psi = psi (mhisto psi) unfix . unfix

-- | Elgot algebras
elgot :: Functor f => (f a -> a) -> (b -> Either a (f b)) -> b -> a
elgot phi psi = h where h = (id ||| phi . fmap h) . psi

-- | Elgot coalgebras: <http://comonad.com/reader/2008/elgot-coalgebras/>
coelgot :: Functor f => ((a, f b) -> b) -> (a -> f a) -> a -> b
coelgot phi psi = h where h = phi . (id &&& fmap h . psi)

-- | Zygohistomorphic prepromorphisms:
--
-- A corrected and modernized version of <http://www.haskell.org/haskellwiki/Zygohistomorphic_prepromorphisms>
zygoHistoPrepro
  :: (Corecursive t, Recursive t)
  => (Base t b -> b)
  -> (forall c. Base t c -> Base t c)
  -> (Base t (EnvT b (Cofree (Base t)) a) -> a)
  -> t
  -> a
zygoHistoPrepro f g t = gprepro (distZygoT f distHisto) g t

-------------------------------------------------------------------------------
-- Not exposed anywhere
-------------------------------------------------------------------------------

-- | Read a list (using square brackets and commas), given a function
-- for reading elements.
_readListWith :: ReadS a -> ReadS [a]
_readListWith rp =
    readParen False (\r -> [pr | ("[",s) <- lex r, pr <- readl s])
  where
    readl s = [([],t) | ("]",t) <- lex s] ++
        [(x:xs,u) | (x,t) <- rp s, (xs,u) <- readl' t]
    readl' s = [([],t) | ("]",t) <- lex s] ++
        [(x:xs,v) | (",",t) <- lex s, (x,u) <- rp t, (xs,v) <- readl' u]