Lazy.hs

{-# LANGUAGE BangPatterns           #-}
{-# LANGUAGE CPP                    #-}
{-# LANGUAGE DataKinds              #-}
{-# LANGUAGE DeriveDataTypeable     #-}
{-# LANGUAGE EmptyCase              #-}
{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE GADTs                  #-}
{-# LANGUAGE RankNTypes             #-}
{-# LANGUAGE Safe                   #-}
{-# LANGUAGE ScopedTypeVariables    #-}
{-# LANGUAGE StandaloneDeriving     #-}
{-# LANGUAGE TypeFamilies           #-}
{-# LANGUAGE UndecidableInstances   #-}
-- | Lazy (in elements and spine) length-indexed list: 'Vec'.
module Data.Vec.Lazy (
    Vec (..),
    -- * Construction
    empty,
    singleton,
    withDict,
    -- * Conversions
    toPull,
    fromPull,
    toList,
    toNonEmpty,
    fromList,
    fromListPrefix,
    reifyList,
    -- * Indexing
    (!),
    tabulate,
    cons,
    snoc,
    head,
    last,
    tail,
    init,
    -- * Reverse
    reverse,
    -- * Concatenation and splitting
    (++),
    split,
    concatMap,
    concat,
    chunks,
    -- * Take and drop
    take,
    drop,
    -- * Folds
    foldMap,
    foldMap1,
    ifoldMap,
    ifoldMap1,
    foldr,
    ifoldr,
    foldl',
    -- * Special folds
    length,
    null,
    sum,
    product,
    -- * Mapping
    map,
    imap,
    traverse,
#ifdef MIN_VERSION_semigroupoids
    traverse1,
#endif
    itraverse,
    itraverse_,
    -- * Zipping
    zipWith,
    izipWith,
    repeat,
    -- * Monadic
    bind,
    join,
    -- * Universe
    universe,
    -- * VecEach
    VecEach (..),
    )  where

import Prelude
       (Bool (..), Eq (..), Functor (..), Int, Maybe (..), Monad (..), Num (..),
       Ord (..), Show (..), id, seq, showParen, showString, uncurry, ($), (.), (&&), Ordering (..))

import Control.Applicative (Applicative (..), (<$>))
import Control.DeepSeq     (NFData (..))
import Control.Lens.Yocto  ((<&>))
import Data.Fin            (Fin (..))
import Data.Hashable       (Hashable (..))
import Data.List.NonEmpty  (NonEmpty (..))
import Data.Monoid         (Monoid (..))
import Data.Nat            (Nat (..))
import Data.Semigroup      (Semigroup (..))
import Data.Typeable       (Typeable)

--- Instances
import qualified Data.Foldable    as I (Foldable (..))
import qualified Data.Traversable as I (Traversable (..))
import qualified Test.QuickCheck  as QC

import qualified Data.Foldable.WithIndex    as WI (FoldableWithIndex (..))
import qualified Data.Functor.WithIndex     as WI (FunctorWithIndex (..))
import qualified Data.Traversable.WithIndex as WI (TraversableWithIndex (..))

import Data.Functor.Classes (Eq1 (..), Ord1 (..), Show1 (..))

#ifdef MIN_VERSION_adjunctions
import qualified Data.Functor.Rep as I (Representable (..))
#endif

#ifdef MIN_VERSION_distributive
import Data.Distributive (Distributive (..))
#endif

#ifdef MIN_VERSION_semigroupoids
import Data.Functor.Apply (Apply (..))

import qualified Data.Functor.Bind          as I (Bind (..))
import qualified Data.Semigroup.Foldable    as I (Foldable1 (..))
import qualified Data.Semigroup.Traversable as I (Traversable1 (..))
#endif

-- vec siblings
import qualified Data.Fin      as F
import qualified Data.Type.Nat as N
import qualified Data.Vec.Pull as P

import qualified Data.Type.Nat.LE          as LE.ZS
import qualified Data.Type.Nat.LE.ReflStep as LE.RS


-- $setup
-- >>> :set -XScopedTypeVariables
-- >>> import Data.Proxy (Proxy (..))
-- >>> import Prelude (Char, not, uncurry, Bool (..), Maybe (..), ($), (<$>), id, (.), Int)
-- >>> import qualified Data.Type.Nat as N
-- >>> import Data.Fin (Fin (..))

-------------------------------------------------------------------------------
-- Type
-------------------------------------------------------------------------------

infixr 5 :::

-- | Vector, i.e. length-indexed list.
data Vec (n :: Nat) a where
    VNil  :: Vec 'Z a
    (:::) :: a -> Vec n a -> Vec ('S n) a
  deriving (Typeable)

-------------------------------------------------------------------------------
-- Instances
-------------------------------------------------------------------------------

deriving instance Eq a => Eq (Vec n a)
deriving instance Ord a => Ord (Vec n a)

instance Show a => Show (Vec n a) where
    showsPrec _ VNil       = showString "VNil"
    showsPrec d (x ::: xs) = showParen (d > 5)
        $ showsPrec 6 x
        . showString " ::: "
        . showsPrec 5 xs

instance Functor (Vec n) where
    fmap = map

instance I.Foldable (Vec n) where
    foldMap = foldMap

    foldr  = foldr
    foldl' = foldl'

#if MIN_VERSION_base(4,8,0)
    null    = null
    length  = length
    sum     = sum
    product = product
#endif

instance I.Traversable (Vec n) where
    traverse = traverse

-- | @since 0.4
instance WI.FunctorWithIndex (Fin n) (Vec n) where
    imap = imap

-- | @since 0.4
instance WI.FoldableWithIndex (Fin n) (Vec n) where
    ifoldMap = ifoldMap
    ifoldr   = ifoldr

-- | @since 0.4
instance WI.TraversableWithIndex (Fin n) (Vec n) where
    itraverse = itraverse

#ifdef MIN_VERSION_semigroupoids
instance n ~ 'S m => I.Foldable1 (Vec n) where
    foldMap1 = foldMap1

instance n ~ 'S m => I.Traversable1 (Vec n) where
    traverse1 = traverse1
#endif

instance NFData a => NFData (Vec n a) where
    rnf VNil       = ()
    rnf (x ::: xs) = rnf x `seq` rnf xs

instance Hashable a => Hashable (Vec n a) where
    hashWithSalt salt VNil = hashWithSalt salt (0 :: Int)
    hashWithSalt salt (x ::: xs) = salt
        `hashWithSalt` x
        `hashWithSalt` xs

instance N.SNatI n => Applicative (Vec n) where
    pure   = repeat
    (<*>)  = zipWith ($)
    _ *> x = x
    x <* _ = x
#if MIN_VERSION_base(4,10,0)
    liftA2 = zipWith
#endif

instance N.SNatI n => Monad (Vec n) where
    return = pure
    (>>=)  = bind
    _ >> x = x

#ifdef MIN_VERSION_distributive
instance N.SNatI n => Distributive (Vec n) where
    distribute f = tabulate (\k -> fmap (! k) f)

#ifdef MIN_VERSION_adjunctions
instance N.SNatI n => I.Representable (Vec n) where
    type Rep (Vec n) = Fin n
    tabulate = tabulate
    index    = (!)
#endif
#endif

instance Semigroup a => Semigroup (Vec n a) where
    (<>) = zipWith (<>)

instance (Monoid a, N.SNatI n) => Monoid (Vec n a) where
    mempty = pure mempty
    mappend = zipWith mappend

#ifdef MIN_VERSION_semigroupoids
instance Apply (Vec n) where
    (<.>)  = zipWith ($)
    _ .> x = x
    x <. _ = x
    liftF2 = zipWith

instance I.Bind (Vec n) where
    (>>-) = bind
    join  = join
#endif

-------------------------------------------------------------------------------
-- Data.Functor.Classes
-------------------------------------------------------------------------------

#ifndef MIN_VERSION_transformers_compat
#define MIN_VERSION_transformers_compat(x,y,z) 0
#endif

#if MIN_VERSION_base(4,9,0)
#define LIFTED_FUNCTOR_CLASSES 1
#else
#if MIN_VERSION_transformers(0,5,0)
#define LIFTED_FUNCTOR_CLASSES 1
#else
#if MIN_VERSION_transformers_compat(0,5,0) && !MIN_VERSION_transformers(0,4,0)
#define LIFTED_FUNCTOR_CLASSES 1
#endif
#endif
#endif

#if LIFTED_FUNCTOR_CLASSES
-- | @since 0.4
instance Eq1 (Vec n) where
    liftEq _eq VNil       VNil       = True
    liftEq  eq (x ::: xs) (y ::: ys) = eq x y && liftEq eq xs ys

-- | @since 0.4
instance Ord1 (Vec n) where
    liftCompare _cmp VNil       VNil       = EQ
    liftCompare  cmp (x ::: xs) (y ::: ys) = cmp x y <> liftCompare cmp xs ys

-- | @since 0.4
instance Show1 (Vec n) where
    liftShowsPrec _  _  _ VNil       = showString "VNil"
    liftShowsPrec sp sl d (x ::: xs) = showParen (d > 5)
        $ sp 6 x
        . showString " ::: "
        . liftShowsPrec sp sl 5 xs
#else
-- | @since 0.4
instance Eq1 (Vec n) where eq1 = (==)

-- | @since 0.4
instance Ord1 (Vec n) where compare1 = compare

-- | @since 0.4
instance Show1 (Vec n) where showsPrec1 = showsPrec
#endif

-------------------------------------------------------------------------------
-- Construction
-------------------------------------------------------------------------------

-- | Empty 'Vec'.
empty :: Vec 'Z a
empty = VNil

-- | 'Vec' with exactly one element.
--
-- >>> singleton True
-- True ::: VNil
--
singleton :: a -> Vec ('S 'Z) a
singleton x = x ::: VNil

-- | /O(n)/. Recover 'N.SNatI' (and 'N.SNatI') dictionary from a 'Vec' value.
--
-- Example: 'N.reflect' is constrained with @'N.SNatI' n@, but if we have a
-- @'Vec' n a@, we can recover that dictionary:
--
-- >>> let f :: forall n a. Vec n a -> N.Nat; f v = withDict v (N.reflect (Proxy :: Proxy n)) in f (True ::: VNil)
-- 1
--
-- /Note:/ using 'N.SNatI' will be suboptimal, as if GHC has no
-- opportunity to optimise the code, the recusion won't be unfold.
-- How bad such code will perform? I don't know, we'll need benchmarks.
--
withDict :: Vec n a -> (N.SNatI n => r) -> r
withDict VNil       r = r
withDict (_ ::: xs) r = withDict xs r

-------------------------------------------------------------------------------
-- Conversions
-------------------------------------------------------------------------------

-- | Convert to pull 'P.Vec'.
toPull :: Vec n a -> P.Vec n a
toPull VNil       = P.Vec F.absurd
toPull (x ::: xs) = P.Vec $ \n -> case n of
    FZ   -> x
    FS m -> P.unVec (toPull xs) m

-- | Convert from pull 'P.Vec'.
fromPull :: forall n a. N.SNatI n => P.Vec n a -> Vec n a
fromPull (P.Vec f) = case N.snat :: N.SNat n of
    N.SZ -> VNil
    N.SS -> f FZ ::: fromPull (P.Vec (f . FS))

-- | Convert 'Vec' to list.
--
-- >>> toList $ 'f' ::: 'o' ::: 'o' ::: VNil
-- "foo"
toList :: Vec n a -> [a]
toList VNil       = []
toList (x ::: xs) = x : toList xs

-- |  
--
-- >>> toNonEmpty $ 1 ::: 2 ::: 3 ::: VNil
-- 1 :| [2,3]
--
-- @since 0.4
toNonEmpty :: Vec ('S n) a -> NonEmpty a
toNonEmpty (x ::: xs) = x :| toList xs

-- | Convert list @[a]@ to @'Vec' n a@.
-- Returns 'Nothing' if lengths don't match exactly.
--
-- >>> fromList "foo" :: Maybe (Vec N.Nat3 Char)
-- Just ('f' ::: 'o' ::: 'o' ::: VNil)
--
-- >>> fromList "quux" :: Maybe (Vec N.Nat3 Char)
-- Nothing
--
-- >>> fromList "xy" :: Maybe (Vec N.Nat3 Char)
-- Nothing
--
fromList :: N.SNatI n => [a] -> Maybe (Vec n a)
fromList = getFromList (N.induction1 start step) where
    start :: FromList 'Z a
    start = FromList $ \xs -> case xs of
        []      -> Just VNil
        (_ : _) -> Nothing

    step :: FromList n a -> FromList ('N.S n) a
    step (FromList f) = FromList $ \xs -> case xs of
        []       -> Nothing
        (x : xs') -> (x :::) <$> f xs'

newtype FromList n a = FromList { getFromList :: [a] -> Maybe (Vec n a) }

-- | Convert list @[a]@ to @'Vec' n a@.
-- Returns 'Nothing' if input list is too short.
--
-- >>> fromListPrefix "foo" :: Maybe (Vec N.Nat3 Char)
-- Just ('f' ::: 'o' ::: 'o' ::: VNil)
--
-- >>> fromListPrefix "quux" :: Maybe (Vec N.Nat3 Char)
-- Just ('q' ::: 'u' ::: 'u' ::: VNil)
--
-- >>> fromListPrefix "xy" :: Maybe (Vec N.Nat3 Char)
-- Nothing
--
fromListPrefix :: N.SNatI n => [a] -> Maybe (Vec n a)
fromListPrefix = getFromList (N.induction1 start step) where
    start :: FromList 'Z a
    start = FromList $ \_ -> Just VNil -- different than in fromList case

    step :: FromList n a -> FromList ('N.S n) a
    step (FromList f) = FromList $ \xs -> case xs of
        []       -> Nothing
        (x : xs') -> (x :::) <$> f xs'

-- | Reify any list @[a]@ to @'Vec' n a@.
--
-- >>> reifyList "foo" length
-- 3
reifyList :: [a] -> (forall n. N.SNatI n => Vec n a -> r) -> r
reifyList []       f = f VNil
reifyList (x : xs) f = reifyList xs $ \xs' -> f (x ::: xs')

-------------------------------------------------------------------------------
-- Indexing
-------------------------------------------------------------------------------

-- | Indexing.
--
-- >>> ('a' ::: 'b' ::: 'c' ::: VNil) ! FS FZ
-- 'b'
--
(!) :: Vec n a -> Fin n -> a
(!) (x ::: _)  FZ     = x
(!) (_ ::: xs) (FS n) = xs ! n
(!) VNil n = case n of {}

-- | Tabulating, inverse of '!'.
--
-- >>> tabulate id :: Vec N.Nat3 (Fin N.Nat3)
-- 0 ::: 1 ::: 2 ::: VNil
--
tabulate :: N.SNatI n => (Fin n -> a) -> Vec n a
tabulate = fromPull . P.tabulate

-- | Cons an element in front of a 'Vec'.
cons :: a -> Vec n a -> Vec ('S n) a
cons = (:::)

-- | Add a single element at the end of a 'Vec'.
--
-- @since 0.2.1
snoc :: Vec n a -> a -> Vec ('S n) a
snoc VNil       x = x ::: VNil
snoc (y ::: ys) x = y ::: snoc ys x

-- | The first element of a 'Vec'.
head :: Vec ('S n) a -> a
head (x ::: _) = x

-- | The last element of a 'Vec'.
--
-- @since 0.4
last :: Vec ('S n) a -> a
last (x ::: VNil) = x
last (_ ::: xs@(_ ::: _)) = last xs

-- | The elements after the 'head' of a 'Vec'.
tail :: Vec ('S n) a -> Vec n a
tail (_ ::: xs) = xs

-- | The elements before the 'last' of a 'Vec'.
--
-- @since 0.4
init :: Vec ('S n) a -> Vec n a
init (_ ::: VNil) = VNil
init (x ::: xs@(_ ::: _)) = x ::: init xs

-------------------------------------------------------------------------------
-- Reverse
-------------------------------------------------------------------------------

-- | Reverse 'Vec'.
--
-- >>> reverse ('a' ::: 'b' ::: 'c' ::: VNil)
-- 'c' ::: 'b' ::: 'a' ::: VNil
--
-- @since 0.2.1
--
reverse :: Vec n a -> Vec n a
reverse VNil       = VNil
reverse (x ::: xs) = snoc (reverse xs) x

-------------------------------------------------------------------------------
-- Concatenation
-------------------------------------------------------------------------------

infixr 5 ++

-- | Append two 'Vec'.
--
-- >>> ('a' ::: 'b' ::: VNil) ++ ('c' ::: 'd' ::: VNil)
-- 'a' ::: 'b' ::: 'c' ::: 'd' ::: VNil
--
(++) :: Vec n a -> Vec m a -> Vec (N.Plus n m) a
VNil       ++ ys = ys
(x ::: xs) ++ ys = x ::: xs ++ ys

-- | Split vector into two parts. Inverse of '++'.
--
-- >>> split ('a' ::: 'b' ::: 'c' ::: VNil) :: (Vec N.Nat1 Char, Vec N.Nat2 Char)
-- ('a' ::: VNil,'b' ::: 'c' ::: VNil)
--
-- >>> uncurry (++) (split ('a' ::: 'b' ::: 'c' ::: VNil) :: (Vec N.Nat1 Char, Vec N.Nat2 Char))
-- 'a' ::: 'b' ::: 'c' ::: VNil
--
split :: N.SNatI n => Vec (N.Plus n m) a -> (Vec n a, Vec m a)
split = appSplit (N.induction1 start step) where
    start :: Split m 'Z a
    start = Split $ \xs -> (VNil, xs)

    step :: Split m n a -> Split m ('S n) a
    step (Split f) = Split $ \(x ::: xs) -> case f xs of
        (ys, zs) -> (x ::: ys, zs)

newtype Split m n a = Split { appSplit :: Vec (N.Plus n m) a -> (Vec n a, Vec m a) }

-- | Map over all the elements of a 'Vec' and concatenate the resulting 'Vec's.
--
-- >>> concatMap (\x -> x ::: x ::: VNil) ('a' ::: 'b' ::: VNil)
-- 'a' ::: 'a' ::: 'b' ::: 'b' ::: VNil
--
concatMap :: (a -> Vec m b) -> Vec n a -> Vec (N.Mult n m) b
concatMap _ VNil       = VNil
concatMap f (x ::: xs) = f x ++ concatMap f xs

-- | @'concatMap' 'id'@
concat :: Vec n (Vec m a) -> Vec (N.Mult n m) a
concat = concatMap id

-- | Inverse of 'concat'.
--
-- >>> chunks <$> fromListPrefix [1..] :: Maybe (Vec N.Nat2 (Vec N.Nat3 Int))
-- Just ((1 ::: 2 ::: 3 ::: VNil) ::: (4 ::: 5 ::: 6 ::: VNil) ::: VNil)
--
-- >>> let idVec x = x :: Vec N.Nat2 (Vec N.Nat3 Int)
-- >>> concat . idVec . chunks <$> fromListPrefix [1..]
-- Just (1 ::: 2 ::: 3 ::: 4 ::: 5 ::: 6 ::: VNil)
--
chunks :: (N.SNatI n, N.SNatI m) => Vec (N.Mult n m) a -> Vec n (Vec m a)
chunks = getChunks $ N.induction1 start step where
    start :: Chunks m 'Z a
    start = Chunks $ \_ -> VNil

    step :: forall m n a. N.SNatI m => Chunks m n a -> Chunks m ('S n) a
    step (Chunks go) = Chunks $ \xs ->
        let (ys, zs) = split xs :: (Vec m a, Vec (N.Mult n m) a)
        in ys ::: go zs

newtype Chunks  m n a = Chunks  { getChunks  :: Vec (N.Mult n m) a -> Vec n (Vec m a) }

-------------------------------------------------------------------------------
-- take and drop
-------------------------------------------------------------------------------

-- |
--
-- >>> let xs = 'a' ::: 'b' ::: 'c' ::: 'd' ::: 'e' ::: VNil
-- >>> take xs :: Vec N.Nat3 Char
-- 'a' ::: 'b' ::: 'c' ::: VNil
--
take :: forall n m a. (LE.ZS.LE n m) => Vec m a -> Vec n a
take = go LE.ZS.leProof where
    go :: LE.ZS.LEProof n' m' -> Vec m' a -> Vec n' a
    go LE.ZS.LEZero _              = VNil
    go (LE.ZS.LESucc p) (x ::: xs) = x ::: go p xs

-- |
--
-- >>> let xs = 'a' ::: 'b' ::: 'c' ::: 'd' ::: 'e' ::: VNil
-- >>> drop xs :: Vec N.Nat3 Char
-- 'c' ::: 'd' ::: 'e' ::: VNil
--
drop :: forall n m a. (LE.ZS.LE n m, N.SNatI m) => Vec m a -> Vec n a
drop = go (LE.RS.fromZeroSucc LE.ZS.leProof) where
    go :: LE.RS.LEProof n' m' -> Vec m' a -> Vec n' a
    go LE.RS.LERefl xs             = xs
    go (LE.RS.LEStep p) (_ ::: xs) = go p xs

-------------------------------------------------------------------------------
-- Mapping
-------------------------------------------------------------------------------

-- | >>> map not $ True ::: False ::: VNil
-- False ::: True ::: VNil
--
map :: (a -> b) -> Vec n a -> Vec n b
map _ VNil       = VNil
map f (x ::: xs) = f x ::: fmap f xs

-- | >>> imap (,) $ 'a' ::: 'b' ::: 'c' ::: VNil
-- (0,'a') ::: (1,'b') ::: (2,'c') ::: VNil
--
imap :: (Fin n -> a -> b) -> Vec n a -> Vec n b
imap _ VNil       = VNil
imap f (x ::: xs) = f FZ x ::: imap (f . FS) xs

-- | Apply an action to every element of a 'Vec', yielding a 'Vec' of results.
traverse :: forall n f a b. Applicative f => (a -> f b) -> Vec n a -> f (Vec n b)
traverse f = go where
    go :: Vec m a -> f (Vec m b)
    go VNil       = pure VNil
    go (x ::: xs) = (:::) <$> f x <*> go xs

#ifdef MIN_VERSION_semigroupoids
-- | Apply an action to non-empty 'Vec', yielding a 'Vec' of results.
traverse1 :: forall n f a b. Apply f => (a -> f b) -> Vec ('S n) a -> f (Vec ('S n) b)
traverse1 f = go where
    go :: Vec ('S m) a -> f (Vec ('S m) b)
    go (x ::: VNil)         = (::: VNil) <$> f x
    go (x ::: xs@(_ ::: _)) = (:::) <$> f x <.> go xs
#endif

-- | Apply an action to every element of a 'Vec' and its index, yielding a 'Vec' of results.
itraverse :: Applicative f => (Fin n -> a -> f b) -> Vec n a -> f (Vec n b)
itraverse _ VNil       = pure VNil
itraverse f (x ::: xs) = (:::) <$> f FZ x <*> itraverse (f . FS) xs

-- | Apply an action to every element of a 'Vec' and its index, ignoring the results.
itraverse_ :: Applicative f => (Fin n -> a -> f b) -> Vec n a -> f ()
itraverse_ _ VNil       = pure ()
itraverse_ f (x ::: xs) = f FZ x *> itraverse_ (f . FS) xs

-------------------------------------------------------------------------------
-- Folding
-------------------------------------------------------------------------------

-- | See 'I.Foldable'.
foldMap :: Monoid m => (a -> m) -> Vec n a -> m
foldMap _ VNil       = mempty
foldMap f (x ::: xs) = mappend (f x) (foldMap f xs)

-- | See 'I.Foldable1'.
foldMap1 :: Semigroup s => (a -> s) -> Vec ('S n) a -> s
foldMap1 f (x ::: VNil)         = f x
foldMap1 f (x ::: xs@(_ ::: _)) = f x <> foldMap1 f xs

-- | See 'I.FoldableWithIndex'.
ifoldMap :: Monoid m => (Fin n -> a -> m) -> Vec n a -> m
ifoldMap _ VNil       = mempty
ifoldMap f (x ::: xs) = mappend (f FZ x) (ifoldMap (f . FS) xs)

-- | There is no type-class for this :(
ifoldMap1 :: Semigroup s => (Fin ('S n) -> a -> s) -> Vec ('S n) a -> s
ifoldMap1 f (x ::: VNil)         = f FZ x
ifoldMap1 f (x ::: xs@(_ ::: _)) = f FZ x <> ifoldMap1 (f . FS) xs

-- | Right fold.
foldr :: forall a b n. (a -> b -> b) -> b -> Vec n a -> b
foldr f z = go where
    go :: Vec m a -> b
    go VNil       = z
    go (x ::: xs) = f x (go xs)

-- | Right fold with an index.
ifoldr :: forall a b n. (Fin n -> a -> b -> b) -> b -> Vec n a -> b
ifoldr _ z VNil       = z
ifoldr f z (x ::: xs) = f FZ x (ifoldr (f . FS) z xs)

-- | Strict left fold.
foldl' :: forall a b n. (b -> a -> b) -> b -> Vec n a -> b
foldl' f z = go z where
    go :: b -> Vec m a -> b
    go !acc VNil       = acc
    go !acc (x ::: xs) = go (f acc x) xs

-- | Yield the length of a 'Vec'. /O(n)/
length :: Vec n a -> Int
length = go 0 where
    go :: Int -> Vec n a -> Int
    go !acc VNil       = acc
    go  acc (_ ::: xs) = go (1 + acc) xs

-- | Test whether a 'Vec' is empty. /O(1)/
null :: Vec n a -> Bool
null VNil      = True
null (_ ::: _) = False

-------------------------------------------------------------------------------
-- Special folds
-------------------------------------------------------------------------------

-- | Non-strict 'sum'.
sum :: Num a => Vec n a -> a
sum VNil       = 0
sum (x ::: xs) = x + sum xs

-- | Non-strict 'product'.
product :: Num a => Vec n a -> a
product VNil       = 1
product (x ::: xs) = x * product xs

-------------------------------------------------------------------------------
-- Zipping
-------------------------------------------------------------------------------

-- | Zip two 'Vec's with a function.
zipWith ::  (a -> b -> c) -> Vec n a -> Vec n b -> Vec n c
zipWith _ VNil       VNil       = VNil
zipWith f (x ::: xs) (y ::: ys) = f x y ::: zipWith f xs ys

-- | Zip two 'Vec's. with a function that also takes the elements' indices.
izipWith :: (Fin n -> a -> b -> c) -> Vec n a -> Vec n b -> Vec n c
izipWith _ VNil       VNil       = VNil
izipWith f (x ::: xs) (y ::: ys) = f FZ x y ::: izipWith (f . FS) xs ys

-- | Repeat a value.
--
-- >>> repeat 'x' :: Vec N.Nat3 Char
-- 'x' ::: 'x' ::: 'x' ::: VNil
--
-- @since 0.2.1
repeat :: N.SNatI n => x -> Vec n x
repeat x = N.induction1 VNil (x :::)

-------------------------------------------------------------------------------
-- Monadic
-------------------------------------------------------------------------------

-- | Monadic bind.
bind :: Vec n a -> (a -> Vec n b) -> Vec n b
bind VNil       _ = VNil
bind (x ::: xs) f = head (f x) ::: bind xs (tail . f)

-- | Monadic join.
--
-- >>> join $ ('a' ::: 'b' ::: VNil) ::: ('c' ::: 'd' ::: VNil) ::: VNil
-- 'a' ::: 'd' ::: VNil
join :: Vec n (Vec n a) -> Vec n a
join VNil       = VNil
join (x ::: xs) = head x ::: join (map tail xs)

-------------------------------------------------------------------------------
-- universe
-------------------------------------------------------------------------------

-- | Get all @'Fin' n@ in a @'Vec' n@.
--
-- >>> universe :: Vec N.Nat3 (Fin N.Nat3)
-- 0 ::: 1 ::: 2 ::: VNil
universe :: N.SNatI n => Vec n (Fin n)
universe = getUniverse (N.induction first step) where
    first :: Universe 'Z
    first = Universe VNil

    step :: Universe m -> Universe ('S m)
    step (Universe go) = Universe (FZ ::: map FS go)

newtype Universe n = Universe { getUniverse :: Vec n (Fin n) }

-------------------------------------------------------------------------------
-- VecEach
-------------------------------------------------------------------------------

-- | Write functions on 'Vec'. Use them with tuples.
--
-- 'VecEach' can be used to avoid "this function won't change the length of the
-- list" in DSLs.
--
-- __bad:__ Instead of
--
-- @
-- [x, y] <- badDslMagic ["foo", "bar"]  -- list!
-- @
--
-- __good:__ we can write
--
-- @
-- (x, y) <- betterDslMagic ("foo", "bar") -- homogenic tuple!
-- @
--
-- where @betterDslMagic@ can be defined using 'traverseWithVec'.
--
-- Moreally @lens@ 'Each' should be a superclass, but
-- there's no strict need for it.
--
class VecEach s t a b | s -> a, t -> b, s b -> t, t a -> s where
    mapWithVec :: (forall n. N.SNatI n => Vec n a -> Vec n b) -> s -> t
    traverseWithVec :: Applicative f => (forall n. N.SNatI n => Vec n a -> f (Vec n b)) -> s -> f t

instance (a ~ a', b ~ b') => VecEach (a, a') (b, b') a b where
    mapWithVec f ~(x, y) = case f (x ::: y ::: VNil) of
        x' ::: y' ::: VNil -> (x', y')

    traverseWithVec f ~(x, y) = f (x ::: y ::: VNil) <&> \res -> case res of
        x' ::: y' ::: VNil -> (x', y')

instance (a ~ a2, a ~ a3, b ~ b2, b ~ b3) => VecEach (a, a2, a3) (b, b2, b3) a b where
    mapWithVec f ~(x, y, z) = case f (x ::: y ::: z ::: VNil) of
        x' ::: y' ::: z' ::: VNil -> (x', y', z')

    traverseWithVec f ~(x, y, z) = f (x ::: y ::: z ::: VNil) <&> \res -> case res of
        x' ::: y' ::: z' ::: VNil -> (x', y', z')

instance (a ~ a2, a ~ a3, a ~ a4, b ~ b2, b ~ b3, b ~ b4) => VecEach (a, a2, a3, a4) (b, b2, b3, b4) a b where
    mapWithVec f ~(x, y, z, u) = case f (x ::: y ::: z ::: u ::: VNil) of
        x' ::: y' ::: z' ::: u' ::: VNil -> (x', y', z', u')

    traverseWithVec f ~(x, y, z, u) = f (x ::: y ::: z ::: u ::: VNil) <&> \res -> case res of
        x' ::: y' ::: z' ::: u' ::: VNil -> (x', y', z', u')

-------------------------------------------------------------------------------
-- QuickCheck
-------------------------------------------------------------------------------

instance N.SNatI n => QC.Arbitrary1 (Vec n) where
    liftArbitrary = liftArbitrary
    liftShrink    = liftShrink

liftArbitrary :: forall n a. N.SNatI n => QC.Gen a -> QC.Gen (Vec n a)
liftArbitrary arb = getArb $ N.induction1 (Arb (return VNil)) step where
    step :: Arb m a -> Arb ('S m) a
    step (Arb rec) = Arb $ (:::) <$> arb <*> rec

newtype Arb n a = Arb { getArb :: QC.Gen (Vec n a) }

liftShrink :: forall n a. N.SNatI n => (a -> [a]) -> Vec n a -> [Vec n a]
liftShrink shr = getShr $ N.induction1 (Shr $ \VNil -> []) step where
    step :: Shr m a -> Shr ('S m) a
    step (Shr rec) = Shr $ \(x ::: xs) ->
        uncurry (:::) <$> QC.liftShrink2 shr rec (x, xs)

newtype Shr n a = Shr { getShr :: Vec n a -> [Vec n a] }

instance (N.SNatI n, QC.Arbitrary a) => QC.Arbitrary (Vec n a) where
    arbitrary = QC.arbitrary1
    shrink    = QC.shrink1

instance (N.SNatI n, QC.CoArbitrary a) => QC.CoArbitrary (Vec n a) where
    coarbitrary v = case N.snat :: N.SNat n of
        N.SZ -> QC.variant (0 :: Int)
        N.SS -> QC.variant (1 :: Int) . (case v of (x ::: xs) -> QC.coarbitrary (x, xs))

instance (N.SNatI n, QC.Function a) => QC.Function (Vec n a) where
    function = case N.snat :: N.SNat n of
        N.SZ -> QC.functionMap (\VNil -> ()) (\() -> VNil)
        N.SS -> QC.functionMap (\(x ::: xs) -> (x, xs)) (\(x,xs) -> x ::: xs)