    {-# LANGUAGE CPP #-}
    {-# LANGUAGE Rank2Types #-}
    {-# LANGUAGE FlexibleContexts #-}
    #ifdef TRUSTWORTHY
    {-# LANGUAGE Trustworthy #-}
    #endif
    -------------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Vector.Generic.Lens
    -- Copyright   :  (C) 2012-2013 Edward Kmett
    -- License     :  BSD-style (see the file LICENSE)
    -- Maintainer  :  Edward Kmett <ekmett@gmail.com>
    -- Stability   :  provisional
    -- Portability :  non-portable
    --
    -- This module provides lenses and traversals for working with generic
    -- vectors.
    -------------------------------------------------------------------------------
    module Data.Vector.Generic.Lens
      ( toVectorOf
      -- * Isomorphisms
      , forced
      , vector
      , asStream
      , asStreamR
      , cloned
      -- * Lenses
      , sliced
      -- * Traversal of individual indices
      , ordinals
      ) where

    import Control.Applicative
    import Control.Lens
    import Data.List (nub)
    import Data.Monoid
    import Data.Vector.Generic as V hiding (zip, filter, indexed)
    import Data.Vector.Fusion.Stream (Stream)
    import Data.Vector.Generic.New (New)
    import Prelude hiding ((++), length, null, head, tail, init, last, map, reverse)

    -- $setup
    -- >>> import Data.Vector as Vector

    -- | @sliced i n@ provides a 'Lens' that edits the @n@ elements starting
    -- at index @i@ from a 'Lens'.
    --
    -- This is only a valid 'Lens' if you do not change the length of the
    -- resulting 'Vector'.
    --
    -- Attempting to return a longer or shorter vector will result in
    -- violations of the 'Lens' laws.
    --
    -- >>> Vector.fromList [1..10] ^. sliced 2 5
    -- fromList [3,4,5,6,7]
    --
    -- >>> Vector.fromList [1..10] & sliced 2 5 . mapped .~ 0
    -- fromList [1,2,0,0,0,0,0,8,9,10]
    sliced :: Vector v a
           => Int -- ^ @i@ starting index
           -> Int -- ^ @n@ length
           -> Lens' (v a) (v a)
    sliced i n f v = f (slice i n v) <&> \ v0 -> v // zip [i..i+n-1] (V.toList v0)
    {-# INLINE sliced #-}

    -- | Similar to 'toListOf', but returning a 'Vector'.
    --
    -- >>> toVectorOf both (8,15) :: Vector.Vector Int
    -- fromList [8,15]
    toVectorOf :: Vector v a => Getting (Endo [a]) s a -> s -> v a
    toVectorOf l s = fromList (toListOf l s)
    {-# INLINE toVectorOf #-}

    -- | Convert a list to a 'Vector' (or back.)
    --
    -- >>> [1,2,3] ^. vector :: Vector.Vector Int
    -- fromList [1,2,3]
    --
    -- >>> Vector.fromList [0,8,15] ^. from vector
    -- [0,8,15]
    vector :: Vector v a => Iso' [a] (v a)
    vector = iso fromList V.toList
    {-# INLINE vector #-}

    -- | Convert a 'Vector' to a finite 'Stream' (or back.)
    asStream :: Vector v a => Iso' (v a) (Stream a)
    asStream = iso stream unstream
    {-# INLINE asStream #-}

    -- | Convert a 'Vector' to a finite 'Stream' from right to left (or
    -- back.)
    asStreamR :: Vector v a => Iso' (v a) (Stream a)
    asStreamR = iso streamR unstreamR
    {-# INLINE asStreamR #-}

    -- | Convert a 'Vector' back and forth to an initializer that when run
    -- produces a copy of the 'Vector'.
    cloned :: Vector v a => Iso' (v a) (New v a)
    cloned = iso clone new
    {-# INLINE cloned #-}

    -- | Convert a 'Vector' to a version that doesn't retain any extra
    -- memory.
    forced :: Vector v a => Iso' (v a) (v a)
    forced = involuted force
    {-# INLINE forced #-}

    -- | This 'Traversal' will ignore any duplicates in the supplied list
    -- of indices.
    --
    -- >>> toListOf (ordinals [1,3,2,5,9,10]) $ Vector.fromList [2,4..40]
    -- [4,8,6,12,20,22]
    ordinals :: Vector v a => [Int] -> IndexedTraversal' Int (v a) a
    ordinals is f v = fmap (v //) $ traverse (\i -> (,) i <$> indexed f i (v ! i)) $ nub $ filter (\i -> 0 <= i && i < l) is where
      l = length v
    {-# INLINE ordinals #-}