-- | This is the main module for end-users of lens-families-core.
-- If you are not building your own lenses or traversals, but just using functional references made by others, this is the only module you need.
module Lens.Family (
-- * Lenses
--
-- | This module provides '^.' for accessing fields and '.~' and '%~' for setting and modifying fields.
-- Lenses are composed with `Prelude..` from the @Prelude@ and `Prelude.id` is the identity lens.
--
-- Lens composition in this library enjoys the following identities.
--
-- * @x^.l1.l2 === x^.l1^.l2@
--
-- * @l1.l2 %~ f === l1 %~ l2 %~ f@
--
-- The identity lens behaves as follows.
--
-- * @x^.id === x@
--
-- * @id %~ f === f@
--
-- The '&' operator, allows for a convenient way to sequence record updating:
--
-- @record & l1 .~ value1 & l2 .~ value2@
--
-- Lenses are implemented in van Laarhoven style.
-- Lenses have type @'Functor' f => (b -> f b) -> a -> f a@ and lens families have type @'Functor' f => (b i -> f (b j)) -> a i -> f (a j)@.
--
-- Keep in mind that lenses and lens families can be used directly for functorial updates.
-- For example, @_2 id@ gives you strength.
--
-- > _2 id :: Functor f => (a, f b) -> f (a, b)
--
-- Here is an example of code that uses the 'Maybe' functor to preserves sharing during update when possible.
--
-- > -- | 'sharedUpdate' returns the *identical* object if the update doesn't change anything.
-- > -- This is useful for preserving sharing.
-- > sharedUpdate :: Eq b => LensLike' Maybe a b -> (b -> b) -> a -> a
-- > sharedUpdate l f a = fromMaybe a (l f' a)
-- >  where
-- >   f' b | fb == b  = Nothing
-- >        | otherwise = Just fb
-- >    where
-- >     fb = f b

-- * Traversals
--
-- | '^.' can be used with traversals to access monoidal fields.
-- The result will be a 'Data.Monid.mconcat' of all the fields referenced.
-- The various @fooOf@ functions can be used to access different monoidal summaries of some kinds of values.
--
-- '^?' can be used to access the first value of a traversal.
-- 'Nothing' is returned when the traversal has no references.
--
-- '^..' can be used with a traversals and will return a list of all fields referenced.
--
-- When '.~' is used with a traversal, all referenced fields will be set to the same value, and when '%~' is used with a traversal, all referenced fields will be modified with the same function.
--
-- Like lenses, traversals can be composed with '.', and because every lens is automatically a traversal, lenses and traversals can be composed with '.' yielding a traversal.
--
-- Traversals are implemented in van Laarhoven style.
-- Traversals have type @'Applicative' f => (b -> f b) -> a -> f a@ and traversal families have type @'Applicative' f => (b i -> f (b j)) -> a i -> f (a j)@.
--
-- For stock lenses and traversals, see "Lens.Family.Stock".
--
-- To build your own lenses and traversals, see "Lens.Family.Unchecked".
--
-- References:
--
-- * <http://www.twanvl.nl/blog/haskell/cps-functional-references>
--
-- * <http://r6.ca/blog/20120623T104901Z.html>
--
-- * <http://comonad.com/reader/2012/mirrored-lenses/>
--
-- * <http://conal.net/blog/posts/semantic-editor-combinators>

-- * Documentation
    to, view, (^.)
  , folding, views, (^..), (^?)
  , toListOf, allOf, anyOf, firstOf, lastOf, sumOf, productOf
  , lengthOf, nullOf
  , backwards
  , over, (%~), set, (.~)
  , (&)
-- * Pseudo-imperatives
  , (+~), (*~), (-~), (//~), (&&~), (||~), (<>~)
-- * Types
  , LensLike, LensLike'
  , FoldLike, FoldLike'
  , ASetter, ASetter'
  , Phantom
  , Constant, Identity
-- * Re-exports
  , Applicative, Foldable, Monoid
  , Backwards, All, Any, First, Last, Sum, Product
  ) where

import Control.Applicative (Applicative)
import Control.Applicative.Backwards (Backwards(..))
import Data.Foldable (Foldable, traverse_)
import Data.Functor.Identity (Identity(..))
import Data.Functor.Constant (Constant(..))
import Data.Monoid ( Monoid, mappend
                   , All(..), Any(..)
                   , First(..), Last(..)
                   , Sum(..), Product(..)
                   )
import Lens.Family.Phantom (Phantom, coerce)
import Lens.Family.Unchecked ( LensLike, LensLike' )

type FoldLike r a a' b b' = LensLike (Constant r) a a' b b'
type FoldLike' r a b = LensLike' (Constant r) a b
type ASetter a a' b b' = LensLike Identity a a' b b'
type ASetter' a b = LensLike' Identity a b

to :: Phantom f => (a -> b) -> LensLike f a a' b b'
-- ^ @
-- to :: (a -> b) -> Getter a a' b b'
-- @
--
-- 'to' promotes a projection function to a read-only lens called a getter.
-- To demote a lens to a projection function, use the section @(^.l)@ or @view l@.
--
-- >>> (3 :+ 4, "example")^._1.to(abs)
-- 5.0 :+ 0.0
to p f = coerce . f . p

view :: FoldLike b a a' b b' -> a -> b
-- ^ @
-- view :: Getter a a' b b' -> a -> b
-- @
--
-- Demote a lens or getter to a projection function.
--
-- @
-- view :: Monoid b => Fold a a' b b' -> a -> b
-- @
--
-- Returns the monoidal summary of a traversal or a fold.
view l = (^.l)

folding :: (Foldable g, Phantom f, Applicative f) => (a -> g b) -> LensLike f a a' b b'
-- ^ @
-- folding :: (a -> [b]) -> Fold a a' b b'
-- @
--
-- 'folding' promotes a \"toList\" function to a read-only traversal called a fold.
--
-- To demote a traversal or fold to a \"toList\" function use the section @(^..l)@ or @toListOf l@.
folding p f = coerce . traverse_ f . p

views :: FoldLike r a a' b b' -> (b -> r) -> a -> r
-- ^ @
-- views :: Monoid r => Fold a a' b b' -> (b -> r) -> a -> r
-- @
--
-- Given a fold or traversal, return the 'foldMap' of all the values using the given function.
--
-- @
-- views :: Getter a a' b b' -> (b -> r) -> a -> r
-- @
--
-- 'views' is not particularly useful for getters or lenses, but given a getter or lens, it returns the referenced value passed through the given function.
--
-- @
-- views l f a = f (view l a)
-- @
views l f = getConstant . l (Constant . f)

toListOf :: FoldLike [b] a a' b b' -> a -> [b]
-- ^ @
-- toListOf :: Fold a a' b b' -> a -> [b]
-- @
--
-- Returns a list of all of the referenced values in order.
toListOf l = views l (:[])

allOf :: FoldLike All a a' b b' -> (b -> Bool) -> a -> Bool
-- ^ @
-- allOf :: Fold a a' b b' -> (b -> Bool) -> a -> Bool
-- @
--
-- Returns true if all of the referenced values satisfy the given predicate.
allOf l p = getAll . views l (All . p)

anyOf :: FoldLike Any a a' b b' -> (b -> Bool) -> a -> Bool
-- ^ @
-- anyOf :: Fold a a' b b' -> (b -> Bool) -> a -> Bool
-- @
--
-- Returns true if any of the referenced values satisfy the given predicate.
anyOf l p = getAny . views l (Any . p)

firstOf :: FoldLike (First b) a a' b b' -> a -> Maybe b
-- ^ @
-- firstOf :: Fold a a' b b' -> a -> Maybe b
-- @
--
-- Returns 'Just' the first referenced value.
-- Returns 'Nothing' if there are no referenced values.
-- See '^?' for an infix version of 'firstOf'
firstOf l = getFirst . views l (First . Just)

lastOf :: FoldLike (Last b) a a' b b' -> a -> Maybe b
-- ^ @
-- lastOf :: Fold a a' b b' -> a -> Maybe b
-- @
--
-- Returns 'Just' the last referenced value.
-- Returns 'Nothing' if there are no referenced values.
lastOf l = getLast . views l (Last . Just)

sumOf :: Num b => FoldLike (Sum b) a a' b b' -> a -> b
-- ^ @
-- sumOf :: Num b => Fold a a' b b' -> a -> b
-- @
--
-- Returns the sum of all the referenced values.
sumOf l = getSum . views l Sum

productOf :: Num b => FoldLike (Product b) a a' b b' -> a -> b
-- ^ @
-- productOf :: Num b => Fold a a' b b' -> a -> b
-- @
--
-- Returns the product of all the referenced values.
productOf l = getProduct . views l Product

lengthOf :: Num r => FoldLike (Sum r) a a' b b' -> a -> r
-- ^ @
-- lengthOf :: Num r => Fold a a' b b' -> a -> r
-- @
--
-- Counts the number of references in a traversal or fold for the input.
lengthOf l = getSum . views l (const (Sum 1))

nullOf :: FoldLike All a a' b b' -> a -> Bool
-- ^ @
-- nullOf :: Fold a a' b b' -> a -> Bool
-- @
--
-- Returns true if the number of references in the input is zero.
nullOf l = allOf l (const False)

infixr 8 ^.

(^.) :: a -> FoldLike b a a' b b' -> b
-- ^ @
-- (^.) :: a -> Getter a a' b b' -> b
-- @
--
-- Access the value referenced by a getter or lens.
--
-- @
-- (^.) :: Monoid b => a -> Fold a a' b b' -> b
-- @
--
-- Access the monoidal summary referenced by a getter or lens.
x^.l = getConstant $ l Constant x

infixr 8 ^..

(^..) :: a -> FoldLike [b] a a' b b' -> [b]
-- ^ @
-- (^..) :: a -> Getter a a' b b' -> [b]
-- @
--
-- Returns a list of all of the referenced values in order.
x^..l = toListOf l x

infixr 8 ^?

(^?) :: a -> FoldLike (First b) a a' b b' -> Maybe b
-- ^ @
-- (^?) :: a -> Fold a a' b b' -> Maybe b
-- @
--
-- Returns 'Just' the first referenced value.
-- Returns 'Nothing' if there are no referenced values.
x^?l = firstOf l x

backwards :: LensLike (Backwards f) a a' b b' -> LensLike f a a' b b'
-- ^ @
-- backwards :: Traversal a a' b b' -> Traversal a a' b b'
-- backwards :: Fold a a' b b' -> Fold a a' b b'
-- @
--
-- Given a traversal or fold, reverse the order that elements are traversed.
--
-- @
-- backwards :: Lens a a' b b' -> Lens a a' b b'
-- backwards :: Getter a a' b b' -> Getter a a' b b'
-- backwards :: Setter a a' b b' -> Setter a a' b b'
-- @
--
-- No effect on lenses, getters or setters.
backwards l f = forwards . l (Backwards . f)

-- | Demote a setter to a semantic editor combinator.
over :: ASetter a a' b b' -> (b -> b') -> a -> a'
over l = (l %~)

infixr 4 %~

-- | Modify all referenced fields.
(%~) :: ASetter a a' b b' -> (b -> b') -> a -> a'
l %~ f = runIdentity . l (Identity . f)

infixr 4 .~

-- | Set all referenced fields to the given value.
(.~) :: ASetter a a' b b' -> b' -> a -> a'
l .~ b = l %~ const b

-- | Set all referenced fields to the given value.
set :: ASetter a a' b b' -> b' -> a -> a'
set = (.~)

infixl 1 &

-- | A flipped version of @($)@.
(&) :: a -> (a -> b) -> b
(&) = flip ($)

infixr 4 +~, -~, *~

(+~), (-~), (*~) :: Num b => ASetter' a b -> b -> a -> a
f +~ b = f %~ (+ b)
f -~ b = f %~ subtract b
f *~ b = f %~ (* b)

infixr 4 //~

(//~) :: Fractional b => ASetter' a b -> b -> a -> a
f //~ b = f %~ (/ b)

infixr 4 &&~, ||~

(&&~), (||~) :: ASetter' a Bool -> Bool -> a -> a
f &&~ b = f %~ (&& b)
f ||~ b = f %~ (|| b)

infixr 4 <>~

-- | Monoidally append a value to all referenced fields.
(<>~) :: (Monoid o) => ASetter' a o -> o -> a -> a
f <>~ o = f %~ (`mappend` o)