--------------------------------------------------------------------------------
-- |
-- Module      :  Control.Lens.Combinators
-- Copyright   :  (C) 2012-13 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  provisional
-- Portability :  portable
--
-- These are general purpose combinators that provide utility for non-lens code
-------------------------------------------------------------------------------
module Control.Lens.Combinators
  ( (&)
  , (<&>)
  , (??)
  ) where

import Data.Functor ((<$>))

-- $setup
-- >>> import Control.Lens
-- >>> import Control.Monad.State
-- >>> import Debug.SimpleReflect.Expr
-- >>> import Debug.SimpleReflect.Vars as Vars hiding (f)
-- >>> let f :: Expr -> Expr; f = Debug.SimpleReflect.Vars.f

infixl 1 &, <&>, ??

-- | Passes the result of the left side to the function on the right side (forward pipe operator).
--
-- This is the flipped version of ('$'), which is more common in languages like F# as (@|>@) where it is needed
-- for inference. Here it is supplied for notational convenience and given a precedence that allows it
-- to be nested inside uses of ('$').
--
-- >>> a & f
-- f a
--
-- >>> "hello" & length & succ
-- 6
--
-- This combinator is commonly used when applying multiple 'Control.Lens.Lens.Lens' operations in sequence.
--
-- >>> ("hello","world") & _1.element 0 .~ 'j' & _1.element 4 .~ 'y'
-- ("jelly","world")
--
-- This reads somewhat similar to:
--
-- >>> flip execState ("hello","world") $ do _1.element 0 .= 'j'; _1.element 4 .= 'y'
-- ("jelly","world")
(&) :: a -> (a -> b) -> b
a & f = f a
{-# INLINE (&) #-}

-- | Infix flipped 'fmap'.
--
-- @
-- ('<&>') = 'flip' 'fmap'
-- @
(<&>) :: Functor f => f a -> (a -> b) -> f b
as <&> f = f <$> as
{-# INLINE (<&>) #-}

-- | This is convenient to 'flip' argument order of composite functions.
--
-- >>> over _2 ?? ("hello","world") $ length
-- ("hello",5)
--
-- >>> over ?? length ?? ("hello","world") $ _2
-- ("hello",5)
(??) :: Functor f => f (a -> b) -> a -> f b
fab ?? a = fmap ($ a) fab
{-# INLINE (??) #-}