Lens.hs

{-# LANGUAGE CPP #-}
-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Complex.Lens
-- Copyright   :  (C) 2012 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  provisional
-- Portability :  Haskell2010
--
----------------------------------------------------------------------------
module Data.Complex.Lens
  ( real, imaginary, polarize
  , traverseComplex
  ) where

import Control.Applicative
import Control.Lens
import Data.Complex

-- | Access the real part of a complex number
--
-- > real :: Functor f => (a -> f a) -> Complex a -> f (Complex a)
#if MIN_VERSION_base(4,4,0)
real :: Simple Lens (Complex a) a
#else
real :: RealFloat a => Simple Lens (Complex a) a
#endif
real f (a :+ b) = (:+ b) <$> f a

-- | Access the imaginary part of a complex number
--
-- > imaginary :: Functor f => (a -> f a) -> Complex a -> f (Complex a)
#if MIN_VERSION_base(4,4,0)
imaginary :: Simple Lens (Complex a) a
#else
imaginary :: RealFloat a => Simple Lens (Complex a) a
#endif
imaginary f (a :+ b) = (a :+) <$> f b

-- | This isn't /quite/ a legal lens. Notably the @view l (set l b a) = b@ law
-- is violated when you set a polar value with 0 magnitude and non-zero phase
-- as the phase information is lost. So don't do that! Otherwise, this is a
-- perfectly cromulent lens.

polarize :: (RealFloat a, RealFloat b) => Iso (Complex a) (Complex b) (a,a) (b,b)
polarize = isos polar (uncurry mkPolar)
                polar (uncurry mkPolar)

-- | Traverse both the real and imaginary parts of a complex number.
--
-- > traverseComplex :: Applicative f => (a -> f b) -> Complex a -> f (Complex b)
#if MIN_VERSION_base(4,4,0)
traverseComplex :: Traversal (Complex a) (Complex b) a b
#else
traverseComplex :: (RealFloat a, RealFloat b) => Traversal (Complex a) (Complex b) a b
#endif
traverseComplex f (a :+ b) = (:+) <$> f a <*> f b