Reverse.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

-------------------------------------------------------------------------------------
-- |
-- Copyright   : (c) Hans Hoglund 2012
--
-- License     : BSD-style
--
-- Maintainer  : hans@hanshoglund.se
-- Stability   : experimental
-- Portability : non-portable (TF,GNTD)
--
-- Provides reversible values.
--
-------------------------------------------------------------------------------------

module Music.Time.Reverse (
        -- * Reversible class
        Reversible(..),

        -- ** Utility
        NoRev(..),
        WithRev(..),
        withRev,
        fromWithRev,
  ) where

import           Data.AffineSpace
import           Data.AffineSpace.Point
import           Data.Map               (Map)
import qualified Data.Map               as Map
import           Data.Ratio
import           Data.Semigroup
import           Data.Set               (Set)
import qualified Data.Set               as Set
import           Data.VectorSpace

import           Music.Time.Delayable
import           Music.Time.Juxtapose
import           Music.Time.Onset
import           Music.Time.Stretchable
import           Music.Time.Time

-- |
-- Reversible values.
--
-- For instances of 'Reversible' and 'HasOnset', the following laws should hold:
--
-- > onset a    = onset (rev a)
-- > duration a = duration (rev a)
--
-- For structural types, 'rev' is applied recursively, hence the constraint on
-- the 'Score' instance. 'rev' is id by default, so for a trivial type @T@ it
-- suffices to write
--
-- > instance Reversible T
--
-- For instances 'U' of 'HasOnset' and 'Transformable', a suitable instance
-- is
--
-- > instance Reversible T where
-- >     rev = withSameOnset (stretch (-1))
--
--
class Reversible a where

    -- |
    -- Reverse a value.
    --
    -- Reverse is an involution, meaning that:
    --
    -- > rev (rev a) = a
    --
    rev :: a -> a
    rev = id

-- instance Reversible Time where
    -- rev t = mirror t

instance Reversible Double
instance Reversible Float
instance Reversible Int
instance Reversible Integer
instance Reversible ()
instance Reversible (Ratio a)

instance Reversible a => Reversible [a] where
    rev = fmap rev

instance (Ord a, Reversible a) => Reversible (Set a) where
    rev = Set.map rev

instance Reversible a => Reversible (Map k a) where
    rev = fmap rev

newtype NoRev a = NoRev { getNoRev :: a }
    deriving (Eq, Ord, Enum, Show, Semigroup, Monoid,
        Delayable, Stretchable, HasOnset, HasOffset, HasDuration)

instance Reversible (NoRev a) where
    rev = id



newtype WithRev a = WithRev (a,a)
    deriving (Eq, Ord, Semigroup, Monoid)

withRev :: Reversible a => a -> WithRev a
withRev x = WithRev (rev x, x)

fromWithRev :: Reversible a => WithRev a -> a
fromWithRev (WithRev (_,x)) = x

instance Reversible a => Reversible (WithRev a) where
    rev (WithRev (r,x)) = WithRev (x,r)