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|
{-# LANGUAGE CPP #-}
{-# LANGUAGE AutoDeriveTypeable #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Functor.Identity
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology 2001
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : ross@soi.city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- The identity functor and monad.
--
-- This trivial type constructor serves two purposes:
--
-- * It can be used with functions parameterized by functor or monad classes.
--
-- * It can be used as a base monad to which a series of monad
-- transformers may be applied to construct a composite monad.
-- Most monad transformer modules include the special case of
-- applying the transformer to 'Identity'. For example, @State s@
-- is an abbreviation for @StateT s 'Identity'@.
--
-- /Since: 4.8.0.0/
-----------------------------------------------------------------------------
module Data.Functor.Identity (
Identity(..),
) where
import Control.Monad.Fix
import Data.Functor
-- | Identity functor and monad. (a non-strict monad)
--
-- /Since: 4.8.0.0/
newtype Identity a = Identity { runIdentity :: a }
deriving (Eq, Ord)
-- | This instance would be equivalent to the derived instances of the
-- 'Identity' newtype if the 'runIdentity' field were removed
instance (Read a) => Read (Identity a) where
readsPrec d = readParen (d > 10) $ \ r ->
[(Identity x,t) | ("Identity",s) <- lex r, (x,t) <- readsPrec 11 s]
-- | This instance would be equivalent to the derived instances of the
-- 'Identity' newtype if the 'runIdentity' field were removed
instance (Show a) => Show (Identity a) where
showsPrec d (Identity x) = showParen (d > 10) $
showString "Identity " . showsPrec 11 x
-- ---------------------------------------------------------------------------
-- Identity instances for Functor and Monad
instance Functor Identity where
fmap f m = Identity (f (runIdentity m))
instance Foldable Identity where
foldMap f (Identity x) = f x
instance Traversable Identity where
traverse f (Identity x) = Identity <$> f x
instance Applicative Identity where
pure a = Identity a
Identity f <*> Identity x = Identity (f x)
instance Monad Identity where
return a = Identity a
m >>= k = k (runIdentity m)
instance MonadFix Identity where
mfix f = Identity (fix (runIdentity . f))
|
