Data/Functor/Identity.hs

    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE DeriveGeneric #-}
    {-# LANGUAGE DeriveTraversable #-}
    {-# LANGUAGE GeneralizedNewtypeDeriving #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    {-# LANGUAGE Trustworthy #-}

    -----------------------------------------------------------------------------
    -- |
    -- 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.Bits (Bits, FiniteBits)
    import Data.Coerce
    import Data.Foldable
    import Data.Functor.Utils ((#.))
    import Foreign.Storable (Storable)
    import GHC.Arr (Ix)
    import GHC.Base ( Applicative(..), Eq(..), Functor(..), Monad(..)
                    , Monoid, Ord(..), ($), (.) )
    import GHC.Enum (Bounded, Enum)
    import GHC.Float (Floating, RealFloat)
    import GHC.Generics (Generic, Generic1)
    import GHC.Num (Num)
    import GHC.Read (Read(..), lex, readParen)
    import GHC.Real (Fractional, Integral, Real, RealFrac)
    import GHC.Show (Show(..), showParen, showString)
    import GHC.Types (Bool(..))

    -- | Identity functor and monad. (a non-strict monad)
    --
    -- @since 4.8.0.0
    newtype Identity a = Identity { runIdentity :: a }
        deriving ( Bits, Bounded, Enum, Eq, FiniteBits, Floating, Fractional
                 , Generic, Generic1, Integral, Ix, Monoid, Num, Ord
                 , Real, RealFrac, RealFloat, Storable)

    -- | This instance would be equivalent to the derived instances of the
    -- 'Identity' newtype if the 'runIdentity' field were removed
    --
    -- @since 4.8.0.0
    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
    --
    -- @since 4.8.0.0
    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

    -- | @since 4.8.0.0
    instance Foldable Identity where
        foldMap                = coerce

        elem                   = (. runIdentity) #. (==)
        foldl                  = coerce
        foldl'                 = coerce
        foldl1 _               = runIdentity
        foldr f z (Identity x) = f x z
        foldr'                 = foldr
        foldr1 _               = runIdentity
        length _               = 1
        maximum                = runIdentity
        minimum                = runIdentity
        null _                 = False
        product                = runIdentity
        sum                    = runIdentity
        toList (Identity x)    = [x]

    -- | @since 4.8.0.0
    instance Functor Identity where
        fmap     = coerce

    -- | @since 4.8.0.0
    instance Applicative Identity where
        pure     = Identity
        (<*>)    = coerce
        liftA2   = coerce

    -- | @since 4.8.0.0
    instance Monad Identity where
        m >>= k  = k (runIdentity m)

    -- | @since 4.8.0.0
    instance MonadFix Identity where
        mfix f   = Identity (fix (runIdentity . f))