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{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Functor.Const
-- Copyright : Conor McBride and Ross Paterson 2005
-- License : BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
-- The 'Const' functor.
--
-- @since 4.9.0.0
module Data.Functor.Const (Const(..)) where
import Data.Foldable (Foldable(foldMap))
import GHC.Base
import GHC.Generics (Generic, Generic1)
import GHC.Read (Read(readsPrec), readParen, lex)
import GHC.Show (Show(showsPrec), showParen, showString)
-- | The 'Const' functor.
newtype Const a b = Const { getConst :: a }
deriving (Generic, Generic1, Monoid, Eq, Ord)
-- | This instance would be equivalent to the derived instances of the
-- 'Const' newtype if the 'runConst' field were removed
instance Read a => Read (Const a b) where
readsPrec d = readParen (d > 10)
$ \r -> [(Const x,t) | ("Const", s) <- lex r, (x, t) <- readsPrec 11 s]
-- | This instance would be equivalent to the derived instances of the
-- 'Const' newtype if the 'runConst' field were removed
instance Show a => Show (Const a b) where
showsPrec d (Const x) = showParen (d > 10) $
showString "Const " . showsPrec 11 x
instance Foldable (Const m) where
foldMap _ _ = mempty
instance Functor (Const m) where
fmap _ (Const v) = Const v
instance Monoid m => Applicative (Const m) where
pure _ = Const mempty
(<*>) = coerce (mappend :: m -> m -> m)
-- This is pretty much the same as
-- Const f <*> Const v = Const (f `mappend` v)
-- but guarantees that mappend for Const a b will have the same arity
-- as the one for a; it won't create a closure to raise the arity
-- to 2.
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