{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE PolyKinds #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  GHC.Internal.Data.Proxy
-- License     :  BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer  :  libraries@haskell.org
-- Stability   :  stable
-- Portability :  portable
--
-- Definition of a Proxy type (poly-kinded in GHC)
--
-- @since base-4.7.0.0
-----------------------------------------------------------------------------

module GHC.Internal.Data.Proxy
  (
        Proxy(..), asProxyTypeOf
      , KProxy(..)
  ) where

import GHC.Internal.Base
import GHC.Internal.Show
import GHC.Internal.Read
import GHC.Internal.Enum
import GHC.Internal.Arr

-- $setup
-- >>> import GHC.Internal.Data.Void
-- >>> import Prelude

-- | 'Proxy' is a type that holds no data, but has a phantom parameter of
-- arbitrary type (or even kind). Its use is to provide type information, even
-- though there is no value available of that type (or it may be too costly to
-- create one).
--
-- Historically, @'Proxy' :: 'Proxy' a@ is a safer alternative to the
-- @'undefined' :: a@ idiom.
--
-- >>> Proxy :: Proxy (Void, Int -> Int)
-- Proxy
--
-- Proxy can even hold types of higher kinds,
--
-- >>> Proxy :: Proxy Either
-- Proxy
--
-- >>> Proxy :: Proxy Functor
-- Proxy
--
-- >>> Proxy :: Proxy complicatedStructure
-- Proxy
data Proxy t = Proxy deriving ( Proxy t
Proxy t -> Proxy t -> Bounded (Proxy t)
forall a. a -> a -> Bounded a
forall k (t :: k). Proxy t
$cminBound :: forall k (t :: k). Proxy t
minBound :: Proxy t
$cmaxBound :: forall k (t :: k). Proxy t
maxBound :: Proxy t
Bounded -- ^ @since base-4.7.0.0
                              , ReadPrec [Proxy t]
ReadPrec (Proxy t)
Int -> ReadS (Proxy t)
ReadS [Proxy t]
(Int -> ReadS (Proxy t))
-> ReadS [Proxy t]
-> ReadPrec (Proxy t)
-> ReadPrec [Proxy t]
-> Read (Proxy t)
forall a.
(Int -> ReadS a)
-> ReadS [a] -> ReadPrec a -> ReadPrec [a] -> Read a
forall k (t :: k). ReadPrec [Proxy t]
forall k (t :: k). ReadPrec (Proxy t)
forall k (t :: k). Int -> ReadS (Proxy t)
forall k (t :: k). ReadS [Proxy t]
$creadsPrec :: forall k (t :: k). Int -> ReadS (Proxy t)
readsPrec :: Int -> ReadS (Proxy t)
$creadList :: forall k (t :: k). ReadS [Proxy t]
readList :: ReadS [Proxy t]
$creadPrec :: forall k (t :: k). ReadPrec (Proxy t)
readPrec :: ReadPrec (Proxy t)
$creadListPrec :: forall k (t :: k). ReadPrec [Proxy t]
readListPrec :: ReadPrec [Proxy t]
Read    -- ^ @since base-4.7.0.0
                              )

-- | A concrete, promotable proxy type, for use at the kind level.
-- There are no instances for this because it is intended at the kind level only
data KProxy (t :: Type) = KProxy

-- It's common to use (undefined :: Proxy t) and (Proxy :: Proxy t)
-- interchangeably, so all of these instances are hand-written to be
-- lazy in Proxy arguments.

-- | @since base-4.7.0.0
instance Eq (Proxy s) where
  Proxy s
_ == :: Proxy s -> Proxy s -> Bool
== Proxy s
_ = Bool
True

-- | @since base-4.7.0.0
instance Ord (Proxy s) where
  compare :: Proxy s -> Proxy s -> Ordering
compare Proxy s
_ Proxy s
_ = Ordering
EQ

-- | @since base-4.7.0.0
instance Show (Proxy s) where
  showsPrec :: Int -> Proxy s -> ShowS
showsPrec Int
_ Proxy s
_ = String -> ShowS
showString String
"Proxy"

-- | @since base-4.7.0.0
instance Enum (Proxy s) where
    succ :: Proxy s -> Proxy s
succ Proxy s
_               = String -> Proxy s
forall a. String -> a
errorWithoutStackTrace String
"Proxy.succ"
    pred :: Proxy s -> Proxy s
pred Proxy s
_               = String -> Proxy s
forall a. String -> a
errorWithoutStackTrace String
"Proxy.pred"
    fromEnum :: Proxy s -> Int
fromEnum Proxy s
_           = Int
0
    toEnum :: Int -> Proxy s
toEnum Int
0             = Proxy s
forall k (t :: k). Proxy t
Proxy
    toEnum Int
_             = String -> Proxy s
forall a. String -> a
errorWithoutStackTrace String
"Proxy.toEnum: 0 expected"
    enumFrom :: Proxy s -> [Proxy s]
enumFrom Proxy s
_           = [Proxy s
forall k (t :: k). Proxy t
Proxy]
    enumFromThen :: Proxy s -> Proxy s -> [Proxy s]
enumFromThen Proxy s
_ Proxy s
_     = [Proxy s
forall k (t :: k). Proxy t
Proxy]
    enumFromThenTo :: Proxy s -> Proxy s -> Proxy s -> [Proxy s]
enumFromThenTo Proxy s
_ Proxy s
_ Proxy s
_ = [Proxy s
forall k (t :: k). Proxy t
Proxy]
    enumFromTo :: Proxy s -> Proxy s -> [Proxy s]
enumFromTo Proxy s
_ Proxy s
_       = [Proxy s
forall k (t :: k). Proxy t
Proxy]

-- | @since base-4.7.0.0
instance Ix (Proxy s) where
    range :: (Proxy s, Proxy s) -> [Proxy s]
range (Proxy s, Proxy s)
_           = [Proxy s
forall k (t :: k). Proxy t
Proxy]
    index :: (Proxy s, Proxy s) -> Proxy s -> Int
index (Proxy s, Proxy s)
_ Proxy s
_         = Int
0
    inRange :: (Proxy s, Proxy s) -> Proxy s -> Bool
inRange (Proxy s, Proxy s)
_ Proxy s
_       = Bool
True
    rangeSize :: (Proxy s, Proxy s) -> Int
rangeSize (Proxy s, Proxy s)
_       = Int
1
    unsafeIndex :: (Proxy s, Proxy s) -> Proxy s -> Int
unsafeIndex (Proxy s, Proxy s)
_ Proxy s
_   = Int
0
    unsafeRangeSize :: (Proxy s, Proxy s) -> Int
unsafeRangeSize (Proxy s, Proxy s)
_ = Int
1

-- | @since base-4.9.0.0
instance Semigroup (Proxy s) where
    Proxy s
_ <> :: Proxy s -> Proxy s -> Proxy s
<> Proxy s
_ = Proxy s
forall k (t :: k). Proxy t
Proxy
    sconcat :: NonEmpty (Proxy s) -> Proxy s
sconcat NonEmpty (Proxy s)
_ = Proxy s
forall k (t :: k). Proxy t
Proxy
    stimes :: forall b. Integral b => b -> Proxy s -> Proxy s
stimes b
_ Proxy s
_ = Proxy s
forall k (t :: k). Proxy t
Proxy

-- | @since base-4.7.0.0
instance Monoid (Proxy s) where
    mempty :: Proxy s
mempty = Proxy s
forall k (t :: k). Proxy t
Proxy
    mconcat :: [Proxy s] -> Proxy s
mconcat [Proxy s]
_ = Proxy s
forall k (t :: k). Proxy t
Proxy

-- | @since base-4.7.0.0
instance Functor Proxy where
    fmap :: forall a b. (a -> b) -> Proxy a -> Proxy b
fmap a -> b
_ Proxy a
_ = Proxy b
forall k (t :: k). Proxy t
Proxy
    {-# INLINE fmap #-}

-- | @since base-4.7.0.0
instance Applicative Proxy where
    pure :: forall a. a -> Proxy a
pure a
_ = Proxy a
forall k (t :: k). Proxy t
Proxy
    {-# INLINE pure #-}
    Proxy (a -> b)
_ <*> :: forall a b. Proxy (a -> b) -> Proxy a -> Proxy b
<*> Proxy a
_ = Proxy b
forall k (t :: k). Proxy t
Proxy
    {-# INLINE (<*>) #-}

-- | @since base-4.9.0.0
instance Alternative Proxy where
    empty :: forall a. Proxy a
empty = Proxy a
forall k (t :: k). Proxy t
Proxy
    {-# INLINE empty #-}
    Proxy a
_ <|> :: forall a. Proxy a -> Proxy a -> Proxy a
<|> Proxy a
_ = Proxy a
forall k (t :: k). Proxy t
Proxy
    {-# INLINE (<|>) #-}

-- | @since base-4.7.0.0
instance Monad Proxy where
    Proxy a
_ >>= :: forall a b. Proxy a -> (a -> Proxy b) -> Proxy b
>>= a -> Proxy b
_ = Proxy b
forall k (t :: k). Proxy t
Proxy
    {-# INLINE (>>=) #-}

-- | @since base-4.9.0.0
instance MonadPlus Proxy

-- | 'asProxyTypeOf' is a type-restricted version of 'const'.
-- It is usually used as an infix operator, and its typing forces its first
-- argument (which is usually overloaded) to have the same type as the tag
-- of the second.
--
-- >>> import GHC.Internal.Word
-- >>> :type asProxyTypeOf 123 (Proxy :: Proxy Word8)
-- asProxyTypeOf 123 (Proxy :: Proxy Word8) :: Word8
--
-- Note the lower-case @proxy@ in the definition. This allows any type
-- constructor with just one argument to be passed to the function, for example
-- we could also write
--
-- >>> import GHC.Internal.Word
-- >>> :type asProxyTypeOf 123 (Just (undefined :: Word8))
-- asProxyTypeOf 123 (Just (undefined :: Word8)) :: Word8
asProxyTypeOf :: a -> proxy a -> a
asProxyTypeOf :: forall a (proxy :: * -> *). a -> proxy a -> a
asProxyTypeOf = a -> proxy a -> a
forall a b. a -> b -> a
const
{-# INLINE asProxyTypeOf #-}