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{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE MagicHash #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Type.Coercion
-- License : BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : not portable
--
-- Definition of representational equality ('Coercion').
--
-- @since 4.7.0.0
-----------------------------------------------------------------------------
module Data.Type.Coercion
( Coercion(..)
, coerceWith
, gcoerceWith
, sym
, trans
, repr
, TestCoercion(..)
) where
import qualified Data.Type.Equality as Eq
import Data.Maybe
import GHC.Enum
import GHC.Show
import GHC.Read
import GHC.Base
-- | Representational equality. If @Coercion a b@ is inhabited by some terminating
-- value, then the type @a@ has the same underlying representation as the type @b@.
--
-- To use this equality in practice, pattern-match on the @Coercion a b@ to get out
-- the @Coercible a b@ instance, and then use 'coerce' to apply it.
--
-- @since 4.7.0.0
data Coercion a b where
Coercion :: Coercible a b => Coercion a b
-- with credit to Conal Elliott for 'ty', Erik Hesselink & Martijn van
-- Steenbergen for 'type-equality', Edward Kmett for 'eq', and Gabor Greif
-- for 'type-eq'
-- | Type-safe cast, using representational equality
coerceWith :: Coercion a b -> a -> b
coerceWith Coercion x = coerce x
-- | Generalized form of type-safe cast using representational equality
--
-- @since 4.10.0.0
gcoerceWith :: Coercion a b -> (Coercible a b => r) -> r
gcoerceWith Coercion x = x
-- | Symmetry of representational equality
sym :: Coercion a b -> Coercion b a
sym Coercion = Coercion
-- | Transitivity of representational equality
trans :: Coercion a b -> Coercion b c -> Coercion a c
trans Coercion Coercion = Coercion
-- | Convert propositional (nominal) equality to representational equality
repr :: (a Eq.:~: b) -> Coercion a b
repr Eq.Refl = Coercion
-- | @since 4.7.0.0
deriving instance Eq (Coercion a b)
-- | @since 4.7.0.0
deriving instance Show (Coercion a b)
-- | @since 4.7.0.0
deriving instance Ord (Coercion a b)
-- | @since 4.7.0.0
deriving instance Coercible a b => Read (Coercion a b)
-- | @since 4.7.0.0
instance Coercible a b => Enum (Coercion a b) where
toEnum 0 = Coercion
toEnum _ = errorWithoutStackTrace "Data.Type.Coercion.toEnum: bad argument"
fromEnum Coercion = 0
-- | @since 4.7.0.0
deriving instance Coercible a b => Bounded (Coercion a b)
-- | This class contains types where you can learn the equality of two types
-- from information contained in /terms/. Typically, only singleton types should
-- inhabit this class.
class TestCoercion f where
-- | Conditionally prove the representational equality of @a@ and @b@.
testCoercion :: f a -> f b -> Maybe (Coercion a b)
-- | @since 4.7.0.0
instance TestCoercion ((Eq.:~:) a) where
testCoercion Eq.Refl Eq.Refl = Just Coercion
-- | @since 4.10.0.0
instance TestCoercion ((Eq.:~~:) a) where
testCoercion Eq.HRefl Eq.HRefl = Just Coercion
-- | @since 4.7.0.0
instance TestCoercion (Coercion a) where
testCoercion Coercion Coercion = Just Coercion
|