Data/Typeable.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude
               , OverlappingInstances
               , ScopedTypeVariables
               , FlexibleInstances
               , TypeOperators
               , PolyKinds
               , GADTs
               , MagicHash
      #-}
    {-# OPTIONS_GHC -funbox-strict-fields #-}

    -- The -XOverlappingInstances flag allows the user to over-ride
    -- the instances for Typeable given here.  In particular, we provide an instance
    --      instance ... => Typeable (s a)
    -- But a user might want to say
    --      instance ... => Typeable (MyType a b)

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Typeable
    -- Copyright   :  (c) The University of Glasgow, CWI 2001--2004
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- The 'Typeable' class reifies types to some extent by associating type
    -- representations to types. These type representations can be compared,
    -- and one can in turn define a type-safe cast operation. To this end,
    -- an unsafe cast is guarded by a test for type (representation)
    -- equivalence. The module "Data.Dynamic" uses Typeable for an
    -- implementation of dynamics. The module "Data.Data" uses Typeable
    -- and type-safe cast (but not dynamics) to support the \"Scrap your
    -- boilerplate\" style of generic programming.
    --
    -- == Compatibility Notes
    --
    -- Since GHC 7.8, 'Typeable' is poly-kinded. The changes required for this might
    -- break some old programs involving 'Typeable'. More details on this, including
    -- how to fix your code, can be found on the
    -- <https://ghc.haskell.org/trac/ghc/wiki/GhcKinds/PolyTypeable PolyTypeable wiki page>
    --
    -----------------------------------------------------------------------------

    module Data.Typeable
      (
            -- * The Typeable class
            Typeable,
            typeRep,

            -- * Propositional equality
            (:~:)(Refl),

            -- * For backwards compatibility
            typeOf, typeOf1, typeOf2, typeOf3, typeOf4, typeOf5, typeOf6, typeOf7,
            Typeable1, Typeable2, Typeable3, Typeable4, Typeable5, Typeable6,
            Typeable7,

            -- * Type-safe cast
            cast,
            eqT,
            gcast,                  -- a generalisation of cast

            -- * Generalized casts for higher-order kinds
            gcast1,                 -- :: ... => c (t a) -> Maybe (c (t' a))
            gcast2,                 -- :: ... => c (t a b) -> Maybe (c (t' a b))

            -- * A canonical proxy type
            Proxy (..),

            -- * Type representations
            TypeRep,        -- abstract, instance of: Eq, Show, Typeable
            showsTypeRep,

            TyCon,          -- abstract, instance of: Eq, Show, Typeable
            tyConString,
            tyConPackage,
            tyConModule,
            tyConName,

            -- * Construction of type representations
            -- mkTyCon,        -- :: String  -> TyCon
            mkTyCon3,       -- :: String  -> String -> String -> TyCon
            mkTyConApp,     -- :: TyCon   -> [TypeRep] -> TypeRep
            mkAppTy,        -- :: TypeRep -> TypeRep   -> TypeRep
            mkFunTy,        -- :: TypeRep -> TypeRep   -> TypeRep

            -- * Observation of type representations
            splitTyConApp,  -- :: TypeRep -> (TyCon, [TypeRep])
            funResultTy,    -- :: TypeRep -> TypeRep   -> Maybe TypeRep
            typeRepTyCon,   -- :: TypeRep -> TyCon
            typeRepArgs,    -- :: TypeRep -> [TypeRep]
      ) where

    import Data.Typeable.Internal hiding (mkTyCon)
    import Data.Type.Equality

    import Unsafe.Coerce
    import Data.Maybe
    import GHC.Base

    -------------------------------------------------------------
    --
    --              Type-safe cast
    --
    -------------------------------------------------------------

    -- | The type-safe cast operation
    cast :: forall a b. (Typeable a, Typeable b) => a -> Maybe b
    cast x = if typeRep (Proxy :: Proxy a) == typeRep (Proxy :: Proxy b)
               then Just $ unsafeCoerce x
               else Nothing

    -- | Extract a witness of equality of two types
    --
    -- /Since: 4.7.0.0/
    eqT :: forall a b. (Typeable a, Typeable b) => Maybe (a :~: b)
    eqT = if typeRep (Proxy :: Proxy a) == typeRep (Proxy :: Proxy b)
          then Just $ unsafeCoerce Refl
          else Nothing

    -- | A flexible variation parameterised in a type constructor
    gcast :: forall a b c. (Typeable a, Typeable b) => c a -> Maybe (c b)
    gcast x = fmap (\Refl -> x) (eqT :: Maybe (a :~: b))

    -- | Cast over @k1 -> k2@
    gcast1 :: forall c t t' a. (Typeable t, Typeable t')
           => c (t a) -> Maybe (c (t' a))
    gcast1 x = fmap (\Refl -> x) (eqT :: Maybe (t :~: t'))

    -- | Cast over @k1 -> k2 -> k3@
    gcast2 :: forall c t t' a b. (Typeable t, Typeable t')
           => c (t a b) -> Maybe (c (t' a b))
    gcast2 x = fmap (\Refl -> x) (eqT :: Maybe (t :~: t'))