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{-# LANGUAGE Unsafe #-} {-# LANGUAGE NoImplicitPrelude, MagicHash #-}

-- | -- Module : Unsafe.Coerce -- Copyright : Malcolm Wallace 2006 -- License : BSD-style (see the LICENSE file in the distribution)

-- Maintainer : libraries@haskell.org -- Stability : experimental -- Portability : portable

-- The highly unsafe primitive 'unsafeCoerce' converts a value from any -- type to any other type. Needless to say, if you use this function, -- it is your responsibility to ensure that the old and new types have -- identical internal representations, in order to prevent runtime corruption.

-- The types for which 'unsafeCoerce' is representation-safe may differ -- from compiler to compiler (and version to version).

-- * Documentation for correct usage in GHC will be found under -- 'unsafeCoerce#' in GHC.Base (around which 'unsafeCoerce' is just a -- trivial wrapper).

-- * In nhc98, the only representation-safe coercions are between Enum -- types with the same range (e.g. Int, Int32, Char, Word32), -- or between a newtype and the type that it wraps.

module Unsafe.Coerce (unsafeCoerce) where

import GHC.Integer () -- for build ordering import GHC.Natural () -- for build ordering import GHC.Prim (unsafeCoerce#)

local_id :: a -> a local_id x = x -- See Note [Mega-hack for coerce]

{- Note [Mega-hack for coerce]

If we just say unsafeCoerce x = unsafeCoerce# x then the simple-optimiser that the desugarer runs will eta-reduce to unsafeCoerce :: forall (a:) (b:). a -> b unsafeCoerce = unsafeCoerce# But we shouldn't be calling unsafeCoerce# in a higher order way; it has a compulsory unfolding unsafeCoerce# a b x = x |> UnsafeCo a b and we really rely on it being inlined pronto. But the simple-optimiser doesn't. The identity function local_id delays the eta reduction just long enough for unsafeCoerce# to get inlined.

Sigh. This is horrible, but then so is unsafeCoerce. -}

unsafeCoerce :: a -> b unsafeCoerce x = local_id (unsafeCoerce# x) -- See Note [Unsafe coerce magic] in basicTypes/MkId -- NB: Do not eta-reduce this definition (see above)