{-# 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.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# And that, sadly, is ill-typed because unsafeCoerce# has OpenKind type variables And rightly so, because 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, else the type checker -- give usafeCoerce the same (dangerous) type as unsafeCoerce#