Foreign/Marshal/Unsafe.hs

    {-# LANGUAGE Unsafe #-}
    {-# LANGUAGE NoImplicitPrelude #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Foreign.Marshal.Unsafe
    -- Copyright   :  (c) The FFI task force 2003
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  ffi@haskell.org
    -- Stability   :  provisional
    -- Portability :  portable
    --
    -- Marshalling support. Unsafe API.
    --
    -----------------------------------------------------------------------------

    module Foreign.Marshal.Unsafe (
            -- * Unsafe functions
            unsafeLocalState
        ) where

    import GHC.IO

    {- |
    Sometimes an external entity is a pure function, except that it passes
    arguments and/or results via pointers.  The function
    @unsafeLocalState@ permits the packaging of such entities as pure
    functions.

    The only IO operations allowed in the IO action passed to
    @unsafeLocalState@ are (a) local allocation (@alloca@, @allocaBytes@
    and derived operations such as @withArray@ and @withCString@), and (b)
    pointer operations (@Foreign.Storable@ and @Foreign.Ptr@) on the
    pointers to local storage, and (c) foreign functions whose only
    observable effect is to read and/or write the locally allocated
    memory.  Passing an IO operation that does not obey these rules
    results in undefined behaviour.

    It is expected that this operation will be
    replaced in a future revision of Haskell.
    -}
    unsafeLocalState :: IO a -> a
    unsafeLocalState = unsafeDupablePerformIO