Foreign/Marshal.hs

    {-# LANGUAGE PackageImports #-}

    module Foreign.Marshal (
             -- | The module "Foreign.Marshal" re-exports the other modules in the
             -- @Foreign.Marshal@ hierarchy:

             module Foreign.Marshal.Alloc,
             module Foreign.Marshal.Array,
             module Foreign.Marshal.Error,
            -- Not in Haskell 2010:
            -- , module Foreign.Marshal.Pool
             module Foreign.Marshal.Utils,
             -- | and provides one function:
             unsafeLocalState
      ) where

    import "this" Foreign.Marshal.Alloc
    import "this" Foreign.Marshal.Array
    import "this" Foreign.Marshal.Error
    -- Not in Haskell 2010:
    -- import "this" Foreign.Marshal.Pool
    import "this" Foreign.Marshal.Utils

    import "base" System.IO.Unsafe

    {- |
    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 = unsafePerformIO