Chapter 30

module Foreign.Marshal (  
    module Foreign.Marshal.Alloc,  module Foreign.Marshal.Array,  
    module Foreign.Marshal.Error,  module Foreign.Marshal.Utils,  
  ) where

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
module Foreign.Marshal.Utils

and provides one function:

unsafeLocalState :: IO a -> a
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.