Data marshal and stable pointers

The Haskell types Ptr a and FunPtr a represent pointers in foreign, raw memory (outside the Haskell heap). Relevant operations on foreign pointers are provided by the Storable type class, which has instances for primitive marshallable data types. A third pointer type is StablePtr a, which is a pointer to an object in the Haskell heap.

On top of passing primitive values and pointers through the FFI, almost arbitrary data can be marshalled between Haskell and C (and by extension other languages) relatively easily.

Allocating memory outside the heap

A centric type-class in marshalling is Foreign.Storable.Storable. Storable types must provide a length, byte alignment, and the methods peek and poke:

class Storable a where
  sizeOf :: a ->Int
  alignment :: a ->Int
  peek :: Ptr a -> IO a
  poke :: Ptr a -> a -> IO ()
  …

Primitive allocation routines are located in the Foreign.Marshal.Alloc module. Normal dynamic allocations are provided for...