This module provides the Structure type class, enabling you to create a pure data structure, which is available immediately, and then gradually fill it in using the IO monad. This means that you can go ahead and start using the structure before it is fully defined. If a part of the structure is evaluated before it has been written then that part is treated as _|_. This is technically more pure than values from unsafeInterleaveIO. Evaluating the structure, even before it is defined, doesn't perform any IO. All the IO happens in the IO monad.

Here is an example of creating a list of Ints and then writing each level of it step by step. The returned result is [5].

 foo :: IO [Int]
 foo = do

   -- Create the pure list and the handle for writing to it.
   (pxs, xs) <- newStruc

   -- Write the cons constructor to the list. This gives us handles
   -- for writing the head and tail of the list.
   Just (h, t) <- writeStruc pxs ConsC

   -- Write the head of the list.
   writeStruc h 5

   -- Write the tail of the list. This finishes it.
   writeStruc t NilC

   -- Get rid of the Value wrapper on the list's value and return
   -- the resulting list.
   return $ map getValue xs