is there any way to perform a destructive update on a plain ADT? Imagine I have a simple data Tree a = Nil | Node a (Tree a) (Tree a) I would like to be able to modify right subtree of an existing tree.
I can do that for example when using IORefs by changing the datatype to data Tree a = Nil | Node a (IORef (Tree a)) (IORef (Tree a)) and use unsafePerformIO + writeIORef. But the IORefs cause additional complexity when working with the data type.
At the moment I am interested in any GHC solution, be it non-portable or version specific. I would like just to run some benchmarks and see the results.
Cheers, Milan
You can do it if you refer to the children using Array#s. That's what I do in unordered-containers to implement a more efficient fromList.
But the Array# adds another level of indirection, which seem wasteful if it has 2 elements. Also creating and cloning array is calling method in runtime, which is another source of (minor) slowdown.
For arbitrary field types I don't think there's a way (although it would be useful when birthing persistent data structures).
That is my reason for asking this. I was hoping for some Addr# trick or something like that. If I understand the GHC runtime correctly, rewriting a pointer in an ADT should not break any garbage collecting and similar. Cheers, Milan