Re: ArrayArrays

On 27/08/2015 19:36, Edward Kmett wrote:

On Thu, Aug 27, 2015 at 1:24 PM, Edward Z. Yang mailto:ezyang@mit.edu> wrote:

It seems to me that we should take a page from OCaml's playbook and add support for native mutable fields in objects, because this is essentially what a mix of words and pointers is.

That actually doesn't work as well as one might hope.

We currently treat data constructor closures as so much tissue paper around a present. We tear them open, rip out all their contents, scatter them throughout our code and then we build a whole new data constructor closure when we're done, or we just leave them suspended in closures awaiting someone to demand we finally make a new data constructor.

Half the time we don't even give back the data constructor closure and push it into update g frames and we just give back the items on the stack.

With the machinery I mentioned above I get a world where every time I access an object I can know it is evaluated for real, so this means I'm not stuck 'entering an unknown closure', and getting it to give me back a slab of memory that we know is a real data constructor that i can bang away on mutable entries in.

In a world where things in * could hold mutable pointers we have to care a lot more about object identity in deeply uncomfortable ways.

With what I've implemented I only care about object identity between things in # that are gcptrs. The garbage collector may move them around, but it doesn't put in thunks anywhere.

Yeah, I've actually thought about whether we could have mutable fields in constructors a couple of times, and it's far from easy for the reasons you describe. A constructor with mutable fields would need to be an object with identity, with precise control over when it is created. This is nothing like an ordinary constructor. I like the alternative approach in this thread, which is to attack the problem from the other end: start with a primitive object and make it more like a constructor. I don't see any reason why we shouldn't add primops to read/write SmallArray# and other primitive objects in an ArrayArray#. Will someone make a patch? It should be pretty straightforward. Cheers, Simon