[Haskell-cafe] A GC'd heap using weak pointers

[Thomas Koster]

Good afternoon list,

I am implementing a simple, interpreted language that needs a garbage-
collected heap for storage. Like most discussions on memory management,
I use the term "heap" by analogy - the actual data structure is not
necessarily a true heap.

My Heap type is basically a map of "addresses" (Int) to values. Values
may themselves contain addresses, perhaps recursively to each other
(like letrec). The extra field in Heap stores the next unused address,
used to allocate fresh slots, as you can see in "heapAlloc".

> type Address = Int
> data Value = ...things with Addresses...
> data Heap = Heap !Address (Map Address Value)
>
> heapAlloc :: Heap -> (Address, Heap)
> heapAlloc (Heap nextFreeAddr binds) =
>   (nextFreeAddr, Heap (succ nextFreeAddr) binds)

There is also a stack of roots and an expression under evaluation.

> type Stack = [...things with Addresses...]
> data Expr = ...things with Addresses...

The function I need to write is:

> heapGC :: [Address] -> Heap -> Heap
> heapGC roots heap = ...collect unreachable values from heap...

Rather than re-invent this particularly complex wheel, I am wondering
if I could use GHC weak pointers and have the GHC runtime collect values
in my heap for me. Something like this:

> type Address = Int
> data Reference = Reference Address
> data Value = ...things with References...
> data Heap = Heap !Address (Map Address (Weak Value))
>
> heapAlloc :: Heap -> (Reference, Heap)
> heapAlloc (Heap nextFreeAddr binds) =
>   (Reference nextFreeAddr, Heap (succ nextFreeAddr) binds)
>
> heapStore :: Reference -> Value -> Heap -> IO Heap
> heapStore ref@(Reference addr) val (Heap nextFreeAddr binds) = do
>   weakVal <- mkWeak ref val Nothing
>   return $ Heap nextFreeAddr (Map.insert addr weakVal binds)

Note the new type, Reference. This replaces Address everywhere except
in the heap's internal structure, which continues to use Address.
Reference values are opaque, unique and obtained from the heap using
heapAlloc.

The idea is that only two things should cause Values to stay alive:
1. reachable holders of Reference (by GHC's definition of "reachable"),
2. ordinary Haskell closures arising from stepping the interpreter.
"Being in the Heap" should not be enough to keep a Value alive. Then,
my heapGC function should only have to prune dead addresses from the
map.

I am having trouble getting a proof-of-concept working (I am unable to
observe any finalizers running, ever), but before I get into that, is
this idea sound? Has somebody else implemented this in a library
already?

Thanks,
Thomas Koster