[Haskell-cafe] Mistakes in documentation for weak pointers?

Heinrich Apfelmus apfelmus at quantentunnel.de
Tue Sep 10 20:12:24 CEST 2013

Dear café,

I'm currently studying weak pointers in order to implement garbage 
collection for a small JavaScript FFI used by the threepenny-gui library 

While the paper [2] is fairly clear, it seems that the documentation in 
System.Mem.Weak [3] differs in certain aspects. Could someone help me 
and clarify this? I have two questions:

1. The sentence in the documentation

"References from the finalizer to the key are treated in the same way as 
references from the value to the key: they do not keep the key alive."

seems incorrect in the subordinate clause. Namely, the paper only states 
that the weak pointer object does not keep the key alive. However, I 
understood that it is perfectly acceptable for the value to keep the key 
alive, and that this relation is not changed by  mkWeak . Is this still 

(The memo table example in the paper never stores the value itself, only 
a weak pointer to it, so it doesn't matter whether the value is 
reachable from the key or not.)

2. The sentence in the documentation

"A heap object is reachable if: ... It is a weak pointer object whose 
key is reachable."

seems to differ from the paper, which states that while weak pointers 
are always retained if their keys are alive, they can very much be 
unreachable. But if I think about it, the formulation in the 
documentation seems to be equivalent to the paper (as far as the 
semantics are concerned, it does not matter whether unreachable weak 
pointers are tombstoned or not.). Is that correct?

   [1]: http://www.haskell.org/haskellwiki/Threepenny-gui
   [2]: http://community.haskell.org/~simonmar/papers/weak.pdf

Best regards,
Heinrich Apfelmus


More information about the Haskell-Cafe mailing list