[MLton-user] Documentation on finalizable values

[Stephen Weeks]

It is certainly the case that a finalizable has a strong pointer to
the object it contains.  I've tweaked the documentation to make that
clear, as well as added a simple page on reachability.  The underlying
notion of reachability is really the same for weak pointers and
finalizers; it's just that a finalizer isn't a weak pointer.
Hopefully it's all clear now.