A weak pointer object with a key and a value. The value has type v. A weak pointer expresses a relationship between two objects, the
key and the value: if the key is considered to be alive by the
garbage collector, then the value is also alive. A reference from
the value to the key does not keep the key alive. A weak pointer may also have a finalizer of type IO (); if it does,
then the finalizer will be run once, and once only, at a time after
the key has become unreachable by the program ("dead"). The storage
manager attempts to run the finalizer(s) for an object soon after the
object dies, but promptness is not guaranteed. 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. A finalizer may therefore ressurrect the key, perhaps by
storing it in the same data structure. The finalizer, and the relationship between the key and the value,
exist regardless of whether the program keeps a reference to the
Weak object or not. There may be multiple weak pointers with the same key. In this
case, the finalizers for each of these weak pointers will all be
run in some arbitrary order, or perhaps concurrently, when the key
dies. If the programmer specifies a finalizer that assumes it has
the only reference to an object (for example, a file that it wishes
to close), then the programmer must ensure that there is only one
such finalizer. If there are no other threads to run, the runtime system will check
for runnable finalizers before declaring the system to be deadlocked.
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