Sets the weakness state of hash-table. Value may be:
t -- An entry is kept if there is a pointer to the value from another object.
:key -- An entry is kept if there is a pointer to the key from another object.
:both -- An entry is kept if there are pointers to both the key and the value.
:either -- An entry is kept if there is a pointer to either the key or the value.
nil -- Make the hash-table non-weak. All entries are kept.
A designator for a function of two arguments.
Entries that can be removed are removed after a garbage collector (GC) operation which identifies that they can be removed. This means that if the relevant object(s) (the key or the value) have been promoted to a higher generation, a garbage collection (GC) of the higher generation is required to remove them from the table. Note that by default the system does not automatically GC the blocking generation or higher.
The weakness-state of a hash-table can be changed repeatedly, at any time, at any point using any of the weak values listed above. It can also be set by make-hash-table.
free-function can be supplied to specify a free function as described for make-hash-table. It has no effect if weak-kind is
LispWorks User Guide and Reference Manual - 20 Sep 2017