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11.5 Object Hashing

The MIT/GNU Scheme object-hashing facility provides a mechanism for generating a unique hash number for an arbitrary object. This hash number, unlike an object's address, is unchanged by garbage collection. The object-hashing facility is useful in conjunction with hash tables, but it may be used for other things as well. In particular, it is used in the generation of the written representation for many objects (see Custom Output).

All of these procedures accept an optional argument called table; this table contains the object-integer associations. If given, this argument must be an object-hash table as constructed by hash-table/make (see below). If not given, a default table is used.

— procedure: hash object [table]

hash associates an exact non-negative integer with object and returns that integer. If hash was previously called with object as its argument, the integer returned is the same as was returned by the previous call. hash guarantees that distinct objects (in the sense of eq?) are associated with distinct integers.

— procedure: unhash k [table]

unhash takes an exact non-negative integer k and returns the object associated with that integer. If there is no object associated with k, or if the object previously associated with k has been reclaimed by the garbage collector, an error of type condition-type:bad-range-argument is signalled. In other words, if hash previously returned k for some object, and that object has not been reclaimed, it is the value of the call to unhash.

An object that is passed to hash as an argument is not protected from being reclaimed by the garbage collector. If all other references to that object are eliminated, the object will be reclaimed. Subsequently calling unhash with the hash number of the (now reclaimed) object will signal an error. 

     (define x (cons 0 0))             unspecified
     (hash x)                          77
     (eqv? (hash x) (hash x))          #t
     (define x 0)                      unspecified
     (gc-flip)                       ;force a garbage collection
     (unhash 77)                     error-->
— procedure: object-hashed? object [table]

This predicate is true if object has an associated hash number. Otherwise it is false.

— procedure: valid-hash-number? k [table]

This predicate is true if k is the hash number associated with some object. Otherwise it is false.

The following two procedures provide a lower-level interface to the object-hashing mechanism.

— procedure: object-hash object [table [insert?]]

object-hash is like hash, except that it accepts an additional optional argument, insert?. If insert? is supplied and is #f, object-hash will return an integer for object only if there is already an association in the table; otherwise, it will return #f. If insert? is not supplied, or is not #f, object-hash always returns an integer, creating an association in the table if necessary.

object-hash additionally treats #f differently than does hash. Calling object-hash with #f as its argument will return an integer that, when passed to unhash, will signal an error rather than returning #f. Likewise, valid-hash-number? will return #f for this integer.

— procedure: object-unhash k [table]

object-unhash is like unhash, except that when k is not associated with any object or was previously associated with an object that has been reclaimed, object-unhash returns #f. This means that there is an ambiguity in the value returned by object-unhash: if #f is returned, there is no way to tell if k is associated with #f or is not associated with any object at all.

Finally, this procedure makes new object-hash tables:

— procedure: hash-table/make

This procedure creates and returns a new, empty object-hash table that is suitable for use as the optional table argument to the above procedures. The returned table contains no associations.