EQUALP-GENERICEQUAL-STRUCTURE (as ammended) has generated some complaints about its specified behavior on objects with metaclass STRUCTURE-CLASS. Some programs which depend on EQUALP comparing the components of structures are now broken.
There are some who feel that EQUALP could be made much more useful than it currently is by making it user extensible, but there are potential difficulties due to the recent acceptance of Issue HASH-TABLE-TESTS, which added EQUALP as a valid hash-table test.
Specify that EQUALP is a generic function, and define the new generic function EQUALP-HASH. Specify that a necessary requirement on all methods for these functions is that (EQUALP x y) => (= (EQUALP-HASH x t) (EQUALP-HASH y t)). Add a discussion of the constraints on equivelence predicates and their corresponding hash functions, similar to that presented in Issue HASH-TABLE-STABILITY.
EQUALP object1 object2 [Generic Function]
The generic function EQUALP is a predicate which can be called to compare two objects for equivelence. The precise definition of equivelence is specified by the methods defined on this function. The result is either true or false, depending on whether the objects are equivelent.
Method Signatures:
EQUALP (object1 NUMBER) (object2 NUMBER) [Primary Method] Defines equality as being =.
EQUALP (object1 CHARACTER) (object2 CHARACTER) [Primary Method] Defines equality as being CHAR-EQUAL.
EQUALP (object1 CONS) (object2 CONS) [Primary Method] Defines equality as having EQUALP CAR's and EQUALP CDR's.
EQUALP (object1 PATHNAME) (object2 PATHNAME) [Primary Method] Defines equality as being EQUAL.
EQUALP (object1 VECTOR) (object2 VECTOR) [Primary Method] Defines equality as having the same length (observing fill-pointers when present), and every pair of corresponding components are EQUALP.
EQUALP (object1 ARRAY) (object2 ARRAY) [Primary Method] Defines equality as having the same rank and dimensions, and every pair of corresponding components are EQUALP.
EQUALP (object1 HASH-TABLE) (object2 HASH-TABLE) [Primary Method] Defines equality as satisfying the following conditions: 1. Each hash-table uses the same test function. 2. Each hash-table contains entries for the same set of keys, where the test function is used to determine the equivelence of keys. 3. For each key, the associated values from the two tables are EQUALP.
EQUALP (object1 T) (object2 T) [Primary Method] The default method defines equality as being EQ.
Remarks: An error is signaled on an attempt to add a method on any built-in class. Adding a method on a class after ALLOCATE-INSTANCE has been called on the class has undefined consequences. Implementations may be extended in this case.
An implementation may specify additional methods on this function.
EQUALP-HASH object &optional use-pointer depth [Generic Function]
The generic function EQUALP-HASH is a called to compute a hash code for the Object. Use-Pointer is a flag. If true, the memory location of the object may be used to generate the hash code. The default is false.
Depth is used to control the depth of recursion, allowing cuttoff. This prevents infinite recursion in the case of circular references. Callers of this function should not explicitely specify a value for this argument. Methods on this function which need to make recursive calls should simply pass the Depth argument along unchanged.
This function returns two values. The first value is the hash code, which is a non-negative fixnum. The second value is a flag indicating whether the memory location of the object (or any component) was used in generating the hash code. A caller who specifies NIL for the Use-Pointer argument may reliably depend on the second value being false.
This function is used by the implementation to compute hash codes for EQUALP hash-tables. By specifying a NIL value for the second argument, it may also be used to compute a code which is independent of the particular "incarnation" or "core image", just as for SXHASH.
Method Signatures:
EQUALP-HASH (object NUMBER) &optional use-pointer depth [Primary Method] EQUALP-HASH (object CHARACTER) &optional use-pointer depth [Primary Method] EQUALP-HASH (object CONS) &optional use-pointer depth [Primary Method] EQUALP-HASH (object PATHNAME) &optional use-pointer depth [Primary Method] EQUALP-HASH (object VECTOR) &optional use-pointer depth [Primary Method] EQUALP-HASH (object ARRAY) &optional use-pointer depth [Primary Method] EQUALP-HASH (object HASH-TABLE) &optional use-pointer depth [Primary Method] EQUALP-HASH (object T) &optional use-pointer depth [Primary Method]
The manner in which the hash code is computed by the standard methods on this function is implementation dependent.
Remarks: An error is signaled on an attempt to add a method on any built-in class. Adding a method on a class after ALLOCATE-INSTANCE has been called on the class has undefined consequences. Implementations may be extended in this case.
An implementation may specify additional methods on this function.
EQUALP to be a generic function which the user can extend, we allow the user to specify the algorithm for determining equivelence.
The restrictions on adding additional methods are due to the possiblity of existing data structures having been built up using the previous set of methods. One way an implementation could be extended would be to provide a mechanism for marking potentially invalid data structures when new methods are added.
EQUALP, and must have a function similar to EQUALP-HASH in order to implement hash-tables with an EQUALP test function. This proposal merely exposes the hashing function and makes both of these functions user-specializable.EQUALP descending into structures will have to write methods for both functions for the classes they wish to continue to have this behavior.EQUALP or EQUALP hash-tables may be affected due to the differing performance of generic functions compared with normal functions doing type analysis. Presumably the performance can only be worse than what would be the case if this proposal were not adopted, since if defining these functions as generic produced better performance, then the implementation may have already made them generic, possibly without advertising the fact.EQUALP-HASH are possible. The one offered in the proposal is simple and trivial to implement. It is also kind of ugly, and it is possible that some users might want more than is offered for this. Maybe some better alternative can be developed. -------