Any discussion of equality semantics would be incomplete without mentioning Baker's "Equal Rights for Functional Objects": http://home.pipeline.com/~hbaker1/Obj

Any discussion of equality semantics would be incomplete without mentioning Baker's "Equal Rights for Functional Objects": http://home.pipeline.com/~hbaker1/ObjectIdentity.html http://home.pipeline.com/~hbaker1/ObjectIdentity.html tl;dr: meaningful equality predicates can only be defined on immutable values.