I asked here: “Is eqv? the portable way of checking object equality?”.
Grant Rettke wrote: > From what I have read, eq? is a way of determing whether two objects > are the same based on whether they point to the same location in > memory; and generally that eqv? works the same way as eq? but for > numbers and characters; numbers and characters equality will not be > determined consistently across different implementations. There are a few other differences as well, but numbers and characters are indeed the most commonly encountered. > Additionally > I have read that as a result of this one ought to use eqv? as a > portable means for checking for object equality unless performance is > such an issue that eq? should be used instead. That's reasonable, modulo the quibble with "object equality" below. > Is eqv? the portable way of checking object equality? Yes to the first half of that sentence, provided you replace "portable" by "most portable standard". As for the second half of the sentence, I have this quibble: In general, there is no such thing as *the* notion of object equality; the appropriate notion of object equality depends upon the context. In particular, it depends on whether you want to compare two objects with respect to mutability (that is, does a side effect to one object imply the same side effect to the other?) or with respect to their current state. In Scheme (whether IEEE/ANSI/R5RS or R6RS), eqv? is the most implementation-independent of the pre-defined equivalence predicates that are defined on all objects and are guaranteed to distinguish two distinct mutable objects. Similarly, equal? is the pre-defined partial equivalence predicate that distinguishes on the basis of current state but not on the basis of mutation; the R6RS equal? is also total. In Scheme, eq? is basically an efficiency hack that can be used as an alternative to eqv? when you know for sure that at least one of the arguments is a boolean, symbol, empty list, pair, procedure, non-empty string, non-empty vector, non-empty bytevector (R6RS only), or record (R6RS only). The rationale for eq? is that it is often about ten times as fast as eqv?, which is enough to matter for some applications, and the specific list of situations for which eq? is guaranteed to behave the same as eqv? is about as inclusive as it can be without sacrificing speed in typical implementations. Will
Basically, eq? is pointer equality.