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*To*: srfi-69@xxxxxxxxxxxxxxxxx*Subject*: Discussion about your comments*From*: Panu Kalliokoski <atehwa@xxxxxxxx>*Date*: Wed, 4 May 2005 00:19:36 +0300*Delivered-to*: srfi-69@xxxxxxxxxxxxxxxxx*Sender*: Panu Kalliokoski <atehwa@xxxxxxxx>*User-agent*: Mutt/1.5.6+20040907i

This e-mail contains references to points that I find controversial and/or would like to have more information on. ... bounds or no bounds to hash functions? I don't get this argument. For the modular polynomial method to work, it suffices for the mantissa and modulus to be relatively prime. This can achieved easily by picking a prime modulus, but also by picking a prime mantissa. I would have thought it to be more general if the hash function gets to know the modulus on beforehand, not more restrictive. That said, I don't have much reason for requiring the bound parameter. The most important reasons for it were (1) the observation that the hash seems to almost always be converted to some range anyway, so (hash obj bound) seems cleaner than (modulo (hash obj) bound), and (2) the bonus of getting arbitrarily big hash ranges on request (with a fixed range, it is easy to reduce it but not extend it). As any non-bounded hash function h1 can be made into bound-aware hash function h2 by (define (h2 obj bound) (modulo (h1 obj) bound)) one can't say that it makes it much more difficult to define bound-aware hash functions than boundless ones. Am I missing something? ... naming: table or hash-table? (1) "table" does not tell what the data type is supposed to be. (2) (related) a proper name for an indeterminate mapping type is "mapping". (3) the proper way to abstract different implementations of an interface is not to define the implementations mutually exclusively but to define each implementation independently and write glue to unify them under the same abstraction. (4) association lists and search trees are not implementations of the SRFI-69 API, because of failing complexity constraints. In other words, I want the users to know they are using hash tables, and to be able to count on being using hash tables. ... keyword parameters for make-hash-table? In principle, I agree: it would be nice to have an extensible interface for make-hash-table. However, as currently no SRFI defines a syntax for keyword parameters, I will refrain from using them. ... the set of provided hash functions? Yes, hash functions are coupled with equality predicates. However, any hash function for a coarser equality predicate is also acceptable for an equality predicate. Now, in practice people don't use perfect hash functions; that is, hash functions that are equally discriminative as their corresponding equality predicate. This is because such a hash function is really just an isomorphism between the key data type and the subset of integers used, and in such a case, it is better to use an ordinary vector than a full-blown hash table. That is the basic explanation why it is probably not worthwhile to provide generic hash functions of different coarseness: eq?-hash, eqv?-hash and equal?-hash. Practically put, I can't think of any realistic situation where the difference between distribution of eq?-hash and equal?-hash would make any difference for performance. However, for performance it _does_ make sense to provide type-specific hash functions, to avoid the overhead of polymorphism. Lastly, eq?-hash would be nice to provide, because R5RS currently does not provide a way to get the "identity" of an object (other than the object itself), but probably not the business of SRFI-69, but a separate hashing SRFI. Panu -- personal contact: atehwa@xxxxxx, +35841 5323835, +3589 85619369 work contact: pkalliok@xxxxxxxxxxxxxxxx, +35850 3678003 kotisivu (henkkoht): http://www.iki.fi/atehwa/ homepage (technical): http://sange.fi/~atehwa/

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