[r6rs-discuss] [Formal] re: hash-table-hash-function should return a function

[Daniel Villeneuve]

---
This message is a formal comment which was submitted to formal-comment at r6rs.org, following the requirements described at: http://www.r6rs.org/process.html
---
Sorry, the header of the previous mail was messed up (surely
my typing M-q).

submitter's name: Daniel Villeneuve
submitter's email address: daniel_villeneuve at sympatico.ca
type of issue: enhancement
priority: minor
R6RS component: Hash Tables
version of the report: 5.92

SUMMARY

The specification of hash-table-hash-function should not
force implementations to return #f when something more
useful is available.

DESCRIPTION

Quoting from
http://www.r6rs.org/r6rs-editors/2006-March/001047.html, one
can see that there were some arguments about the behavior of
hash-table-hash-function for hash tables built from
make-eq-hash-table and make-eqv-hash-table:

<quote> >>Procedure: hash-table-hash-function hash-table =>
procedure
>>
>>    Returns the hash function used by hash-table.
> 
> 
> Should these procedures be defined on hash tables that
> were created by calling make-eq-hash-table or
> make-eqv-hash-table?

You're right that hash-table-hash-function shouldn't be, so
I've added provisional language to that effect.  </quote>

The current wording on page 52 requires that
hash-table-hash-function return #f for such tables.

I find some problems with this: a) This situation prevents
implementations that can return useful functions to do so.
b) This inhibits having a complete "hash-table-clone"
method, which I think is one of the most useful usages for
such inspection methods: (define (hash-table-clone ht k)
(make-hash-table (hash-table-hash-function ht)
(hash-table-equivalence-function ht) k)) c) On systems that
provide users with public hash-eq and hash-eqv functions,
the current wording forces the implementation to remember if
a hash table was built using (make-eq-hash-table) or
(make-hash-table hash-eq eq?), just for the sake of
complying with the hash-table-hash-function's spec.

PROPOSAL

Alternative 1: on hash tables built using make-eq-hash-table
and make-eqv-hash-table, hash-table-hash-function can return
the implementation's choice of hash-function if this is
available, or #f otherwise.

This addresses issues a) and c), but not b).

Alternative 2: hash-table-hash-function should always return
a function, suitable to be used as a hash function by
make-hash-table.  For tables built using make-eq-hash-table
and make-eqv-hash-table, the returned function can throw an
exception if invoked.

This addresses issues a), b) and c).

--
Daniel Villeneuve