Formal comment #189 (enhancement)

hash-table-hash-function should return a function
Reported by: 	Daniel Villeneuve
Version: 	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).


RESPONSE:

Allowing some implementations to return a procedure in the cases in 
question could encourage dependence on those procedures, resulting
in non-portable programs.  The proposal will not be adopted.