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Cleanup Issue `HASH-TABLE-ACCESS`

Status: Passed, as amended, Mar 89 X3J13 (by vote of 17-0)
Category: ADDITION
References: Hash-tables (Chapter 21 of CLtL)

Problem Description

There are many characteristics of hash-tables which are specified upon creation but are not accessible afterwards.

Proposal

Add the following functions to the language:

HASH-TABLE-REHASH-SIZE hash-table

Returns the current rehash size of a hash table.

HASH-TABLE-REHASH-THRESHOLD hash-table

Returns the current rehash threshold of a hash table.

HASH-TABLE-SIZE hash-table

Returns the current size of a hash table.

HASH-TABLE-TEST hash-table

Returns the test used for comparing keys in the hash table. By default the value will be EQL.

Define that the results of HASH-TABLE-REHASH-SIZE, HASH-TABLE-REHASH-THRESHOLD, and HASH-TABLE-SIZE are suitable for use in a call to MAKE-HASH-TABLE in order to produce a hash table with state corresponding to the current state of the hash table.

Clarify that the result of HASH-TABLE-TEST is always a symbol naming a function rather than the function itself if the test is one of those defined by this standard. (Implementations which provide additional tests for hash tables may determine how this function relates to such extended tests.)

Current Practice

VAX LISP and Lucid 3.0 implement the proposal.

Cost to Implementors

Most of these should be trivial to implement, since the information must be present for nearly all types.

Cost to Users

None. This is an upward-compatible extension.

Cost of Non-Adoption

The benefits would not be available in a portable fashion.

Benefits

Programs would be able to access useful information otherwise hidden. For example, it would allow programs to gain statistics about hash table usage that might enable better tuning.

Discussion

None of these are required to be SETF'able, though some might be reasonable implementation-dependent extensions. Including such modification abilities might constrain some implementations unduly.

This first appeared in ">GLS>clarifications.text" of 12/06/85.

Edit History

13-Sept-88, version 1 by Walter van Roggen
13-Oct-88, version 2 by Walter van Roggen
05-Apr-89, version 3 by Pitman (changes per x3J13)