CLHS: Issue HASH-TABLE-ACCESS Writeup

#

## Issue HASH-TABLE-ACCESS Writeup

    Status:       Passed, as amended, Mar 89 X3J13 (by vote of 17-0)

    Issue:        HASH-TABLE-ACCESS
    References:   Hash-tables (Chapter 21 of CLtL)
    Category:     ADDITION
    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)

    Problem Description:

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

    Proposal: (HASH-TABLE-ACCESS:X3J13-MAR-89)

      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.

Copyright 1996-2005, LispWorks Ltd. All rights reserved.