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.