Issue RESULT-LISTS-SHARED Writeup

Issue:        RESULT-LISTS-SHARED

Forum:	      Editorial

References:   DIRECTORY (p427), LIST-ALL-PACKAGES (p184)

Category:     CLARIFICATION

Edit history: 02-Mar-91, Version 1 by Pitman

	      15-Mar-91, Version 2 by Pitman (repair current practice)

Status:	      For X3J13 consideration


Problem Description:


  CLtL does not specify whether the list returned by each of these

  functions is freshly-consed each time, or whether it must be treated

  as immutable: LIST-ALL-PACKAGES, DIRECTORY.


Proposal (RESULT-LISTS-SHARED:SPECIFY):


  Specify that the lists returned by LIST-ALL-PACKAGES and DIRECTORY are

  freshly consed on every call.  The list resulting from calls to these

  functions may therefore be freely modified by the caller.


Example:


  ;Example of recycling list structure from LIST-ALL-PACKAGES

  (DEFUN ALIST-ALL-PACKAGES ()

    (LET ((P (LIST-ALL-PACKAGES)))

      (DO ((L P (CDR L)))

	  ((NULL L) P)

	(SETF (CAR L)

	      (LIST* (CAR L)

		     (PACKAGE-NAME (CAR L))

		     (PACKAGE-NICKNAMES (CAR L)))))))


  ;Example of recycling list structure from DIRECTORY

  (DEFUN FULL-DIRECTORY (SPEC)

    (LET ((D (DIRECTORY SPEC)))

      (DO ((L D (CDR L)))

          ((NULL L) D)

        (SETF (CAR L)

 	      (LIST (CAR L)

		    :AUTHOR (FILE-AUTHOR (CAR L))

		    :WRITE-DATE (FILE-WRITE-DATE (CAR L))

		    :LENGTH (IGNORE-ERRORS

			      (WITH-OPEN-FILE (S (CAR L))

				(FILE-LENGTH S))))))))


Rationale:


  In most implementations, packages are stored in a hash table for

  faster access by the reader, so the list returned by LIST-ALL-PACKAGES

  is probably freshly consed already.  Also, LIST-ALL-PACKAGES is

  probably called sufficiently seldom that there is no serious

  efficiency issue here.


  In most implementations, information about the file system is external

  to Lisp and must be freshly computed each time a directory listing is

  done anyway.  Better not to force the caller to do a COPY-LIST on a

  call when the list can easily be guaranteed to be fresh to begin with.


  Even in situations where it was possible to return a

  non-freshly-consed list, this might put internal data structures of

  the implementation at risk for no really good efficiency reason.


Current Practice:


  Symbolics Genera freshly conses the list returned by DIRECTORY.

  Symbolics Genera does not freshly cons the list returned by LIST-ALL-PACKAGES.


Cost to Implementors:


  Very small.


Cost to Users:


  None.


Cost of Non-Adoption:


  Users wouldn't know what to expect and would be forced to do a

  COPY-LIST just to be sure when they planned to modify the result.  If

  they forgot, and the implementation decided to return a

  non-freshly-consed list, an attempt to modify that list might

  compromise the integrity of the running Lisp.


Benefits:


  Clearer specfication


Aesthetics:


  Negligible effect.


Discussion:


  Pitman supports this proposal.  (He's more than once wanted to modify

  the result list of DIRECTORY and had to do a gratuitous COPY-LIST just

  to be safe.)