Issue ADJUST-ARRAY-NOT-ADJUSTABLE Writeup

X3J13 accepted v11 Jun 89 on vote of N-1...


Issue:        ADJUST-ARRAY-NOT-ADJUSTABLE

References:   ADJUST-ARRAY (p297), ADJUSTABLE-ARRAY-P (p293),

              MAKE-ARRAY (pp286-289), simple arrays (p28, 289),

              simple strings with fill pointers (p299),

              VECTOR-PUSH-EXTEND (p296)

Category:     CLARIFICATION and CHANGE

Edit history: 22-Apr-87, Version 1 by Pitman

              15-Nov-88, Versions 2a,2b,2c by Pitman

              02-Dec-88, Version 3 by Pitman

              11-Jan-89, Version 4 by Pitman

              16-Jan-89, Version 5, by Gabriel.  Amended at the meeting to shorten.

              23-Jan-89, Version 6, by Moon.  Shorten without the bug introduced

                        by the amendment, add clarification of SIMPLE-ARRAY type.

              15-Feb-89, Version 7, by Pitman. Minor changes per comments from

                        RPG and Dalton.

              11-Mar-89, Version 8, by Pitman. Change category, add endorsements.

              17-Mar-89, Version 9, by Moon, fix wording and examples to make it

                        clear that the semantics of simple-array is unchanged.

               6-Jun-89, Version 10, by Moon and Gabriel, do over.

              23-Jun-89, Version 11, by Moon, two little corrections


Problem Description:


  There are a number of unclear passages in CLtL related to simple arrays

  and adjustable arrays.  There is disagreement on precisely how these

  passages are to be interpreted, and no one is happy with the fact that

  ADJUST-ARRAY works only on an implementation-dependent subset of arrays.


  The description of the :ADJUSTABLE option to MAKE-ARRAY on p288 says that

  ``the argument, if specified and not NIL, indicates that it must be

  possible to alter the array's size dynamically after it is created. This

  argument defaults to NIL.''  The description of the :ADJUSTABLE option

  does not say what MAKE-ARRAY will do if the argument is unsupplied or

  explicitly NIL.


  The description of ADJUSTABLE-ARRAY-P on p293 says that it is true ``if

  the argument (which must be an array) is adjustable, and otherwise

  false.'' However, the description of MAKE-ARRAY makes it clear that this

  is not necessarily the same as asking if the array was created with

  :ADJUSTABLE T.  If ADJUSTABLE-ARRAY-P returns NIL, you know that

  :ADJUSTABLE NIL was supplied (or no :ADJUSTABLE option was supplied), but

  if ADJUSTABLE-ARRAY-P returns T, then there is no information about

  whether :ADJUSTABLE was used.


  The description of ADJUST-ARRAY on pp297-298 says that it is ``not

  permitted to call ADJUST-ARRAY on an array that was not created with the

  :ADJUSTABLE option.'' This is inconsistent with ADJUSTABLE-ARRAY-P.


  The definition of SIMPLE-ARRAY on p.28 says ``an array that is not

  displaced to another array, has no fill pointer, and is not to have its

  size adjusted dynamically after creation is called a simple array.''

  It is left unclear whether this is an implication or an equivalence,

  i.e. whether there can be other simple arrays as well.

  CLtL p.299 appears to refer to simple strings with fill pointers,

  suggesting that it is an implication, but similar language is used for

  equivalences in other parts of CLtL.


Proposal (ADJUST-ARRAY-NOT-ADJUSTABLE:IMPLICIT-COPY)


  1. If MAKE-ARRAY is called with the :ADJUSTABLE, :FILL-POINTER, 

  and :DISPLACED-TO arguments each either unspecified or false, the

  resulting array is a simple array.  (This just repeats what CLtL

  says on page 289, it's here to aid in understanding the next point.)

      

  2. If MAKE-ARRAY is called with one or more of the :ADJUSTABLE,

  :FILL-POINTER, or :DISPLACED-TO arguments true, whether the

  resulting array is simple is unspecified.


  3. It is permitted to call ADJUST-ARRAY on any array.  (Remove the

  restriction documented at the bottom of p.297.)


  4. If ADJUST-ARRAY is applied to an array created with :ADJUSTABLE true,

  the array returned is EQ to its first argument.  It is not specified

  whether ADJUST-ARRAY returns an array EQ to its first argument for any

  other arrays.  If the array returned by ADJUST-ARRAY is not EQ to its

  first argument, the original array is unchanged and does not share

  storage with the new array.


  5. The predicate ADJUSTABLE-ARRAY-P is true if and only if ADJUST-ARRAY

  will return a value EQ to this array when given this array as its first

  argument.


Clarifications and Logical Consequences:


  a. There is no specified way to create an array for which ADJUSTABLE-ARRAY-P

     definitely returns NIL.


  b. There is no specified way to create an array that is non-simple.


  c. The definition of SIMPLE-ARRAY on p.28 is taken to be an implication,

     not an equivalence.  This is either a clarification or a change depending

     on one's prior reading of that definition.


  d. The meaning of ADJUSTABLE-ARRAY-P is changed.


  e. As with such functions as DELETE and NCONC, textbooks should

     instruct programmers to be careful to receive the value returned by

     ADJUST-ARRAY, as it might not be EQ to the first argument.


  f. VECTOR-PUSH-EXTEND still signals an error if given a non-adjustable

     array.  ADJUST-ARRAY's new feature of making a copy cannot be used

     by VECTOR-PUSH-EXTEND, since there is no way to return the copy to

     the caller.


Rationale:


  Points 3 and 4 eliminate the problem of ADJUST-ARRAY only working on a

  subset of arrays, by changing it to work on all arrays.  It remains

  implementation-dependent whether the array is modified in place or

  copied, i.e. whether the result is EQ to the argument, however many other

  functions in Common Lisp have similar implementation-dependent behavior.

  Implementation-dependent storage allocation or reuse is considered

  more benign than implementation-dependent applicability of an operation.


  Point 3 recognizes that ADJUST-ARRAY offers features that are offered by

  no other function and which are useful in cases involving non-adjustable

  arrays (for what amounts to copying).  This change would allow an

  expression such as:


    (SETQ X (ADJUST-ARRAY X ...))


  to work reliably. Those desiring the old behavior could do:


    (IF (OR (NOT (ADJUSTABLE-ARRAY-P X))

            (NOT (EQUAL (ARRAY-RANK X) (LENGTH NEW-DIMENSIONS))))

        (ERROR "Array cannot be adjusted."))

  

  to get the old style error checking.


  Point 5 recycles the name ADJUSTABLE-ARRAY-P as a test for whether an

  array is adjusted in place or by copying.


  Point 2 preserves the raison d'etre of simple arrays, which is to provide

  a portable interface to implementation-dependent specialized arrays that

  trade decreased functionality for faster access.  A proposed alternative

  was to specify a way to create an array that is guaranteed not to be

  simple.  This would have made (typep (make-array ...) 'simple-array)

  return the same value in all implementations, but would have required

  large changes to some implementations and would be of little benefit to

  users.  Users need to know that certain arrays are simple, so they can

  put in declarations and get higher performance, but users have no need to

  be able to create arrays that are definitely non-simple (for lower

  performance) or definitely non-adjustable.


Examples:


  1. The following program is conforming.

  

    (defun double (a)

      (adjust-array a (* (length a) 2)))

  

    (double (make-array 30))


  2. The following program is conforming.  In no implementation is the

  type declaration violated.


    (let ((a (make-array 100)))

      (declare (simple-array a))

      (frob a))


  3. The following program is non-conforming.  The consequences of this

  program are undefined because the type declaration is violated in some

  implementations.


    (let ((a (make-array 100 :adjustable t)))

      (declare (simple-array a))

      (frob a))


Current Practice:


  Every correct CLtL implementation conforms to points 1 and 2.  It is

  unlikely that any implementation currently exists that conforms to points

  3, 4, and 5.  Points 3 and 4 involve additions to an implementation to

  support the copying form of ADJUST-ARRAY.  Point 5 may involve a change

  to ADJUSTABLE-ARRAY-P or may be able to use the existing implementation

  of the function.


  Symbolics Genera makes :ADJUSTABLE NIL arrays adjustable in most cases,

  and ignores adjustability in deciding whether an array is a SIMPLE-ARRAY.

  The arrays that are internally simple in Symbolics Genera are a different

  subset of arrays from the type SIMPLE-ARRAY, because simplicity in that

  implementation depends on the rank and total-size as well as on the

  fill-pointer and displacement, thus Genera does not use the type

  SIMPLE-ARRAY for anything.


  Lucid, IIM, Ibuki, and Symbolics Cloe make :ADJUSTABLE NIL arrays

  non-adjustable in all cases, and make every array non-simple that CLTL

  does not require to be simple.


  Macintosh Allegro Common Lisp v1.2 makes :ADJUSTABLE NIL arrays

  non-adjustable in all cases, makes all arrays of rank other than 1

  non-simple (violating point 1), and makes every array non-simple that

  CLTL does not require to be simple.


Cost to Implementors:


  The change to ADJUSTABLE-ARRAY-P is easy.  The change to ADJUST-ARRAY may

  involve some complex coding but should not be a large task.  No changes

  are required to anything connected with SIMPLE-ARRAY.


Cost to Users:


  None in code that does not call ADJUSTABLE-ARRAY-P.  This is a fully

  upward-compatible change from the user's standpoint.


Benefits:


  Programs that use simple arrays and/or adjust arrays will be easier

  to port, as the language specification for these features will be

  clearer.  More programs will be able to call ADJUST-ARRAY, as its use

  will not be restricted to a subset of arrays.


Non-Benefits:


  Users who expect adjusting arrays created with :ADJUSTABLE NIL to signal

  an error would not get the desired signal.  A few programs might have

  porting problems due to variation among implementations of whether the

  result of ADJUST-ARRAY is EQ to the first argument.


Aesthetics:


  Most people believe the status quo is unaesthetic.  Having an aspect of

  the language more clearly specified is an aesthetic improvement.

  Allowing ADJUST-ARRAY on all arrays is an aesthetic improvement.


Discussion:


  There are at least 110 messages of discussion preceding this version of the

  proposal.  It does not seem feasible to summarize them here.


  Dick Gabriel, Dave Moon, and Guy Steele support this proposal.


  Some commentors would like to get rid of ADJUSTABLE-ARRAY-P, since

  ADJUST-ARRAY now works on all arrays.  Other commentors have said that

  ADJUSTABLE-ARRAY-P is still needed in some applications, such as user

  written functions that behave like VECTOR-PUSH-EXTEND, and hence should

  be kept; the concept of "adjustable array" is still meaningful.