Issue ADJUST-ARRAY-FILL-POINTER Writeup

Issue:        ADJUST-ARRAY-FILL-POINTER

References:   ADJUST-ARRAY (p297)

Category:     CLARIFICATION

Edit history: 15-Mar-88, Version 1 by Pitman


Problem Description:


  CLtL does not specify clearly the effect of ADJUST-ARRAY on the fill

  pointer.


Proposal (ADJUST-ARRAY-FILL-POINTER:MINIMAL):


  Clarify that the :FILL-POINTER keyword argument to ADJUST-ARRAY is treated

  as follows...


  If :FILL-POINTER argument is not given, then the fill-pointer of the array

  to be adjusted (if any) is left alone. It is an error to adjust an array

  to a size smaller than its fill pointer without specifying the :FILL-POINTER

  option so that its fill-pointer is properly adjusted in the process.


  If supplied, the :FILL-POINTER argument must be either an integer between 0

  and the new size of the array, the symbol T (indicating that the new size

  of the array should be used), or the symbol NIL (indicating that the fill

  pointer should left as it is -- as if the :FILL-POINTER option had not been

  specified).


  An error is signalled if a non-NIL value for :FILL-POINTER is supplied and

  the array to be adjusted does not already have a fill pointer.


Test Case:


  (SETQ A1 (MAKE-ARRAY 5 :FILL-POINTER 3 :ADJUSTABLE T))

  (SETQ A2 (ADJUST-ARRAY A1 4))

  (FILL-POINTER A1) => 3

  (FILL-POINTER A2) => 3


  (SETQ B1 (MAKE-ARRAY 5 :FILL-POINTER 3 :ADJUSTABLE T))

  (SETQ B2 (ADJUST-ARRAY B1 2 :FILL-POINTER 1))

  (FILL-POINTER B1) => 1

  (FILL-POINTER B2) => 1


  (SETQ C1 (MAKE-ARRAY 5 :FILL-POINTER 3 :ADJUSTABLE T))

  (SETQ C2 (ADJUST-ARRAY C1 2 :FILL-POINTER T))

  (FILL-POINTER C1) => 2

  (FILL-POINTER C2) => 2


  (SETQ D1 (MAKE-ARRAY 5 :ADJUSTABLE T))

  (SETQ D2 (ADJUST-ARRAY D1 2 :FILL-POINTER 2)) signals an error.


  (SETQ E1 (MAKE-ARRAY 5 :FILL-POINTER T :ADJUSTABLE T))

  (SETQ E2 (ADJUST-ARRAY E1 4)) is an error.


Rationale:


  This behavior must be more clearly defined if portable programs are going

  to be able to depend on it.


Current Practice:


  Symbolics Lisp implements the proposal. In case "E", it does not signal an

  error. It simply leaves the illegal fill-pointer in place so that the

  programmer can correct it using (SETF (FILL-POINTER E2) ...)


Cost to Implementors:


  Probably most implementations do not deviate significantly from the proposed

  behavior. The cost to implementors is probably very slight.


Cost to Users:


  None. This clarifies a fuzzy area in the manual which users cannot currently

  depend upon.


Cost of Non-Adoption:


  The interaction between ADJUST-ARRAY and fill-pointers would continue to be

  hazy, and portable programs would not be able to rely upon that behavior

  being consistent across implementations.


Benefits:


  The cost of non-adoption would be avoided.


Aesthetics:


  There is little if any aesthetic impact of this change.


Discussion:


  Pitman volunteered to write this issue up for the Cleanup committee. He's

  not very passionate about the details one way or another, but figures it's

  a good idea that they be clarified.



   The cleanup committee didn't object.