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Cleanup Issue `ADJUST-ARRAY-FILL-POINTER`

Status: passed, 1988 (not sure which meeting)
Category: CLARIFICATION
References: ADJUST-ARRAY (p297)

Problem Description

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

Proposal (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 Cases

  (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.

Edit History

15-Mar-88, Version 1 by Pitman