References: ADJUST-ARRAY (p297)
Edit history: 15-Mar-88, Version 1 by Pitman
CLtL does not specify clearly the effect of ADJUST-ARRAY on the fill
Clarify that the :FILL-POINTER keyword argument to ADJUST-ARRAY is treated
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
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.
(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.
This behavior must be more clearly defined if portable programs are going
to be able to depend on it.
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
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.
The cost of non-adoption would be avoided.
There is little if any aesthetic impact of this change.
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.