Related issues: ADJUST-ARRAY-DISPLACEMENT
Edit history: v1, 13 Feb 1991, Sandra Loosemore
v2, 11 Mar 1991, Sandra Loosemore
v3, 26 Mar 1991, Sandra Loosemore (amendment from meeting)
Status: v3 (proposal ADD) accepted by X3J13, Mar 91
There is no way to determine whether an array is displaced to another
array. Having this information available would be helpful to some
applications that want to dump and restore array contents, either to
allow them to detect when sharing of displaced arrays is lost or to
reconstruct the arrays in such a way that sharing is preserved.
Add a function:
ARRAY-DISPLACEMENT <array> [Function]
The <array> argument must be an array. If the array is a displaced
array, two values are returned: the array to which it is displaced
corresponding to the :DISPLACED-TO argument to MAKE-ARRAY, and an
integer corresponding to the :DISPLACED-INDEX-OFFSET argument to
MAKE-ARRAY. If the array is not a displaced array, this function
returns two values NIL and 0.
If ARRAY-DISPLACEMENT is called on an array for which a non-NIL
object was provided as the :DISPLACED-TO argument to MAKE-ARRAY
or ADJUST-ARRAY, it must return that object as its first value.
It is implementation-dependent whether ARRAY-DISPLACEMENT returns
a non-NIL first value for any other array.
(array-displacement '#(1 2 3))
=> the values are not specified, but are probably NIL and 0 in many
(array-displacement (make-array 2 :displaced-to '#1=#(1 2 3)))
=> (values #1# 0)
Providing the function solves the problem.
Some implementations implicitly displace some arrays. (For example,
adjustable arrays might be represented as displaced arrays.) Permitting
ARRAY-DISPLACEMENT to return non-NIL for those arrays allows those
implementations not to have to record some additional information
in the array about whether an explicit :DISPLACED-TO was provided.
Lucid version 4.0 and Apple's Macintosh Common Lisp implement such
a function, but call it DISPLACED-ARRAY-P.
Symbolics has an ARRAY-DISPLACED-P predicate but it returns neither
the array nor an index, just a boolean.
Cost to Implementors:
It's probably easy to add.
Cost to Users:
None, this is a new feature.
Cost of non-adoption:
Users can't do some kind of manipulations on displaced arrays in a
The cost of non-adoption is avoided.
Looks OK to me.
The addition of such a function was mentioned in the writeup for
proposal ADJUST-ARRAY-DISPLACEMENT as something that had been
considered at that time. I don't recall what the arguments were
A more extreme proposal would also define ARRAY-DISPLACEMENT as
a SETF place, to allow these parameters of displaced arrays to
The original name suggested for this function was DISPLACED-ARRAY-P.
The reason Lucid has DISPLACED-ARRAY-P is that is the name Guy Steele
gave it back it 6-Dec-85 -- the "Clarifications" and
"non-controversial ..." lists. Quite possibly, it had been suggested
already in the common-lisp@su-ai forum.