AREF-1D (Version 6)

[Masinter.pa@Xerox.COM]

This revision clarifies the proposal by giving arguments and some
equivalence relations. I added that row-major-aref was usable with SETF
(which did not appear in the original proposal although it was implied
in one of the examples.)
!
Issue:        AREF-1D
References:   Arrays (pp286-298)
Category:     ENHANCEMENT
Edit history: 22-Apr-87, Version 1 by Pitman
              02-Jun-87, Version 2 by Pitman (ROW-MAJOR-AREF)
              6-Jun-87, Versions 3, 4 by Masinter (editorial)
              11-Jun-87, Version 5, to X3J13 (no changes)
               6-Jul-87, Version 6, by Masinter

Problem Description:

It's hard to write functions like Maclisp's LISTARRAY and FILLARRAY
efficiently in Common Lisp because they take arguments of varying rank.
Currently, you have to make a displaced array to work with temporarily
and then throw away the displaced array when you're done. In many cases,
this is bothersome because there is no a priori reason why they should
have to cons at all.

Proposal (AREF-1D:ROW-MAJOR-AREF):

Introduce a new function ROW-MAJOR-AREF that allows one-dimensional
access to the storage backing up a given array assuming the normal
row-major storage layout.

ROW-MAJOR-AREF is valid for use with SETF.

row-major-aref array index              [Function]

This accesses and returns the element of array specified by index when
the elements of array are considered in row-major order. Array may be an
array of any dimensionality. row-major-aref may be used with setf. For
reference, the following sets of expressions are equivalent:

(row-major-aref array index) ==
    (aref (make-array (array-total-size array)
                      :displaced-to array
                      :element-type (array-element-type array))
          index)

and

(aref array .. subscripts ..) ==
    (row-major-aref array (array-row-major-index array .. subscripts
..))

Rationale:

Common Lisp requires row-major storage layout of arrays and has a number
of operators that allow users to exploit that order. ROW-MAJOR-AREF is a
useful, simple addition.

LISTARRAY and FILLARRAY, for example, could be trivially defined by
loops that had the following form:

    (DOTIMES (I (ARRAY-TOTAL-SIZE ARRAY))
      ... (ROW-MAJOR-AREF ARRAY I) ...)

Currently, the only really efficient way to write this would involve
something like:

    (ECASE (ARRAY-RANK ARRAY1)
      ((0) (SETF (AREF ARRAY1) (AREF ARRAY2)))
      ((1) (DOTIMES (I (ARRAY-DIMENSION ARRAY 0))
	     (SETF (AREF ARRAY1 I) (AREF ARRAY2 I))))
      ((2) (DOTIMES (I (ARRAY-DIMENSION ARRAY 0))
	     (DOTIMES (I (ARRAY-DIMENSION ARRAY 1))
	       (SETF (AREF ARRAY1 I J) (AREF ARRAY2 I J)))))
      ...some finite number of clauses...)

Current Practice:

Many implementations have this primitive under some other name for use
internally. In Symbolics systems, for example, it is SYS:%1D-AREF.

Adoption Cost:

This change is fairly localized. In implementations that already use
this primitive internally, it's little more than a matter of changing
the name of or otherwise releasing the existing primitive. In some
implementations, it might involve writing a small amount of code or
compiler work to make ROW-MAJOR-AREF work efficiently.

Benefits:

This gives users efficient access to something to which they already
have inefficient access.

Conversion Cost:

This is an upward-compatible change; the name ROW-MAJOR-AREF is unlikely
to be used by any current program.

Aesthetics:

This allows certain programs to be written in a more aesthetic way.

Discussion:

The cleanup committee generally supports this enhancement. Version 2 was
endorsed (assuming change to function name ROW-MAJOR-AREF.)

     ----- End Forwarded Messages -----