13-224
To: J3
From: Dan Nagle
Subject: duplicate array bounds
Date: 2013 December 28


Fortran 1x proposal: Provide a simple syntax to declare an array
with the same bounds as a previously-declared array.

Problem Statement:

Often subprograms must declare an automatic array with the same bounds
as a dummy-argument array.  Or, arrays differing in type or
other attributes must have the same bounds; these arrays
cannot be declared on the same declarative statement so the bounds
must be repeated.

If the lower bound of each extent may be assumed to be 1,
the size inquiry intrinsic may be specified to duplicate the bounds.
For the more general case of arbitrary lower bounds,
both lbound and ubound must be specified.  When the arrays are
of higher rank, this can produce a lengthy <array-spec>.
Lengthy expressions can also occur when the name of the array
whose bounds are to be duplicated is long (perhaps it is a component
of a derived type).  As these factors accumulate, the specification
of the <array-spec> can quickly become tedious, error-prone, and hard
to read and maintain.

Proposed Solution:

A simple, compact declaration would therefore be helpful when writing
and maintaining programs.  For example, if something like

real, dimension( lbound( a%b, dim= 1): ubound( a%b, dim= 1), &
                 lbound( a%b, dim= 2): ubound( a%b, dim= 2)) :: tmp

could be replaced by something like

real, dimension( bounds( a%b)) :: tmp

or similarly

real :: tmp( bounds( a%b))

then the resulting code is clearer, less error-prone, and easier
to read and maintain.

Care should be taken so that, in the future, the agreed notation
could be generalized to cases where only part of the bounds information
is needed (perhaps only the first few extents), or to cases where
the resulting whole <array-spec> is to express a higher rank than
the original (that is, more extents could be added), or to a form
that could be used in an allocate statement or other context.  However,
this proposal is for the exact duplication of <array-spec> only,
in a declaration only.  Enhancements are possibly too complex for f1x.