add istable() function

inst/istable.m

@@ -0,0 +1,64 @@
+## Copyright (C) 2019, 2023 Andrew Janke <floss@apjanke.net>
+##
+## This file is part of Octave.
+##
+## Octave is free software: you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## Octave is distributed in the hope that it will be useful, but
+## WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with Octave; see the file COPYING.  If not, see
+## <https://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function} {@var{out} =} istable (@var{x})
+##
+## True if input is a @code{table} array or other table-like type, false
+## otherwise.
+##
+## Respects @code{istable} override methods on user-defined classes, even if
+## they do not inherit from @code{table} or were known to Tablicious at
+## authoring time.
+##
+## User-defined classes should only override @code{istable} to return true if
+## they conform to the @code{table} public interface. That interface is not
+## well-defined or documented yet, so maybe you don't want to do that yet. 
+##
+## Returns a scalar logical.
+##
+## @end deftypefn
+
+# Developers' note: Other packages that wish to support tables and be compatible
+# with Tablicious, but not take a mandatory dependency on Tablicious, may want
+# to use istable() to test for table inputs. This function is simple, well-defined,
+# and likely stable enough for other packages to just copy-paste an exact copy
+# of it in to their own `inst/` directory, so there will be an istable() available
+# whether or not Tablicious is installed and loaded.
+
+function out = istable (x)
+  %ISTABLE True for table arrays or table-like arrays.
+  if isa (x, 'table')
+    % Main case: table arrays are tables.
+    % This is actually redundant with the generic isobject/method test below,
+    % but written separately for readability.
+    out = true;
+  elseif isobject (x)
+    % Respect istable methods on classes. Normally, those methods would "grab"
+    % this call. We check for it here so that these method overrides are respected
+    % even if this istable function was called through a function handle or 
+    % similar mechanism, bypassing the regular method dispatch mechanism.
+    if ismember ('istable', methods (x))
+      out = istable (x);
+    else
+      out = false;
+    end
+  else
+    out = false;
+  end
+end