add idempotent one-arg constructor forms for string, datetime, etc (b…

…ut not table)

The table class does not get one, because that's not how Matlab's table(x) constructor works, and would be a type discontinuity. Instead, it creates a new one-variable table whose variable contains the table passed to the constructor. (That is, it creates a table containing a nested table.)

inst/calendarDuration.m

@@ -125,6 +125,11 @@
       # this = calendarDuration (..., 'Format',displayFormat)
 
       args = varargin;
+      if (nargin == 1 && isa (args{1}, 'calendarDuration'))
+        this = args{1};
+        return
+      endif
+
       # Peel off options
       knownOptions = {'Format'};
       opts = struct;

inst/categorical.m

@@ -238,6 +238,10 @@
       if (nargin == 0)
         return
       endif
+      if (nargin == 1 && isa (x, 'categorical'))
+        this = x;
+        return
+      end
 
       validOptions = {'Ordinal', 'Protected'};
       [opts, args] = peelOffNameValueOptions (varargin, validOptions);

inst/datetime.m

@@ -129,9 +129,9 @@
 
       # Peel off options
       args = varargin;
-      knownOptions = {'Format','InputFormat','Locale','PivotYear','TimeZone'};
+      knownOptions = {"Format", "InputFormat", "Locale", "PivotYear", "TimeZone"};
       opts = struct;
-      while (numel (args) >= 3 && isa (args{end-1}, 'char') ...
+      while (numel (args) >= 3 && isa (args{end-1}, "char") ...
           && ismember (args{end-1}, knownOptions))
         opts.(args{end-1}) = args{end};
         args(end-1:end) = [];
@@ -147,7 +147,10 @@
           dnums = now;
         case 1
           x = args{1};
-          if (isnumeric (x))
+          if (isa (x, 'datetime'))
+            this = x;
+            return
+          elseif (isnumeric (x))
             # Convert date vectors
             dnums = datenum (x);
           elseif (ischar (x) || iscellstr (x) || isa (x, 'string'))

inst/duration.m

@@ -96,7 +96,7 @@
       #DURATION Construct a new duration array
       args = varargin;
       # Peel off options
-      knownOptions = {'InputFormat','Format'};
+      knownOptions = {'InputFormat', 'Format'};
       opts = struct;
       while (numel (args) >= 3 && isa (args{end-1}, 'char') ...
           && ismember (args{end-1}, knownOptions))
@@ -109,7 +109,10 @@
           return
         case 1
           in = args{1};
-          if (isnumeric (in))
+          if (isa (in, 'duration'))
+            this = in;
+            return
+          elseif (isnumeric (in))
             switch (size (in, 2))
               case 3
                 [H, MI, S] = deal (in(:,1), in(:,2), in(:,3));

inst/localdate.m

@@ -105,7 +105,10 @@
           dnums = floor (now);
         case 1
           x = args{1};
-          if (isnumeric (x))
+          if (isa (x, 'localdate'))
+            this = x;
+            return
+          elseif (isnumeric (x))
             # Convert datenums
             mustBeIntOrNanOrInf (x, 'input');
             dnums = double (x);

inst/string.m

@@ -174,23 +174,15 @@
     ## @end itemize
     ##
     ## @end deftypefn
-    function this = string(in, varargin)
-      # TODO: Support a 'MapMissing' option to map "standard missing values"
-      # (empty strings, NaN, NaT) to string missings.
-      #
+    function this = string(in)
       # TODO: Maybe fall back to calling cellstr() on arbitrary input objects.
       if (nargin == 0)
         return
       endif
       if (isa (in, "string"))
-        # Copy properties, because I don't know if a full-array pass-through
-        # works in Octave. -apj
-        # this = in; % That is, don't do this.
-        this.strs = in.strs;
-        this.tfMissing = in.tfMissing;
+        this = in;
         return
-      endif
-      if (ischar (in))
+      elseif (ischar (in))
         this.strs = cellstr (in);
         this.tfMissing = false (size (this.strs));
       elseif (iscell (in))