is_char_device: use missing

Author: scivision

+stdlib/+python/is_char_device.m

@@ -4,7 +4,7 @@
   y = py.pathlib.Path(file).is_char_device();
 catch e
   pythonException(e)
-  y = logical([]);
+  y = missing;
 end
 
 end

+stdlib/is_char_device.m

@@ -25,7 +25,7 @@
 end
 
 
-i = logical([]);
+i = missing;
 
 for b = backend
   switch b
@@ -39,7 +39,7 @@
       error('stdlib:is_char_device:ValueError', 'Unknown backend: %s', b)
   end
 
-  if ~isempty(i)
+  if ~ismissing(i)
     return
   end
 end

AGENTS.md

@@ -45,7 +45,8 @@ These rules apply under the namespace "stdlib" (directory +stdlib/) and all its
 - The code syntax must work for Matlab >= R2020b
 - The code must not require any Matlab toolboxes, only base Matlab functionality
 - Prohibited to use Matlab factory function "isMATLABReleaseOlderThan()" as it is slow, except in ./test_main.m or ./buildfile.m. Instead we use stdlib.matlabOlderThan() which is like 200x faster than isMATLABReleaseOlderThan() and works for Matlab >= R2016b
-- When an exception is encountered, we generally desire that the code return an "empty" value of the appropriate type. In certain cases we may throw or rethrow an error.
+- When an exception is encountered, we generally desire that the code return "missing". In certain cases we may throw or rethrow an error.
+- When a backend is not available, we require that the code return "missing". The previous behavior of returning empty array or empty char or empty string is deprecated and being transitioned.
 - where the output represents a filesystem path, the output should be a string type if any input is string type, otherwise char type.
 
 ## Dev notes

test/TestBackend.m

@@ -96,7 +96,7 @@ function test_backend(tc)
 
 [i,b] = stdlib.is_char_device('.');
 tc.assertThat(b, IsSubsetOf(stdlib.Backend('is_char_device').backends));
-tc.verifyNotEmpty(i);
+tc.verifyNotEqual(i, missing);
 
 [i, b] = stdlib.is_dev_drive('.');
 tc.assertThat(b, IsSubsetOf(stdlib.Backend('is_dev_drive').backends))

test/TestExists.m

@@ -66,14 +66,14 @@ function test_is_char_device(tc, B_is_char_device)
 
 [r, b] = stdlib.is_char_device(n, B_is_char_device);
 tc.assertMatches(b, B_is_char_device)
-tc.assertClass(r, 'logical')
 
 if ismember(B_is_char_device, stdlib.Backend().select('is_char_device'))
+  tc.assertClass(r, 'logical')
   tc.verifyTrue(r, n)
 
   tc.verifyTrue(stdlib.is_char_device(string(n), B_is_char_device))
 else
-  tc.verifyEmpty(r)
+  tc.verifyEqual(r, missing)
 end
 end
 end