Interfaces cannot inherit from Classes

lib/interface.js

@@ -22,7 +22,8 @@
  * @package core
  */
 
-var util = require( './util' );
+var util  = require( './util' ),
+    Class = require( './class' );
 
 
 /**
@@ -51,6 +52,9 @@ function extend()
         base      = args.pop() || Interface,
         prototype = new base();
 
+    // sanity check
+    inheritCheck( prototype );
+
     var new_interface = function() {};
 
     util.propCopy( props, prototype, {
@@ -77,6 +81,39 @@ function extend()
 }
 
 
+/**
+ * Assures that the parent object is a valid object to inherit from
+ *
+ * This method allows inheriting from any object (note that it will likely cause
+ * errors if not an interface), but will place restrictions on objects like
+ * Classes that do not make sense to inherit from. This will provide a more
+ * friendly error, with suggestions on how to resolve the issue, rather than a
+ * cryptic error resulting from inheritance problems.
+ *
+ * This method will throw an exception if there is a violation.
+ *
+ * @param  {Object}  prototype  prototype to check for inheritance flaws
+ *
+ * @return  {undefined}
+ */
+function inheritCheck( prototype )
+{
+    // if we're inheriting from another interface, then we're good
+    if ( prototype instanceof Interface )
+    {
+        return;
+    }
+
+    if ( Class.isClassInstance( prototype ) )
+    {
+        throw new TypeError(
+            "Interfaces cannot extend from classes. Try creating an " +
+                "abstract class instead."
+        );
+    }
+}
+
+
 /**
  * Attaches extend method to the given function (interface)
  *

test/test-interface-extend-permitted.js

@@ -0,0 +1,39 @@
+/**
+ * Tests to ensure interfaces can only extend from interfaces
+ *
+ * Interface can extend from any object, so long as it only contains abstract
+ * methods. However, the system checks for situations that don't make sense in
+ * order to provide more meaningful error messages.
+ *
+ *  Copyright (C) 2010 Mike Gerwitz
+ *
+ *  This file is part of ease.js.
+ *
+ *  ease.js is free software: you can redistribute it and/or modify it under the
+ *  terms of the GNU Lesser General Public License as published by the Free
+ *  Software Foundation, either version 3 of the License, or (at your option)
+ *  any later version.
+ *
+ *  This program 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 Lesser General Public License
+ *  for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * @author  Mike Gerwitz
+ * @package test
+ */
+
+var common    = require( './common' ),
+    assert    = require( 'assert' ),
+    Class     = common.require( 'class' );
+    Interface = common.require( 'interface' );
+
+
+assert.throws( function()
+{
+    Interface.extend( Class.extend(), {} );
+}, TypeError, "Interfaces cannot extend from classes" );
+