Interfaces permit only abstract methods

lib/interface.js

@@ -36,6 +36,9 @@ exports.extend = function()
 }
 
 
+exports.abstractMethod = util.createAbstractMethod;
+
+
 /**
  * Default interface implementation
  *
@@ -71,10 +74,10 @@ function prop_check( props )
 {
     for ( prop in props )
     {
-        if ( !( props[ prop ] instanceof Function ) )
+        if ( util.isAbstractMethod( props[ prop ] ) === false )
         {
             throw new Error(
-                "Only methods are permitted within Interface definitons"
+                "Only abstract methods are permitted within Interface definitons"
             );
         }
     }

test/test-interface.js

@@ -26,7 +26,8 @@ require( './common' );
 
 var assert         = require( 'assert' ),
     Class          = require( 'class' ),
-    Interface      = require( 'interface' );
+    Interface      = require( 'interface' ),
+    abstractMethod = Interface.abstractMethod;
 
 var FooType = Interface.extend();
 
@@ -50,13 +51,26 @@ assert.throws( function()
         // properties (non-methods) are not permitted
         prop: 'not permitted',
     });
-}, Error, "Only methods are permitted within Interface definitions" );
+}, Error, "Properties are not permitted within Interface definitions" );
 
-assert.doesNotThrow( function()
+assert.throws( function()
 {
     Interface.extend(
     {
-        method: function() {},
+        // concrete method
+        method: function() {}
     });
-}, Error, "Method declarations are allowed within Interface definitions" );
+}, Error, "Concrete methods are not permitted within Interface definitions" );
+
+assert.doesNotThrow(
+    function()
+    {
+        Interface.extend(
+        {
+            method: abstractMethod(),
+        });
+    },
+    Error,
+    "Abstract method declarations are allowed within Interface definitions"
+);