Abstract mixin initial implementation

lib/class.js

@@ -34,6 +34,7 @@ var _console = ( typeof console !== 'undefined' ) ? console : undefined;
 
 var util         = require( './util' ),
     ClassBuilder = require( './ClassBuilder' ),
+    Interface    = require( './interface' ),
 
     warn        = require( './warn' ),
     Warning     = warn.Warning,
@@ -472,6 +473,11 @@ function createImplement( base, ifaces, cname )
  * be explicit: in this case, any instantiation attempts will result in an
  * exception being thrown.
  *
+ * This staging object may be used as a base for extending.  Note, however,
+ * that its metadata are unavailable at the time of definition---its
+ * contents are marked as "lazy" and must be processed using the mixin's
+ * eventual metadata.
+ *
  * @param  {function()}        basef    returns base from which to lazily
  *                                       extend
  * @param  {Array.<Function>}  traits   traits to mix in
@@ -486,6 +492,13 @@ function createUse( basef, traits, nonbase )
     // invoking the partially applied class will immediately complete its
     // definition and instantiate it with the provided constructor arguments
     var partial = function()
+    {
+        return partialClass()
+            .apply( null, arguments );
+    };
+
+
+    var partialClass = function()
     {
         // this argument will be set only in the case where an existing
         // (non-base) class is extended, meaning that an explict Class or
@@ -498,8 +511,7 @@ function createUse( basef, traits, nonbase )
             );
         }
 
-        return createMixedClass( basef(), traits )
-            .apply( null, arguments );
+        return createMixedClass( basef(), traits );
     };
 
 
@@ -545,6 +557,20 @@ function createUse( basef, traits, nonbase )
         return partial.extend( {} );
     };
 
+    partial.asPrototype = function()
+    {
+        return partialClass().asPrototype();
+    };
+
+    partial.__isInstanceOf = Interface.isInstanceOf;
+
+    // allow the system to recognize this object as a viable base for
+    // extending, but mark the metadata as lazy: since we defer all
+    // processing for mixins, we cannot yet know all metadata
+    // TODO: `_lazy' is a kluge
+    ClassBuilder.masquerade( partial );
+    ClassBuilder.getMeta( partial )._lazy = true;
+
     return partial;
 }
 

test/Trait/AbstractTest.js

@@ -1,7 +1,7 @@
 /**
  * Tests abstract trait definition and use
  *
- *  Copyright (C) 2014 Free Software Foundation, Inc.
+ *  Copyright (C) 2015 Free Software Foundation, Inc.
  *
  *  This file is part of GNU ease.js.
  *
@@ -360,4 +360,70 @@ require( 'common' ).testCase(
             _self.Class.use( Ta ).extend();
         } );
     },
+
+
+    /**
+     * Before traits, the only way to make an abstract class concrete, or
+     * vice versa, was by extending.  Now, however, a mixing in a trait can
+     * introduce abstract or concrete methods.  This poses a problem, since
+     * the syntax for providing self-documenting AbstractClass definitions
+     * no longer works: invoking `AbstractClass.use' produces different
+     * results than invoking `SomeAbstractClass.use', with the goal of
+     * extending it.
+     *
+     * Consider this issue: we wish to mix some trait T into abstract class
+     * AC.  Trait T does not provide a concrete implementation of the
+     * abstract methods in AT, and so the resulting class after the final
+     * `#extend' call would be abstract.
+     *
+     * We have no choice but to allow extending the intermediate object
+     * produced by a class's `#use' method; otherwise, any call to `#extend'
+     * on the intermediate object would result in an error, because the
+     * class would still have abstract members, but has not been declared to
+     * be abstract.  Handling abstract classes in this manner would be
+     * consistent with all other scenarios, and would be transparent: why
+     * should the user care that there is some odd intermediate object being
+     * used rather than an actual class?
+     */
+    'Abstract classes can be derived from intermediates': function()
+    {
+        var chk = [{}];
+
+        var AC = this.AbstractClass( { 'abstract foo': [] } ),
+            T  = this.Sut( { moo: function() { return chk; } } );
+
+        // mix trait into an abstract class
+        var M = this.AbstractClass.extend(
+            AC.use( T ),
+            {}
+        );
+
+        this.assertOk( this.Class.isClass( M ) );
+        this.assertOk( M.isAbstract() );
+
+        var inst = M.extend( { foo: function() {} } )();
+
+        // we should not have lost the original abstract class
+        this.assertOk(
+            this.Class.isA( AC, inst )
+        );
+
+        // not strictly necessary; comfort/sanity check: if this succeeds
+        // but the next fails, then there's a problem marking the
+        // implemented types
+        this.assertStrictEqual(
+            chk,
+            inst.moo()
+        );
+
+        // the trait should have been applied (see above note if this
+        // fails); if this does fail, note that, without
+        // AbstractClass.extend, we have (correctly):
+        // isA( T, AC.use( T ).extend( ... )() )
+        this.assertOk(
+            this.Class.isA( T, inst ),
+            'Instance is not recognized as having mixed in type T, but ' +
+                'incorporates its definition; metadata bug?'
+        );
+    },
 } );