Trait can now implement interfaces

Note the incomplete test case: the next commit will introduce the ability
for mixins to override methods that may have already been defined.

lib/Trait.js

@@ -74,6 +74,9 @@ function createNamedTrait( name, dfn )
 
 Trait.extend = function( dfn )
 {
+    // we may have been passed some additional metadata
+    var meta = this.__$$meta || {};
+
     // store any provided name, since we'll be clobbering it (the definition
     // object will be used to define the hidden abstract class)
     var name = dfn.__name || '(Trait)';
@@ -91,8 +94,15 @@ Trait.extend = function( dfn )
         throw Error( "Cannot instantiate trait" );
     };
 
+    // implement interfaces if indicated
+    var base = AbstractClass;
+    if ( meta.ifaces )
+    {
+        base = base.implement.apply( null, meta.ifaces );
+    }
+
     // and here we can see that traits are quite literally abstract classes
-    var tclass = AbstractClass( dfn );
+    var tclass = base.extend( dfn );
 
     TraitType.__trait  = true;
     TraitType.__acls   = tclass;
@@ -114,10 +124,33 @@ Trait.extend = function( dfn )
         mixin( TraitType, dfn, tc );
     };
 
+    // mixes in implemented types
+    TraitType.__mixinImpl = function( dest_meta )
+    {
+        mixinImpl( tclass, dest_meta );
+    };
+
     return TraitType;
 };
 
 
+Trait.implement = function()
+{
+    var ifaces = arguments;
+
+    return {
+        extend: function()
+        {
+            // pass our interface metadata as the invocation context
+            return Trait.extend.apply(
+                { __$$meta: { ifaces: ifaces } },
+                arguments
+            );
+        },
+    };
+};
+
+
 Trait.isTrait = function( trait )
 {
     return !!( trait || {} ).__trait;
@@ -260,16 +293,71 @@ function createVirtProxy( acls, dfn )
 function mixin( trait, dfn, tc )
 {
     // the abstract class hidden within the trait
-    var acls    = trait.__acls,
+    var acls = trait.__acls;
-        methods = acls.___$$methods$$;
 
     // retrieve the private member name that will contain this trait object
     var iname = addTraitInst( trait, dfn, tc );
 
+    // recursively mix in trait's underlying abstract class (ensuring that
+    // anything that the trait inherits from is also properly mixed in)
+    mixinCls( acls, dfn, iname );
+    return dfn;
+}
+
+
+/**
+ * Recursively mix in class methods
+ *
+ * If CLS extends another class, its methods will be recursively processed
+ * to ensure that the entire prototype chain is properly proxied.
+ *
+ * For an explanation of the iname parameter, see the mixin function.
+ *
+ * @param  {Class}   cls    class to mix in
+ * @param  {Object}  dfn    definition object to merge into
+ * @param  {string}  iname  trait object private member instance name
+ *
+ * @return {undefined}
+ */
+function mixinCls( cls, dfn, iname )
+{
+    var methods = cls.___$$methods$$;
+
     mixMethods( methods['public'], dfn, 'public', iname );
     mixMethods( methods['protected'], dfn, 'protected', iname );
 
-    return dfn;
+    // if this class inherits from another class that is *not* the base
+    // class, recursively process its methods; otherwise, we will have
+    // incompletely proxied the prototype chain
+    var parent = methods['public'].___$$parent$$;
+    if ( parent && ( parent.constructor !== ClassBuilder.ClassBase ) )
+    {
+        mixinCls( parent.constructor, dfn, iname );
+    }
+}
+
+
+/**
+ * Mix implemented types into destination object
+ *
+ * The provided destination object will ideally be the `implemented' array
+ * of the destination class's meta object.
+ *
+ * @param  {Class}   cls        source class
+ * @param  {Object}  dest_meta  destination object to copy into
+ *
+ * @return {undefined}
+ */
+function mixinImpl( cls, dest_meta )
+{
+    var impl = ClassBuilder.getMeta( cls ).implemented || [],
+        i    = impl.length;
+
+    while ( i-- )
+    {
+        // TODO: this could potentially result in duplicates
+        dest_meta.push( impl[ i ] );
+    }
 }
 
 

lib/class.js

@@ -431,9 +431,11 @@ function createMixedClass( base, traits )
 
     // add each trait to the list of implemented types so that the
     // class is considered to be of type T in traits
+    var impl = meta.implemented;
     for ( var i = 0, n = traits.length; i < n; i++ )
     {
-        meta.implemented.push( traits[ i ] );
+        impl.push( traits[ i ] );
+        traits[ i ].__mixinImpl( impl );
     }
 
     return C;

test/Trait/ClassVirtualTest.js

@@ -0,0 +1,184 @@
+/**
+ * Tests overriding virtual class methods using mixins
+ *
+ *  Copyright (C) 2014 Mike Gerwitz
+ *
+ *  This file is part of GNU ease.js.
+ *
+ *  ease.js is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU 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 General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * These tests vary from those in VirtualTest in that, rather than a class
+ * overriding a virtual method defined within a trait, a trait is overriding
+ * a method in the class that it is mixed into. In particular, since
+ * overrides require that the super method actually exist, this means that a
+ * trait must implement or extend a common interface.
+ *
+ * It is this very important (and powerful) system that allows traits to be
+ * used as stackable modifications, similar to how one would use the
+ * decorator pattern (but more tightly coupled).
+ */
+
+require( 'common' ).testCase(
+{
+    caseSetUp: function()
+    {
+        this.Sut           = this.require( 'Trait' );
+        this.Class         = this.require( 'class' );
+        this.AbstractClass = this.require( 'class_abstract' );
+        this.Interface     = this.require( 'interface' );
+    },
+
+
+    /**
+     * A trait may implement an interface I for a couple of reasons: to have
+     * the class mixed into be considered to of type I and to override
+     * methods. But, regardless of the reason, let's start with the
+     * fundamentals.
+     */
+    'Traits may implement an interface': function()
+    {
+        var _self = this;
+
+        // simply make sure that the API is supported; nothing more.
+        this.assertDoesNotThrow( function()
+        {
+           _self.Sut.implement( _self.Interface( {} ) ).extend( {} );
+        } );
+    },
+
+
+    /**
+     * We would expect that the default behavior of implementing an
+     * interface I into a trait would create a trait with all abstract
+     * methods defined by I.
+     */
+    'Traits implementing interfaces define abstract methods': function()
+    {
+        var I = this.Interface( { foo: [], bar: [] } ),
+            T = this.Sut.implement( I ).extend( {} );
+
+        var Class         = this.Class,
+            AbstractClass = this.AbstractClass;
+
+        // T should contain both foo and bar as abstract methods, which we
+        // will test indirectly in the assertions below
+
+        // should fail because of abstract foo and bar
+        this.assertThrows( function()
+        {
+            Class.use( T ).extend( {} );
+        } );
+
+        // should succeed, since we can have abstract methods within an
+        // abstract class
+        this.assertDoesNotThrow( function()
+        {
+            AbstractClass.use( T ).extend( {} );
+        } );
+
+        // one remaining abstract method
+        this.assertDoesNotThrow( function()
+        {
+            AbstractClass.use( T ).extend( { foo: function() {} } );
+        } );
+
+        // both concrete
+        this.assertDoesNotThrow( function()
+        {
+            Class.use( T ).extend(
+            {
+                foo: function() {},
+                bar: function() {},
+            } );
+        } );
+    },
+
+
+    /**
+     * Just as classes implementing interfaces may choose to immediately
+     * provide concrete definitions for the methods declared in the
+     * interface (instead of becoming an abstract class), so too may traits.
+     */
+    'Traits may provide concrete methods for interfaces': function()
+    {
+        var called = false;
+
+        var I = this.Interface( { foo: [] } ),
+            T = this.Sut.implement( I ).extend(
+            {
+                foo: function()
+                {
+                    called = true;
+                },
+            } );
+
+        var Class = this.Class;
+        this.assertDoesNotThrow( function()
+        {
+            // should invoke concrete foo; class definition should not fail,
+            // because foo is no longer abstract
+            Class.use( T )().foo();
+        } );
+
+        this.assertOk( called );
+    },
+
+
+    /**
+     * Instances of class C mixing in some trait T implementing I will be
+     * considered to be of type I, since any method of I would either be
+     * defined within T, or would be implicitly abstract in T, requiring its
+     * definition within C; otherwise, C would have to be declared astract.
+     */
+    'Instance of class mixing in trait implementing I is of type I':
+    function()
+    {
+        var I = this.Interface( {} ),
+            T = this.Sut.implement( I ).extend( {} );
+
+        this.assertOk(
+            this.Class.isA( I, this.Class.use( T )() )
+        );
+    },
+
+
+    /**
+     * The API for multiple interfaces should be the same for traits as it
+     * is for classes.
+     */
+    'Trait can implement multiple interfaces': function()
+    {
+        var Ia = this.Interface( {} ),
+            Ib = this.Interface( {} ),
+            T  = this.Sut.implement( Ia, Ib ).extend( {} ),
+            o  = this.Class.use( T ).extend( {} )();
+
+        this.assertOk( this.Class.isA( Ia, o ) );
+        this.assertOk( this.Class.isA( Ib, o ) );
+    },
+
+
+    /**
+     * This is a concept borrowed from Scala: consider class C and trait T,
+     * both implementing interface I which declares method M. T should be
+     * able to override C.M so long as it is concrete, but to do so, we need
+     * some way of telling ease.js that we are overriding at time of mixin;
+     * otherwise, override does not make sense, because I.M is clearly
+     * abstract and there is nothing to override.
+     */
+    'Trait can override virtual concrete interface methods at mixin':
+    function()
+    {
+    },
+} );