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<title>Virtual Traits for Scala</title>
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<meta name="author" content="Anders Bach Nielsen"/>
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<h1>Virtual Traits for Scala</h1>
<h2>Abstract</h2>
<p>This document describes the design, integration and implementation
of Virtual Traits (VT) into the Scala language. Virtual traits in
Scala are modeled after the BETA [<a
href="#madsen93:_objec_orien_progr_beta_progr_languag">1</a>] and
gbeta [<a href="#ernst99b">3</a>] notion of virtual classes [<a
href="#Mads89a">2</a>, <a href="#1111062">4</a>]. </p>
<h2>Contents</h2>
<ul>
<li><a href="#motivation">Motivation</a>
<ul>
<li><a href="#goals">Goals</a></li>
<li><a href="#usecases">Use cases</a></li>
<li><a href="#usecasedetail">Use cases in detail</a></li>
<li><a href="#futuregoals">Future goals that extend beyond this proposal</a></li>
</ul>
</li>
<li><a href="#languagespec">Language specification changes</a></li>
<li><a href="#implementation">Implementation</a></li>
<li><a href="#references">References</a></li>
</ul>
<a name="motivation"></a>
<h2>Motivation</h2>
<p>The Scala language already has abstract type members that behave
like the types of virtual classes, however this is only for
types. Scala supports both classes and traits, where traits act as
mixins that can be composed together to create classes. The reason
why there are both classes and traits, is to support the
interoperability with Java which has only classes and for this
reason classes are treated specially in the compiler.</p>
<p>Using abstract type members and a complicated rewriting scheme it
is possible to create virtual classes manually, but there are
several typing issues which will be covered later. We will in this
proposal show the extensions to the Scala language specification to
include virtual traits, the rewriting scheme for translating virtual
trait programs into regular Scala programs and discuss the
implementation of this in the Scala compiler.</p>
<p>This proposal builds upon the parametrized traits.</p>
<a name="goals"></a>
<h3>Goals</h3>
<ul>
<li>Present the additions and changes to the Scala Language
Specification to cover Virtual Traits.</li>
<li>Show in detail the rewriting scheme used in the translation of
Scala programs with virtual traits to programs in Scala
proper.</li>
<li>Present a sample implementation in the Scala compiler.</li>
</ul>
<a name="usecases"></a>
<h3>Use cases</h3>
<ol>
<li><a href="#usecase1">Initial binding of virtual trait</a></li>
<li><a href="#usecase2">Further binding of a virtual trait</a></li>
<li><a href="#usecase3">Final binding of a virtual trait</a></li>
<li><a href="#usecase4">Diamond inheritance involving virtual traits</a></li>
<li><a href="#usecase5">Virtual traits with nested virtual traits</a></li>
</ol>
<a name="usecasedetail"></a>
<h3>Use cases in detail</h3>
<a name="usecase1"></a>
<h4>(1) Initial binding of virtual trait</h4>
<p></p>
<pre>
<b>trait</b> A <: { ... }
<b>trait</b> B <: A { ... }
<b>trait</b> C <: Foo <b>with</b> B { ... }
</pre>
<a name="usecase2"></a>
<h4>(2) Further binding of a virtual trait</h4>
<p></p>
<pre>
<b>override trait</b> A <: { ... }
<b>override trait</b> B <: { ... }
<b>override trait</b> C <: Bar { ... }
</pre>
<a name="usecase3"></a>
<h4>(3) Final binding of a virtual trait</h4>
<p></p>
<pre>
<b>override trait</b> A
<b>override trait</b> B { ... }
<b>override trait</b> C <b>extends</b> Baz { ... }
</pre>
<a name="usecase4"></a>
<h4>(4) Diamond inheritance involving virtual traits</h4>
<p></p>
<pre>
<b>trait</b> A {
<b>trait</b> V <: { ... }
}
<b>trait</b> B <b>extends</b> A {
<b>override trait</b> V <: { ... }
}
<b>trait</b> C <b>extends</b> A {
<b>override trait</b> V <: { ... }
}
<b>trait</b> D <b>extends</b> B <b>with</b> C {
<b>override trait</b> V <: { ... }
}
</pre>
<a name="usecase5"></a>
<h4>(5) Virtual traits with nested virtual traits</h4>
<p></p>
<pre>
<b>trait</b> A {
<b>trait</b> V <: {
<b>trait</b> W <: { ... }
}
}
<b>trait</b> B <b>extends</b> A {
<b>override trait</b> V <: {
<b>override trait</b> W <: { ... }
}
}
</pre>
<a name="futuregoals"></a>
<h3>Future goals that extend beyond this proposal</h3>
<ul>
<li>Having virtual classes, these would have to be converted from
classes to traits and so</li>
</ul>
<a name="languagespec"></a>
<h2>Language specification changes</h2>
This section will cover the changes to the Scala language, both the
syntactic and sematic changes that are nessesary to make virtual
traits work under Scala.
<p>
Normal declaration of a trait in Scala.
</p>
<pre>
trait Foo { ... }
</pre>
<pre>
TmplDef ::= ['case'] 'class' ClassDef
| ['case'] 'object' ObjectDef
| ['override'] 'trait' TraitDef
TraitDef ::= id [TypeParamClause] TraitTemplateOpt
TraitTemplateOpt ::= TraitExtends TraitTemplate
| [['extends'] TemplateBody ]
| '<:' TemplateBody
TraitExtends ::= 'extends' | '<:'
---------- FLAGS -------------
TRAIT + DEFERED + ! OVERRIDE => Initial binding
TRAIT + DEFERED + OVERRIDE => Further binding
TRAIT + ! DEFERED + OVERRIDE => Final binding
classes that extend virtuals are marked with a VIRTUALSUBCLASS
</pre>
<h3>Syntactic</h3>
From the virtual class litterateur there is a distinction between
the initial binding and the further binding of a virtual class. To
have the same expressiveness in Scala this is the way to declare a
initial binding of a virtual trait.
<pre>
trait Bar <: { ... }
</pre>
To be able to tell the difference between a initial and a further
binding of a trait on the syntactical level, further bindings are
written as.
<pre>
override trait Bar <: { ... }
</pre>
<h3>Semantic</h3>
<a name="implementation"></a>
<h2>Implementation</h2>
<a name="references"></a>
<h2>References</h2>
<table>
<tr valign="top">
<td align="right">
[<a name="madsen93:_objec_orien_progr_beta_progr_languag">1</a>]
</td>
<td>
Ole Lehrmann Madsen, Kristen Nygaard, and Birger Møller-Pedersen.
<em>Object-Oriented Programming in The Beta Programming Language</em>.
Addison-Wesley, 1993.
[ <a href="sip-0000X-1.xhtml#madsen93:_objec_orien_progr_beta_progr_languag">bib</a> ]
</td>
</tr>
<tr valign="top">
<td align="right">
[<a name="Mads89a">2</a>]
</td>
<td>
Ole Lehrmann Madsen and Birger Møller-Pedersen.
Virtual classes: A powerful mechanism in object-oriented
programming.
In <em>Proceedings OOPSLA'89, ACM SIGPLAN Notices</em>, volume 24,
10, pages 397-406, October 1989.
[ <a href="sip-0000X-1.xhtml#Mads89a">bib</a> ]
</td>
</tr>
<tr valign="top">
<td align="right">
[<a name="ernst99b">3</a>]
</td>
<td>
Erik Ernst.
<em>gbeta - a Language with Virtual Attributes, Block Structure,
and Propagating, Dynamic Inheritance</em>.
PhD thesis, Department of Computer Science, University of Aarhus,
Århus, Denmark, 1999.
[ <a href="sip-0000X-1.xhtml#ernst99b">bib</a> ]
</td>
</tr>
<tr valign="top">
<td align="right">
[<a name="1111062">4</a>]
</td>
<td>
Erik Ernst, Klaus Ostermann, and William R. Cook.
A virtual class calculus.
In <em>POPL '06: Conference record of the 33rd ACM SIGPLAN-SIGACT
symposium on Principles of programming languages</em>, pages 270-282, New York,
NY, USA, 2006. ACM Press.
[ <a href="sip-0000X-1.xhtml#1111062">bib</a> ]
</td>
</tr>
<tr valign="top">
<td align="right">
[<a name="Erns01a">5</a>]
</td>
<td>
Erik Ernst.
Family polymorphism.
In J. L. Knudsen, editor, <em>ECOOP 2001</em>, number 2072 in LNCS,
pages 303-326. Springer Verlag, 2001.
[ <a href="sip-0000X-1.xhtml#Erns01a">bib</a> ]
</td>
</tr>
<tr valign="top">
<td align="right">
[<a name="EE99">6</a>]
</td>
<td>
Erik Ernst.
Propagating class and method combination.
In Rachid Guerraoui, editor, <em>Proceedings ECOOP'99</em>, LNCS 1628,
pages 67-91, Lisboa, Portugal, June 1999. Springer-Verlag.
[ <a href="sip-0000X-1.xhtml#EE99">bib</a> ]
</td>
</tr>
<tr valign="top">
<td align="right">
[<a name="aracic06:_overv_of_caesarj">7</a>]
</td>
<td>
Ivica Aracic, Vaidas Gasiunas, Mira Mezini, and Klaus Ostermann.
An overview of caesarj.
In <em>Transactions on Aspect-Oriented Software Development I</em>,
volume 3880/2006 of <em>Lecture Notes in Computer Science</em>, pages 135-173.
Springer Berlin / Heidelberg, 2006.
[ <a href="sip-0000X-1.xhtml#aracic06:_overv_of_caesarj">bib</a> ]
</td>
</tr>
</table>
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