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                   <h1>&sect;9&nbsp;Value Dependent Classes</h1>
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             <div id="toc-box">
                <ul class="toc-box">
                   <li><a href="s9.html">&sect;9&nbsp;Value Dependent Classes</a></li>
                   <li><a href="#s9.1">&sect;9.1&nbsp;Defining classes with value parameters</a></li>
                   <li><a href="#s9.2">&sect;9.2&nbsp;Using classes with value parameters</a></li>
                   <li><a href="#s9.3">&sect;9.3&nbsp;Restrictions and limitations</a></li>
                </ul>
             </div>
             <div class="intro">
                <h3>Generalizing externalized roles</h3>
                <div class="line"></div>
                <div class="term">Type Value Parameter</div>
                <div class="termdesc">In addition to regular generics, a class may declare parameters that represent an object value.
                   				Such a value parameter is called the <strong>type anchor</strong> for this class,
                   				the class's type is said to be <strong>anchored</strong> to this parameter.
                </div>
                <div class="line"></div>
                <div class="term">Value Dependent Classes</div>
                <div class="termdesc">A class that declares one or more value parameters depends on the
                   				runtime instance(s) denoted by its anchor(s).
                </div>
                <div class="line"></div>
                <div class="term">Externalized Roles</div>
                <div class="termdesc">The concept of externalized roles (<a href="s1.2.2.html" title="&sect;1.2.2&nbsp;Externalized roles" class="sect">&sect;1.2.2</a>)
                   				is a special case of the concepts presented here.
                </div>
                <div class="line"></div>
             </div>
             <div class="sect depth2" id="s9.1">
                <h2 class="sect">&sect;9.1&nbsp;Defining classes with value parameters<a class="img" href="s9.1.html"
                      title="PermaLink to &sect;9.1&nbsp;Defining classes with value parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a><span class="toplink"><a href="#top">&uarr;&nbsp;&sect;9</a></span></h2>
                <div class="syntaxlink"><a href="sA.html#sA.9.1" title="&sect;A.9.1&nbsp;TypeParameter" class="syntax">&rarr;&nbsp;Syntax&nbsp;&sect;A.9.1</a></div>
                <div class="subsect depth3" id="s9.1.a">
                   <h4 class="subsect">(a)&nbsp;<span class="title">Value parameter declaration</span><a class="img" href="s9.1.a.html"
                         title="PermaLink to (a)&nbsp;Value parameter declaration"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>Within the angle brackets that mark the parameters of a generic class also value parameters
                      		can be declared. In contrast to a type parameter, a value parameter is denoted as a pair
                      		of two identifiers: a type and a free name, e.g.,

                   </p>
                   <div class="listing plain"><pre><b>class</b> MyClass<em>&lt;YourType aName&gt;</em> { ...</pre></div>
                   <p>Note that value parameters are valid for classes only, not for interfaces.

                   </p>
                </div>
                <div class="subsect depth3" id="s9.1.b">
                   <h4 class="subsect">(b)&nbsp;<span class="title">Value parameter application</span><a class="img" href="s9.1.b.html"
                         title="PermaLink to (b)&nbsp;Value parameter application"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>Within the given class (<code>MyClass</code>) the parameter name (<code>aName</code>) can be used
                      		like a final field of the given type (<code>YourType</code>). In contrast to regular final fields
                      		the assignment to this name occurs even before the constructor is executed.

                   </p>
                </div>
                <div class="subsect depth3" id="s9.1.c">
                   <h4 class="subsect">(c)&nbsp;<span class="title">Role types as dependent types</span><a class="img" href="s9.1.c.html"
                         title="PermaLink to (c)&nbsp;Role types as dependent types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>Any role type can be interpreted as a value dependent type, however, in the declaration
                      		of a role type the value parameter remains implicit: it is identical to the enclosing team instance.

                   </p>
                </div>
             </div>
             <div class="sect depth2" id="s9.2">
                <h2 class="sect">&sect;9.2&nbsp;Using classes with value parameters<a class="img" href="s9.2.html"
                      title="PermaLink to &sect;9.2&nbsp;Using classes with value parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a><span class="toplink"><a href="#top">&uarr;&nbsp;&sect;9</a></span></h2>
                <div class="syntaxlink"><a href="sA.html#sA.9.2" title="&sect;A.9.2&nbsp;ActualTypeArgument"
                      class="syntax">&rarr;&nbsp;Syntax&nbsp;&sect;A.9.2</a></div>
                <p>When using a class which declares one or more value parameters (type anchors)
                   	a corresponding <strong>anchor value</strong> has to be provided.

                </p>
                <div class="sect depth3" id="s9.2.1">
                   <h3 class="sect">&sect;9.2.1&nbsp;Parameter substitution<a class="img" href="s9.2.1.html"
                         title="PermaLink to &sect;9.2.1&nbsp;Parameter substitution"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a><span class="toplink"><a href="#s9.2">&uarr;&nbsp;&sect;9.2</a></span></h3>
                   <p>Substitution of a type anchor of a class <code>MyClass&lt;YourType p&gt;</code> is denoted as
                      		<code>MyClass&lt;@v&gt;</code>.
                      	In this term <code>v</code> must be a value which is conform to the declaration of the value
                      	parameter "<code>YourType p</code>", ie., <code>v</code> must have the static type <code>YourType</code>.
                      	<br />
                      The value passed for substituting a type anchor must be a path of variables declared as <code>final</code>.
                      Obviously, only the first element in such a path can be a local variable or a method argument,
                      all other elements have to be fields. The reason for requiring final variables is
                      in type checking as discussed next.

                   </p>
                   <div class="note">
                      <h5>Note:</h5>
                      				Externalized roles as defined in <a href="s1.2.2.b.html"
                         title="&sect;1.2.2.(b)&nbsp;Declaration with anchored type"
                         class="sect">&sect;1.2.2.(b)</a> are
                      				a special case of types with a value parameter, where the value
                      				is an instance of the enclosing team.

                   </div>
                   <div class="subsect depth4" id="s9.2.1.a">
                      <h4 class="subsect">(a)&nbsp;<span class="title">Instance constrained type parameters</span><a class="img" href="s9.2.1.a.html"
                            title="PermaLink to (a)&nbsp;Instance constrained type parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                                 alt="" /></a></h4>
                      <p>In addition to normal usage, a value parameter can be applied nested to a regular type parameter:

                      </p>
                      <div class="listing plain"><pre><b>class</b> MyClass&lt;YourType aName, DependentParam&lt;<em>@aName&gt;</em>&gt; { ...</pre></div>
                      <p>
                         					Here the type parameter <code>DependentParam</code> is constrained to be anchored to <code>aName</code>.

                      </p>
                      <p>If a value parameter is used as a constraint for a regular type parameter
                         					any substitution for the type parameter must also supply a value matching the value parameter.
                         				    The class from above could be applied like this:

                      </p>
                      <div class="listing plain"><pre><b>final</b> YourType anchor = <b>new</b> YourType();
 MyClass <em>&lt;@anchor, YourDependent&lt;<strong>@anchor</strong>&gt;&gt;</em></pre></div>
                      <p>Within the declaring element (class or method)
                         				   applications of the type variable representing the instance constrained type parameter
                         				   must repeat the anchor verbatim, i.e., no substitutions are performed here.

                      </p>
                   </div>
                </div>
                <div class="sect depth3" id="s9.2.2">
                   <h3 class="sect">&sect;9.2.2&nbsp;Type conformance<a class="img" href="s9.2.2.html"
                         title="PermaLink to &sect;9.2.2&nbsp;Type conformance"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a><span class="toplink"><a href="#s9.2">&uarr;&nbsp;&sect;9.2</a></span></h3>
                   <p>Two value dependent types (anchored types) are considered conform only if the anchors
                      		of both types refer to <i>the same object(s)</i>.  The compiler must be able to statically
                      		analyze this anchor identity.

                   </p>
                   <div class="subsect depth4" id="s9.2.2.a">
                      <h4 class="subsect">(a)&nbsp;<span class="title">Substitutions for type anchors</span><a class="img" href="s9.2.2.a.html"
                            title="PermaLink to (a)&nbsp;Substitutions for type anchors"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                                 alt="" /></a></h4>
                      <p>Only two substitutions are considered for determining anchor identity:

                      </p>
                      <ol>
                         <li>If a method signature uses <code>this</code> as the anchor of any of its types,
                            		type checking an application of this method performs the following substitutions:
                            		<br />
                            A simple <code>this</code> expression is substituted by the actual call target
                            of the method application.
                            <br />
                            A qualified <code>Outer.this</code> expression is substituted by the corresponding
                            enclosing instance of the call target.
                         </li>
                         <li>Assignments from a <code>final</code> identifier to another <code>final</code>
                            					identifier are transitively followed, i.e., if <code>t1, t2</code> are final,
                            					after an assignment <code>t1=t2</code> the types <code>C&lt;@t1&gt;</code> and
                            					<code>C&lt;@t2&gt;</code> are considered identical. Otherwise <code>C&lt;@t1&gt;</code>
                            					and <code>C&lt;@t2&gt;</code> are incommensurable.
                            					<br />
                            Attaching an actual parameter to a formal parameter in a method call is also considered
                            as an assignment with respect to this rule.
                         </li>
                      </ol>
                   </div>
                   <div class="subsect depth4" id="s9.2.2.b">
                      <h4 class="subsect">(b)&nbsp;<span class="title">Conformance of raw types</span><a class="img" href="s9.2.2.b.html"
                            title="PermaLink to (b)&nbsp;Conformance of raw types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                                 alt="" /></a></h4>
                      <p>After anchors have been proven identical, the raw types are checked for compatibility
                         			using the standard Java rules.

                      </p>
                   </div>
                </div>
             </div>
             <div class="sect depth2" id="s9.3">
                <h2 class="sect">&sect;9.3&nbsp;Restrictions and limitations<a class="img" href="s9.3.html"
                      title="PermaLink to &sect;9.3&nbsp;Restrictions and limitations"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a><span class="toplink"><a href="#top">&uarr;&nbsp;&sect;9</a></span></h2>
                <div class="subsect depth3" id="s9.3.a">
                   <h4 class="subsect">(a)&nbsp;<span class="title">No overriding</span><a class="img" href="s9.3.a.html" title="PermaLink to (a)&nbsp;No overriding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>Types with value parameters that are declared outside a team cannot be overridden,
                      		as roles can be. Therefor, implicit inheritance does not apply for these types.

                   </p>
                </div>
                <div class="subsect depth3" id="s9.3.b">
                   <h4 class="subsect">(b)&nbsp;<span class="title">Only first parameter</span><a class="img" href="s9.3.b.html"
                         title="PermaLink to (b)&nbsp;Only first parameter"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <div class="note">
                      <p>Currently only the first parameter of a class may be a value parameter.
                         				This restriction may be removed in the future.
                      </p>
                   </div>
                </div>
             </div>
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	<td class="back"><a id="top"></a><a href="s8.html" rel="prev"><< §8 Join Point Queries</a></td>
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	<div class="chapter" id="s9">
	<div class="headl">
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	<h1>§9 Value Dependent Classes</h1>
	</div>
	</div>
	<div id="toc-box">
	<ul class="toc-box">
	<li><a href="s9.html">§9 Value Dependent Classes</a></li>
	<li><a href="#s9.1">§9.1 Defining classes with value parameters</a></li>
	<li><a href="#s9.2">§9.2 Using classes with value parameters</a></li>
	<li><a href="#s9.3">§9.3 Restrictions and limitations</a></li>
	</ul>
	</div>
	<div class="intro">
	<h3>Generalizing externalized roles</h3>
	<div class="line"></div>
	<div class="term">Type Value Parameter</div>
	<div class="termdesc">In addition to regular generics, a class may declare parameters that represent an object value.
	Such a value parameter is called the <strong>type anchor</strong> for this class,
	the class's type is said to be <strong>anchored</strong> to this parameter.
	</div>
	<div class="line"></div>
	<div class="term">Value Dependent Classes</div>
	<div class="termdesc">A class that declares one or more value parameters depends on the
	runtime instance(s) denoted by its anchor(s).
	</div>
	<div class="line"></div>
	<div class="term">Externalized Roles</div>
	<div class="termdesc">The concept of externalized roles (<a href="s1.2.2.html" title="§1.2.2 Externalized roles" class="sect">§1.2.2</a>)
	is a special case of the concepts presented here.
	</div>
	<div class="line"></div>
	</div>
	<div class="sect depth2" id="s9.1">
	<h2 class="sect">§9.1 Defining classes with value parameters<a class="img" href="s9.1.html"
	title="PermaLink to §9.1 Defining classes with value parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#top">↑ §9</a></span></h2>
	<div class="syntaxlink"><a href="sA.html#sA.9.1" title="§A.9.1 TypeParameter" class="syntax">→ Syntax §A.9.1</a></div>
	<div class="subsect depth3" id="s9.1.a">
	<h4 class="subsect">(a) <span class="title">Value parameter declaration</span><a class="img" href="s9.1.a.html"
	title="PermaLink to (a) Value parameter declaration"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Within the angle brackets that mark the parameters of a generic class also value parameters
	can be declared. In contrast to a type parameter, a value parameter is denoted as a pair
	of two identifiers: a type and a free name, e.g.,

	</p>
	<div class="listing plain"><pre><b>class</b> MyClass<em><YourType aName></em> { ...</pre></div>
	<p>Note that value parameters are valid for classes only, not for interfaces.

	</p>
	</div>
	<div class="subsect depth3" id="s9.1.b">
	<h4 class="subsect">(b) <span class="title">Value parameter application</span><a class="img" href="s9.1.b.html"
	title="PermaLink to (b) Value parameter application"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Within the given class (<code>MyClass</code>) the parameter name (<code>aName</code>) can be used
	like a final field of the given type (<code>YourType</code>). In contrast to regular final fields
	the assignment to this name occurs even before the constructor is executed.

	</p>
	</div>
	<div class="subsect depth3" id="s9.1.c">
	<h4 class="subsect">(c) <span class="title">Role types as dependent types</span><a class="img" href="s9.1.c.html"
	title="PermaLink to (c) Role types as dependent types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Any role type can be interpreted as a value dependent type, however, in the declaration
	of a role type the value parameter remains implicit: it is identical to the enclosing team instance.

	</p>
	</div>
	</div>
	<div class="sect depth2" id="s9.2">
	<h2 class="sect">§9.2 Using classes with value parameters<a class="img" href="s9.2.html"
	title="PermaLink to §9.2 Using classes with value parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#top">↑ §9</a></span></h2>
	<div class="syntaxlink"><a href="sA.html#sA.9.2" title="§A.9.2 ActualTypeArgument"
	class="syntax">→ Syntax §A.9.2</a></div>
	<p>When using a class which declares one or more value parameters (type anchors)
	a corresponding <strong>anchor value</strong> has to be provided.

	</p>
	<div class="sect depth3" id="s9.2.1">
	<h3 class="sect">§9.2.1 Parameter substitution<a class="img" href="s9.2.1.html"
	title="PermaLink to §9.2.1 Parameter substitution"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#s9.2">↑ §9.2</a></span></h3>
	<p>Substitution of a type anchor of a class <code>MyClass<YourType p></code> is denoted as
	<code>MyClass<@v></code>.
	In this term <code>v</code> must be a value which is conform to the declaration of the value
	parameter "<code>YourType p</code>", ie., <code>v</code> must have the static type <code>YourType</code>.
	<br />
	The value passed for substituting a type anchor must be a path of variables declared as <code>final</code>.
	Obviously, only the first element in such a path can be a local variable or a method argument,
	all other elements have to be fields. The reason for requiring final variables is
	in type checking as discussed next.

	</p>
	<div class="note">
	<h5>Note:</h5>
	Externalized roles as defined in <a href="s1.2.2.b.html"
	title="§1.2.2.(b) Declaration with anchored type"
	class="sect">§1.2.2.(b)</a> are
	a special case of types with a value parameter, where the value
	is an instance of the enclosing team.

	</div>
	<div class="subsect depth4" id="s9.2.1.a">
	<h4 class="subsect">(a) <span class="title">Instance constrained type parameters</span><a class="img" href="s9.2.1.a.html"
	title="PermaLink to (a) Instance constrained type parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>In addition to normal usage, a value parameter can be applied nested to a regular type parameter:

	</p>
	<div class="listing plain"><pre><b>class</b> MyClass<YourType aName, DependentParam<<em>@aName></em>> { ...</pre></div>
	<p>
	Here the type parameter <code>DependentParam</code> is constrained to be anchored to <code>aName</code>.

	</p>
	<p>If a value parameter is used as a constraint for a regular type parameter
	any substitution for the type parameter must also supply a value matching the value parameter.
	The class from above could be applied like this:

	</p>
	<div class="listing plain"><pre><b>final</b> YourType anchor = <b>new</b> YourType();
	MyClass <em><@anchor, YourDependent<<strong>@anchor</strong>>></em></pre></div>
	<p>Within the declaring element (class or method)
	applications of the type variable representing the instance constrained type parameter
	must repeat the anchor verbatim, i.e., no substitutions are performed here.

	</p>
	</div>
	</div>
	<div class="sect depth3" id="s9.2.2">
	<h3 class="sect">§9.2.2 Type conformance<a class="img" href="s9.2.2.html"
	title="PermaLink to §9.2.2 Type conformance"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#s9.2">↑ §9.2</a></span></h3>
	<p>Two value dependent types (anchored types) are considered conform only if the anchors
	of both types refer to <i>the same object(s)</i>. The compiler must be able to statically
	analyze this anchor identity.

	</p>
	<div class="subsect depth4" id="s9.2.2.a">
	<h4 class="subsect">(a) <span class="title">Substitutions for type anchors</span><a class="img" href="s9.2.2.a.html"
	title="PermaLink to (a) Substitutions for type anchors"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Only two substitutions are considered for determining anchor identity:

	</p>
	<ol>
	<li>If a method signature uses <code>this</code> as the anchor of any of its types,
	type checking an application of this method performs the following substitutions:
	<br />
	A simple <code>this</code> expression is substituted by the actual call target
	of the method application.
	<br />
	A qualified <code>Outer.this</code> expression is substituted by the corresponding
	enclosing instance of the call target.
	</li>
	<li>Assignments from a <code>final</code> identifier to another <code>final</code>
	identifier are transitively followed, i.e., if <code>t1, t2</code> are final,
	after an assignment <code>t1=t2</code> the types <code>C<@t1></code> and
	<code>C<@t2></code> are considered identical. Otherwise <code>C<@t1></code>
	and <code>C<@t2></code> are incommensurable.
	<br />
	Attaching an actual parameter to a formal parameter in a method call is also considered
	as an assignment with respect to this rule.
	</li>
	</ol>
	</div>
	<div class="subsect depth4" id="s9.2.2.b">
	<h4 class="subsect">(b) <span class="title">Conformance of raw types</span><a class="img" href="s9.2.2.b.html"
	title="PermaLink to (b) Conformance of raw types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>After anchors have been proven identical, the raw types are checked for compatibility
	using the standard Java rules.

	</p>
	</div>
	</div>
	</div>
	<div class="sect depth2" id="s9.3">
	<h2 class="sect">§9.3 Restrictions and limitations<a class="img" href="s9.3.html"
	title="PermaLink to §9.3 Restrictions and limitations"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#top">↑ §9</a></span></h2>
	<div class="subsect depth3" id="s9.3.a">
	<h4 class="subsect">(a) <span class="title">No overriding</span><a class="img" href="s9.3.a.html" title="PermaLink to (a) No overriding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Types with value parameters that are declared outside a team cannot be overridden,
	as roles can be. Therefor, implicit inheritance does not apply for these types.

	</p>
	</div>
	<div class="subsect depth3" id="s9.3.b">
	<h4 class="subsect">(b) <span class="title">Only first parameter</span><a class="img" href="s9.3.b.html"
	title="PermaLink to (b) Only first parameter"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<div class="note">
	<p>Currently only the first parameter of a class may be a value parameter.
	This restriction may be removed in the future.
	</p>
	</div>
	</div>
	</div>
	</div>
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