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<td class="back"><a id="top"></a><a href="s9.1.html" rel="prev">&lt;&lt;&nbsp;&sect;9.1&nbsp;Defining classes with value parameters</a></td>
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<div class="breadcrumb"><a class="nav" href="s9.html" rel="section">&sect;9&nbsp;Value Dependent Classes</a></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></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="#top">&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="#top">&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>
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