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<div class="breadcrumb"><a class="nav" href="s2.html" rel="section">§2 Role Binding</a> > <a class="nav" href="s2.3.html" rel="section">§2.3 Lifting</a> > <a class="nav" href="s2.3.4.html" rel="section">§2.3.4 Binding ambiguities</a></div>

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Stephan Herrmann

109893e

2010-05-18 22:02:53 +0000

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<h4 class="subsect">§2.3.4.(a) <span class="title">Potential ambiguity</span><a class="img" href="s2.3.4.a.html"

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title="PermaLink to (a) Potential ambiguity"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"

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alt="" /></a></h4>

Stephan Herrmann

0166441

2010-04-01 20:28:43 +0000

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<p>A <strong>potential ambiguity</strong> is given,

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if two role classes <code>R1</code> and <code>R2</code>

exist such that

</p>

<ul>

<li><code>R1</code> and <code>R2</code> are played by the

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same base class <code>B</code>, and

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</li>

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<li><code>R1</code> and <code>R2</code> have a common

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super role <code>R0</code>,

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which is also bound to a base class <code>B0</code>, and

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</li>

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<li>neither role class <code>R1</code> nor

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<code>R2</code> is a (indirect) sub-class of the other.

</li>

</ul>

According to <a href="s2.1.c.html" title="§2.1.(c) Covariant refinement"

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class="sect">§2.1.(c)</a>, if <code>B</code> is distinct from <code>B0</code>

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it has to be a sub-class of <code>B0</code>.

</div>

<h5>Effect:</h5>

In this case the compiler issues a warning, stating that the <code>B</code><em> may not be liftable,</em> because both role classes <code>R1</code>

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and <code>R2</code> are candidates and there is no reason to prefer one over the other.

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<br /><strong>If no potential ambiguity is detected, lifting will always be unambiguous.</strong></div>

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<p>In the above situation, trying to lift an instance of type <code>B</code> to the role type

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<code>R0</code> is an <strong>illegal lifting request</strong>. If <code>R0</code> is bound

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to the same base class <code>B</code> as its sub-roles <code>R1</code> and <code>R2</code> are,

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role <code>R0</code> is <strong>unliftable</strong>, meaning that no instance of <code>R0</code>

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can ever by obtained by lifting.

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</p>

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<h5 class="listing">Example code (Potential Ambiguity):</h5>

<td><pre><b>team</b> <b>class</b> MyTeam {</pre></td>

</tr>

<td><pre> <b>public</b> <b>class</b> SuperRole <b>playedBy</b> MyBase {...}</pre></td>

</tr>

<td><pre> <b>public</b> <b>class</b> SubRoleA <b>extends</b> SuperRole {...}</pre></td>

</tr>

<td><pre> <b>public</b> <b>class</b> SubRoleB <b>extends</b> SuperRole {...}</pre></td>

</tr>

</tr>

</table>

</div>

</div>

<tr>

<td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td>

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<div class="breadcrumb"><a class="nav" href="s2.html" rel="section">§2 Role Binding</a> > <a class="nav" href="s2.3.html" rel="section">§2.3 Lifting</a> > <a class="nav" href="s2.3.4.html" rel="section">§2.3.4 Binding ambiguities</a></div>

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Stephan Herrmann

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2010-05-18 22:02:53 +0000

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Stephan Herrmann

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2010-06-08 18:24:55 +0000

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2010-04-01 20:28:43 +0000

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