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| <td class="back"><a id="top"></a><a href="s4.10.d.html" rel="prev"><< §4.10.(d) Binding to a type parameter</a></td> |
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| <div class="breadcrumb"><a class="nav" href="s4.html" rel="section">§4 Callin Binding</a> > <a class="nav" href="s4.10.html" rel="section">§4.10 Generic callin bindings</a></div> |
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| <h4 class="subsect">§4.10.(e) <span class="title">Propagating type parameters</span><a class="img" href="s4.10.e.html" |
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| <p> |
| If a callin binding binds to a generic base method, |
| any type parameter(s) of the base method must be propagated into the role method |
| by declaring the callin binding with type parameters, too. |
| By matching a type parameter of a base method with a type variable |
| of the callin binding, this genericity is propagated through the callin binding. |
| |
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| <div class="listing frame"> |
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| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>class</b> MyBase {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <T> T getIt(T it) { <b>return</b> it; }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre>}</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre><b>team</b> <b>class</b> MyTeam {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <b>protected</b> <b>class</b> MyRole <b>playedBy</b> MyBase {</pre></td> |
| </tr> |
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| <td class="ln">6</td> |
| <td><pre> <b>callin</b> <U> U rm(U a) { <b>return</b> base.rm(a); }</pre></td> |
| </tr> |
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| <td class="ln">7</td> |
| <td><pre> <U> U rm(U a) <b><-</b> <b>replace</b> U getIt(U it);</pre></td> |
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| <td class="ln">8</td> |
| <td><pre> }</pre></td> |
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| <td class="ln">9</td> |
| <td><pre>}</pre></td> |
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| <div class="codecomment"> |
| <h5>Explanation:</h5> |
| The callin binding declares a type parameter <code><U></code> |
| which is used to match all occurrences of <code>T</code> in the signature of <code>getIt</code>. |
| Thus the implementation of <code>rm</code> uses the type <code>U</code> |
| in exactly the same generic way as <code>getIt</code> uses <code>T</code>. |
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