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| <td class="back"><a id="top"></a><a href="s4.9.html" rel="prev"><< §4.9 Callin inheritance</a></td> |
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| <div class="breadcrumb"><a class="nav" href="s4.html" rel="section">§4 Callin Binding</a></div> |
| <div class="sect depth2" id="s4.10"> |
| <h2 class="sect">§4.10 Generic callin bindings<a class="img" href="s4.10.html" |
| title="PermaLink to §4.10 Generic callin bindings"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h2> |
| <p> |
| As mentioned in <a href="s4.5.d.html" title="§4.5.(d) Typing rules" class="sect">§4.5.(d)</a> replace bindings do not support subtype polymorphism |
| in either direction. |
| On the other hand, binding several base methods to the same <code>callin</code> method |
| may require some more flexibility if these base methods have different signatures. |
| This is where type parameter come to the rescue to allow for generic <code>callin</code> methods |
| and their binding to base methods with different signatures.<br /> |
| Note that this rule is a generalization of rule <a href="s4.9.3.c.html" title="§4.9.3.(c) Covariant replace binding" |
| class="sect">§4.9.3.(c)</a>. |
| |
| </p> |
| <p> |
| Additionally, any callin binding (before,replace,after) may declare one or more type parameters |
| for propagating type parameters of the bound base method(s) (<a href="#s4.10.e" title="§4.10.(e) Propagating type parameters" |
| class="sect">§4.10.(e)</a>). |
| |
| </p> |
| <div class="subsect depth3" id="s4.10.a"> |
| <h4 class="subsect">(a) <span class="title">Fresh type parameter</span><a class="img" href="s4.10.a.html" |
| title="PermaLink to (a) Fresh type parameter"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| If a callin method declares a type parameter <code><T></code> for |
| capturing a covariant return type |
| this type <code>T</code> |
| can be used for specifying the type of exactly one parameter or the return type. |
| If a type parameter is used in more than one position of a <code>callin</code> method |
| it is not considered a <em>fresh type parameter</em> |
| and can thus not be bound to a covariant return type (see <a href="#s4.10.d" title="§4.10.(d) Binding to a type parameter" |
| class="sect">§4.10.(d)</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.10.b"> |
| <h4 class="subsect">(b) <span class="title">Type bounds</span><a class="img" href="s4.10.b.html" title="PermaLink to (b) Type bounds"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| The type parameter of a callin binding may be bounded by an upper bound as in <code><T extends C></code>. |
| In this case <code>T</code> can only be instantiated by types conforming to the upper bound <code>C</code>. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.10.c"> |
| <h4 class="subsect">(c) <span class="title">Generic replace binding</span><a class="img" href="s4.10.c.html" |
| title="PermaLink to (c) Generic replace binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| A generic <code>callin</code> method according to the above rules is bound using a replace |
| binding that declares the same number of type parameters, |
| where type parameters of the binding and its callin method are identified. |
| If the <code>callin</code> method declares bounds for its type parameters |
| so should the replace binding. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.10.d"> |
| <h4 class="subsect">(d) <span class="title">Binding to a type parameter</span><a class="img" href="s4.10.d.html" |
| title="PermaLink to (d) Binding to a type parameter"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| A fresh type parameter can be used to capture arbitrary types in the base methods to be bound. |
| The type parameter may be instantiated differently for each bound base method. |
| By such type parameter instantiation the types in role and base signatures are actually identical, |
| thus satisfying the requirement of two-way substitutability. |
| |
| </p> |
| </div> |
| <p> |
| Within the body of a generic <code>callin</code> method no further rules have to be followed, |
| because the fresh type variable actually guarantees, that the role method cannot replace the |
| original value (initial argument or base-call result) with a different object, |
| because no type exists that is guaranteed to conform to the type parameters. |
| Yet, the type bound allows the role method to invoke methods of the provided object. |
| |
| </p> |
| <h5 class="listing">Generic replace binding</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> MyTeam {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>protected</b> <b>class</b> R <b>playedBy</b> Figures {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>callin</b> <E <b>extends</b> Shape, F <b>extends</b> Shape> E ci(F arg) {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> E result= base.ci(arg);</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <span class="error">result= arg; // illegal, types E and F are incommensurable</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> <span class="error">arg= result; // illegal, types E and F are incommensurable</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> <b>int</b> size= arg.getSize(); <span class="comment">// getSize() is availabel on F via type bound Shape</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> result.resize(size); <span class="comment"> // resize() is available on E via type bound Shape</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> <b>return</b> result; <span class="comment">// only two legal values exist: <code>result</code> and <code>null</code></span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">11</td> |
| <td><pre> <E <b>extends</b> Shape, F <b>extends</b> Shape> </pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">12</td> |
| <td><pre> E ci(F arg) <b><-</b> <b>replace</b> Rectangle getBoundingBox(Shape original), </pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">13</td> |
| <td><pre> Rectangle stretch(Square original);</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">14</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">15</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Explanation:</h5> |
| These declaration generate two version of the <code>callin</code> method <code>ci</code>: |
| |
| <ol> |
| <li><code>Rectangle ci (Shape arg)</code></li> |
| <li><code>Rectangle ci (Square arg)</code></li> |
| </ol> |
| Within the <code>callin</code> method the following observations hold: |
| |
| <ul> |
| <li>Line 5 is illegal for the first signature as <code>Shape</code> is not conform to <code>Rectangle</code></li> |
| <li>Line 6 is illegal for the second signature as <code>Rectangle</code> is not conform to <code>Square</code></li> |
| <li>Everything else is type-safe.</li> |
| </ul> |
| </div> |
| <div class="subsect depth3" id="s4.10.e"> |
| <h4 class="subsect">(e) <span class="title">Propagating type parameters</span><a class="img" href="s4.10.e.html" |
| title="PermaLink to (e) Propagating type parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <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. |
| |
| </p> |
| <div class="listing frame"> |
| <table class="listing"> |
| <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> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> <b>callin</b> <U> U rm(U a) { <b>return</b> base.rm(a); }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> <U> U rm(U a) <b><-</b> <b>replace</b> U getIt(U it);</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <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>. |
| |
| </div> |
| </div> |
| </div> |
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| <td class="back"><a href="s4.9.html" rel="prev"><< §4.9 Callin inheritance</a></td> |
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