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| <td class="back"><a id="top"></a><a href="s3.html" rel="prev"><< §3 Callout Binding</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s5.html" rel="next">§5 Team Activation >></a></td> |
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| <div class="chapter" id="s4"> |
| <div class="headl"> |
| <div class="headr"> |
| <h1>§4 Callin Binding</h1> |
| </div> |
| </div> |
| <div id="toc-box"> |
| <ul class="toc-box"> |
| <li><a href="s4.html">§4 Callin Binding</a></li> |
| <li><a href="#s4.1">§4.1 Callin method binding</a></li> |
| <li><a href="#s4.2">§4.2 Callin modifiers (before, after, replace)</a></li> |
| <li><a href="#s4.3">§4.3 Base calls</a></li> |
| <li><a href="#s4.4">§4.4 Callin parameter mapping</a></li> |
| <li><a href="#s4.5">§4.5 Lifting and lowering</a></li> |
| <li><a href="#s4.6">§4.6 Overriding access restrictions</a></li> |
| <li><a href="#s4.7">§4.7 Callin binding with static methods</a></li> |
| <li><a href="#s4.8">§4.8 Callin precedence</a></li> |
| <li><a href="#s4.9">§4.9 Callin inheritance</a></li> |
| <li><a href="#s4.10">§4.10 Generic callin bindings</a></li> |
| </ul> |
| </div> |
| <div class="intro"> |
| <h3>Notion of callin binding</h3> |
| <p> |
| Callin bindings realize a forwarding in the direction opposite to callout bindings |
| (see <a href="s3.html" title="§3 Callout Binding" class="sect">§3</a>). |
| Both terms are chosen from the perspective of a role, which controls its communication with |
| an associated base object. |
| Technically, callin bindings are equivalent to weaving additional code (triggers) |
| into existing base methods. |
| |
| </p> |
| <div class="line"></div> |
| <div class="term">Callin</div> |
| <div class="termdesc">Methods of a base class may be <strong>intercepted</strong> by a callin binding |
| (<em>the base method "calls into" the role</em>). |
| </div> |
| <div class="line"></div> |
| <div class="term">Before/after/replace</div> |
| <div class="termdesc">The modifiers <strong>before, after, replace</strong> control the composition |
| of original method and callin method. |
| </div> |
| <div class="line"></div> |
| <div class="term">Activation</div> |
| <div class="termdesc">Callin bindings may be active or inactive according to |
| <a href="s5.html" title="§5 Team Activation" class="sect">§5</a>. |
| </div> |
| <div class="line"></div> |
| </div> |
| <div class="sect depth2" id="s4.1"> |
| <h2 class="sect">§4.1 Callin method binding<a class="img" href="s4.html#s4.1" |
| title="PermaLink to §4.1 Callin method binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <div class="syntaxlink"><a href="sA.html#sA.3.3" title="§A.3.3 Callin binding" |
| class="syntax">→ Syntax §A.3.3</a></div> |
| <div class="subsect depth3" id="s4.1.a"> |
| <h4 class="subsect">(a) <span class="title">Method call interception</span><a class="img" href="s4.html#s4.1.a" |
| title="PermaLink to (a) Method call interception"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A role method may intercept calls to a base method by |
| a callin binding. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.b"> |
| <h4 class="subsect">(b) <span class="title">Prerequisite: Class binding</span><a class="img" href="s4.html#s4.1.b" |
| title="PermaLink to (b) Prerequisite: Class binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A callin binding requires the enclosing class to be a role class |
| bound to a base class according to <a href="s2.html#s2.1" title="§2.1 playedBy relation" class="sect">§2.1</a>. |
| An <em>unliftable</em> role (see <a href="s2.html#s2.3.4.a" title="§2.3.4.(a) Potential ambiguity" |
| class="sect">§2.3.4.(a)</a>) should not define callin bindings. |
| In this case callin bindings can only safely be introduced in sub-roles |
| which (by an appropriately refined <code>playedBy</code> clause) disambiguate the lifting translation. |
| |
| </p> |
| <p> |
| For corner cases the above rule can be overridden by suppressing the corresponding error |
| using the <code>"hidden-lifting-problem"</code> token (see <a href="s2.html#s2.3.5" |
| title="§2.3.5 Consequences of lifting problems" |
| class="sect">§2.3.5</a>). |
| This will allow callin bindings to be defined even for unliftable roles expecting that lifting |
| may still succeed by one of the patterns desribed in <a href="s2.html#s2.3.4.b" title="§2.3.4.(b) Definite ambiguity" |
| class="sect">§2.3.4.(b)</a>. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.c"> |
| <h4 class="subsect">(c) <span class="title">Callin declaration</span><a class="img" href="s4.html#s4.1.c" |
| title="PermaLink to (c) Callin declaration"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A callin binding composes an existing role method with a |
| given base method. |
| It may appear within the role class at any place where feature |
| declarations are allowed. It is denoted by |
| |
| </p> |
| <div class="listing plain"><pre><i>role_method_designator</i> <b><-</b> <i>callin_modifier </i><i>base_method_designator;</i></pre></div> |
| <p>Just like with callout bindings, method designators may |
| or may not contain parameters lists and return type but no modifiers; |
| also, each method designator must exactly and uniquely select one method |
| (cf. <a href="s3.html#s3.1.c" |
| title="§3.1.(c) Kinds of method designators" |
| class="sect">§3.1.(c)</a>). |
| <br /> |
| For <em>callin modifiers</em> see <a href="#s4.2" |
| title="§4.2 Callin modifiers (before, after, replace)" |
| class="sect">below (§4.2)</a>. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.d"> |
| <h4 class="subsect">(d) <span class="title">Multiple base methods</span><a class="img" href="s4.html#s4.1.d" |
| title="PermaLink to (d) Multiple base methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Base method designators may furthermore enumerate a |
| list of methods. If multiple base methods are bound in one |
| callin declaration generally all signatures in this binding must be conform. |
| <br /> |
| However, <em>extraneous parameters</em> from base methods may be ignored |
| at the role. |
| <br /> |
| For <em>result types</em> different rules exist, depending on the |
| applied callin modifier (see next). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.e"> |
| <h4 class="subsect">(e) <span class="title">Named callin binding</span><a class="img" href="s4.html#s4.1.e" |
| title="PermaLink to (e) Named callin binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Any callin binding may be labeled with a name. The name of a callin binding is used |
| for declaring <em>precedence</em> (<a href="#s4.8" title="§4.8 Callin precedence" class="sect">§4.8</a>). |
| A named callin binding <em>overrides</em> any inherited callin binding |
| (explicit and implicit (<a href="s1.html#s1.3.1" |
| title="§1.3.1 Acquisition and implicit inheritance of role classes" |
| class="sect">§1.3.1</a>)) with the same name. |
| <br /> |
| It is an error to use the same callin name more than once within the same role class. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.f"> |
| <h4 class="subsect">(f) <span class="title">Callin to final</span><a class="img" href="s4.html#s4.1.f" |
| title="PermaLink to (f) Callin to final"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>When binding to a final base method, the enclosing role must be played by the exact base class |
| declaring the final method. I.e., callin binding to a final method inherited from the |
| base class's super-class is not allowed. This is motivated by the fact that no sub-class may have |
| a version of a final method with different semantics. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.g"> |
| <h4 class="subsect">(g) <span class="title">Declared exceptions</span><a class="img" href="s4.html#s4.1.g" |
| title="PermaLink to (g) Declared exceptions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>It is an error if a role method to be bound by callin declares in its <tt>throws</tt> clause |
| any exceptions that are not declared by the corresponding base method(s). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.h"> |
| <h4 class="subsect">(h) <span class="title">Method of enclosing class</span><a class="img" href="s4.html#s4.1.h" |
| title="PermaLink to (h) Method of enclosing class"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>In a <code>before</code> or <code>after</code> callin binding the left hand side may alternatively |
| resolve to a method of an enclosing class rather than the current role. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.1.i"> |
| <h4 class="subsect">(i) <span class="title">Callin to constructor</span><a class="img" href="s4.html#s4.1.i" |
| title="PermaLink to (i) Callin to constructor"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A callin binding may refer to a constructor of the bound base class by using the |
| constructor's source name (identical to the name of the base class). |
| In this case only an <strong>after</strong> binding (<a href="#s4.2.a" title="§4.2.(a) Method composition" class="sect">§4.2.(a)</a>) is allowed. |
| |
| </p> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.2"> |
| <h2 class="sect">§4.2 Callin modifiers (before, after, replace)<a class="img" href="s4.html#s4.2" |
| title="PermaLink to §4.2 Callin modifiers (before, after, replace)"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <div class="subsect depth3" id="s4.2.a"> |
| <h4 class="subsect">(a) <span class="title">Method composition</span><a class="img" href="s4.html#s4.2.a" |
| title="PermaLink to (a) Method composition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The kind of method composition is controlled by adding one |
| of the modifiers <strong>before, after</strong> or <strong>replace</strong> after the |
| "<code><-</code>" token of the binding declaration. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.2.b"> |
| <h4 class="subsect">(b) <span class="title">Additive composition</span><a class="img" href="s4.html#s4.2.b" |
| title="PermaLink to (b) Additive composition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The <code>before</code> and <code>after</code> modifiers have the |
| effect of adding a call to the role method at the beginning or end |
| of the base method, resp.<br /> |
| In this case no data are transferred from the role to the base, |
| so if the role method has a result, this will always be ignored. |
| |
| </p> |
| <h5 class="listing">Example code (Callin):</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>team</b> <b>class</b> Company {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>protected</b> <b>class</b> Employee <b>playedBy</b> Person {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>public</b> <b>void</b> recalculateIncome() { ... }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> recalculateIncome <em><b><-</b> after</em> haveBirthday; <span class="comment">// callin binding</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <p>Line 4 declares a callin binding for the role method <code>recalculateIncome()</code> |
| defined in line 3. In combination with the role binding in line 2 this has the following effect: |
| |
| </p> |
| <ul> |
| <li><strong>After</strong> every call of the method <code>Person.haveBirthday</code> |
| the method <code>Company.recalculateIncome</code> is called. |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="subsect depth3" id="s4.2.c"> |
| <h4 class="subsect">(c) <span class="title">Replacing composition</span><a class="img" href="s4.html#s4.2.c" |
| title="PermaLink to (c) Replacing composition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The <code>replace</code> modifier causes <em>only</em> the role method to be |
| invoked, replacing the base method. |
| <br /> |
| In this case, if the base method declares a result, this should be provided by the role method. |
| Special cases of return values in callin bindings are discussed in <a href="#s4.3.e" title="§4.3.(e) Fragile callin binding" |
| class="sect">§4.3.(e)</a></p> |
| </div> |
| <div class="subsect depth3" id="s4.2.d"> |
| <h4 class="subsect">(d) <span class="title">Callin methods</span><a class="img" href="s4.html#s4.2.d" |
| title="PermaLink to (d) Callin methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Role methods to be bound by a callin replacement binding must have |
| the modifier <code>callin</code>. This modifier is only allowed for methods |
| of a role class.<br /> |
| A method with the <code>callin</code> modifier can only be called |
| |
| </p> |
| <ul> |
| <li>via a callin replace binding</li> |
| <li>by a <code>super</code> or <code>tsuper</code> call from an overriding callin method. |
| </li> |
| </ul> |
| <p>It is illegal for a <code>callin</code> method |
| </p> |
| <ul> |
| <li>to be called directly,</li> |
| <li>to be bound using a callout binding, and</li> |
| <li>to be bound to a base method using a <code>before</code> or <code>after</code> callin binding. |
| </li> |
| </ul> |
| <p>Despite these rules a second level role — which is played by the current role — |
| can intercept the execution of a callin method using any form of callin binding.</p> |
| <p>A callin method cannot override a regular method and vice versa, however, |
| overriding one callin method with another callin method is legal and |
| dynamic binding applies to callin method just like regular methods. |
| <br /> |
| A callin method must not declare its visibility using any of the modifiers <code>public</code>, |
| <code>protected</code> or <code>private</code>. Since callin methods can only be invoked via callin bindings |
| such visibility control would not be useful. |
| |
| </p> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.3"> |
| <h2 class="sect">§4.3 Base calls<a class="img" href="s4.html#s4.3" |
| title="PermaLink to §4.3 Base calls"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <div class="syntaxlink"><a href="sA.html#sA.5.3" title="§A.5.3 BaseCall" class="syntax">→ Syntax §A.5.3</a></div> |
| <p>Role methods with a <code>callin</code> modifier should contain |
| a <em>base call</em> which uses the special name <code>base</code> |
| in order to invoke the original base method (original means: before replacement). |
| |
| </p> |
| <div class="subsect depth3" id="s4.3.a"> |
| <h4 class="subsect">(a) <span class="title">Syntax</span><a class="img" href="s4.html#s4.3.a" title="PermaLink to (a) Syntax"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The syntax for base calls is <code>base.m()</code>, |
| which is in analogy to super calls. |
| A <code>base.m()</code> call must use the same name and signature |
| as the enclosing method. This again follows the rule, that roles |
| should never explicitly use base names, except in binding declarations. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.3.b"> |
| <h4 class="subsect">(b) <span class="title">Missing base call</span><a class="img" href="s4.html#s4.3.b" |
| title="PermaLink to (b) Missing base call"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>For each callin method, the compiler uses some flow analysis to check whether |
| a base call will be invoked on each path of execution (analysis is very similar |
| to the analysis for definite assignment regarding final variables - |
| <a href="http://java.sun.com/docs/books/jls/third_edition/html/defAssign.html" |
| class="ext">JLS §16</a>). |
| The compiler will issue a warning if a base call is missing either |
| on each path (definitely missing) or on some paths (potentially missing). |
| Instead of directly invoking a base call, a callin method may also call |
| its explicit or implicit super version using <code>super.m()</code> or <code>tsuper.m()</code> (see <a href="s1.html#s1.3.1.f" title="§1.3.1.(f) tsuper" class="sect">§1.3.1.(f)</a>). |
| In this case the flow analysis will transitively include the called super/tsuper version. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.3.c"> |
| <h4 class="subsect">(c) <span class="title">Duplicate base call</span><a class="img" href="s4.html#s4.3.c" |
| title="PermaLink to (c) Duplicate base call"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a callin method contains several base calls, the compiler gives |
| a warning if this will result in duplicate base call invocations on all |
| paths (definitely duplicate) or on some paths (potentially duplicate). |
| Again super/tsuper calls are included in the flow analysis (see 4.3(b)). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.3.d"> |
| <h4 class="subsect">(d) <span class="title">Parameter tunneling</span><a class="img" href="s4.html#s4.3.d" |
| title="PermaLink to (d) Parameter tunneling"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a base method has more parameters |
| than a callin method to which it is composed, additional |
| parameters are implicitly passed unchanged from the original |
| call to the base call (original means: before interception). |
| I.e., a call <code>base.m()</code> may invisibly pass additional |
| parameters that were provided by the caller, but are hidden from the |
| role method. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.3.e"> |
| <h4 class="subsect">(e) <span class="title">Fragile callin binding</span><a class="img" href="s4.html#s4.3.e" |
| title="PermaLink to (e) Fragile callin binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a role method returns void, but the bound base method declares a |
| non-void result, this is reported as a <em>fragile callin binding</em>: |
| The result can still be provided by the base call, but omitting the |
| base call may cause problems depending on the return type: |
| |
| </p> |
| <ul> |
| <li>For reference return types <code>null</code> will be returned |
| in this case. |
| </li> |
| <li>In the case of primitive return types this will cause a |
| <code>ResultNotProvidedException</code> at run-time. |
| </li> |
| </ul> |
| <p>It is an error if a callin method involved in a fragile callin binding |
| has definitely no base call. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.3.f"> |
| <h4 class="subsect">(f) <span class="title">Base super calls</span><a class="img" href="s4.html#s4.3.f" |
| title="PermaLink to (f) Base super calls"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a callin method <code>rm</code> is bound to a base method <code>B1.m</code> |
| that in turn overrides an inherited method <code>B0.m</code> |
| (<code>B0</code> is a super class of <code>B1</code>), |
| the callin method may use a special form of a base call denoted as |
| |
| </p> |
| <div class="listing plain"><pre><strong>base.super</strong>.rm();</pre></div> |
| <p>Such base super call invokes the super method of the bound base method, |
| here <code>B0.m</code>. This invocation is not affected by any further callin binding. |
| |
| </p> |
| <p>A base super call bypasses both the original method <code>B1.m</code> and |
| also other callin bindings that would be triggered by a regular base call. |
| For this reason any application of this construct is flagged by a decapsulation warning |
| (see <a href="s3.html#s3.4" title="§3.4 Overriding access restrictions" |
| class="sect">§3.4</a>). |
| |
| </p> |
| </div> |
| <div class="note"> |
| <h5>Comment:</h5> |
| Base calls can occur in callin methods that are not |
| yet bound. These methods have no idea of the names of base methods that |
| a sub-role will bind to them. Also multiple base methods may be bound |
| to the same callin method. |
| Hence the use of the role method's own name and signature. |
| The language implementation translates the method name and signature |
| back to the base method that has originally been invoked. |
| |
| </div> |
| <h5 class="listing">Example code (Base Call):</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>public</b> <b>class</b> ValidatorRole <b>playedBy</b> Po<b>int</b> {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <em>callin</em> <b>void</b> checkCoordinate(<b>int</b> value) {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>if</b> (value < 0)</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> <em>base</em>.checkCoordinate(-value);</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> else</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> <em>base</em>.checkCoordinate(value);</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> checkCoordinate <b><-</b> <b>replace</b> setX, setY;</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Effects:</h5> |
| <ul> |
| <li>Line 2 defines a callin method which is bound to two methods |
| of the base class <code>Point</code> (see line 8). |
| </li> |
| <li>The value passed to either setX or setY is checked if it is |
| positive (line 3). |
| </li> |
| <li>Lines 4 and 6 show calls of the original method (base calls). |
| While line 6 passes the original value, in the negative case (line 4) |
| the passed value is made positive. |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.4"> |
| <h2 class="sect">§4.4 Callin parameter mapping<a class="img" href="s4.html#s4.4" |
| title="PermaLink to §4.4 Callin parameter mapping"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <div class="subsect depth3" id="s4.4.a"> |
| <h4 class="subsect">(a) <span class="title">General case parameter mapping</span><a class="img" href="s4.html#s4.4.a" |
| title="PermaLink to (a) General case parameter mapping"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The rules for mapping callin parameters and result type |
| are mainly the same as for callout bindings (<a href="s3.html#s3.2" title="§3.2 Callout parameter mapping" |
| class="sect">§3.2</a>) |
| except for reversing the <code>-></code> and <code><-</code> tokens and |
| swapping left hand side and right hand side. |
| <br /> |
| Callin bindings using <code>before</code> have no result mapping. |
| For <code>result</code> in <code>after</code> callin bindings |
| see <a href="#s4.4.c" |
| title="§4.4.(c) Mapping the result of a base method" |
| class="sect">§4.4.(c)</a> below. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.4.b"> |
| <h4 class="subsect">(b) <span class="title">Restrictions for callin replace bindings</span><a class="img" href="s4.html#s4.4.b" |
| title="PermaLink to (b) Restrictions for callin replace bindings"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The right-hand side of a parameter mapping may either be |
| the simple name of a base method argument without further computation, |
| or an arbitrary expression <em>not</em> containing any base method argument. |
| <br /> |
| Each base method argument must either appear as a simple name in exactly one parameter mapping |
| or not be mapped at all. In the latter case, the original argument is "tunneled" to |
| the base call, meaning, the callin method does not see the argument, but it is passed |
| to the base method as expected. |
| <br /> |
| If the base method declares a result, then |
| |
| </p> |
| <ul> |
| <li>if the role method also declares a result, <code>result</code> must be mapped to itself: |
| <br /><code>result -> result</code></li> |
| <li>if the role method does not declare a result, an arbitrary expression may be mapped to result: |
| <br /><code><em>expression</em> -> result</code><br /> |
| If in this situation no result mapping exists, the result of the base call |
| is "tunneled" and passed to the original caller (see <a href="#s4.3.e" title="§4.3.(e) Fragile callin binding" |
| class="sect"> |
| fragile callin binding (§4.3.(e))</a> above). |
| |
| </li> |
| </ul> |
| <p>These rules ensure that these bindings are reversible |
| for the sake of base calls (<a href="#s4.3" title="§4.3 Base calls" class="sect">§4.3</a>). |
| |
| </p> |
| <p>As stated above a fragile callin binding (<a href="#s4.3.e" title="§4.3.(e) Fragile callin binding" |
| class="sect">§4.3.(e)</a>) |
| is not allowed with a callin method that definitely has no base call |
| (<a href="#s4.3.b" title="§4.3.(b) Missing base call" class="sect">§4.3.(b)</a>). A callin replace binding is not fragile if it provides |
| the base result using a result mapping. |
| |
| </p> |
| <p>A callin method bound with replace to a base method returning |
| void must not declare a non-void result. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.4.c"> |
| <h4 class="subsect">(c) <span class="title">Mapping the result of a base method</span><a class="img" href="s4.html#s4.4.c" |
| title="PermaLink to (c) Mapping the result of a base method"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>In an <code>after</code> callin binding, the right-hand side |
| of a parameter mapping may use the identifier <code>result</code> |
| to refer to the result of the base method. |
| |
| </p> |
| <p> |
| An <code>after</code> callin binding |
| can, however, not <em>influence</em> the result of the base method, |
| thus mappings with the <code>-></code> token are not allowed for after callin bindings. |
| For <code>before</code> mappings using the <code>-></code> token is already ruled out by <a href="#s4.4.a" title="§4.4.(a) General case parameter mapping" |
| class="sect">§4.4.(a)</a></p> |
| </div> |
| <div class="subsect depth3" id="s4.4.d"> |
| <h4 class="subsect">(d) <span class="title">Multiple base methods</span><a class="img" href="s4.html#s4.4.d" |
| title="PermaLink to (d) Multiple base methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A callin binding listing more than one base method may use parameter |
| mappings with only the following restriction: if any base parameter should be mapped this parameter |
| must have the same name and type in all listed base method designators. |
| However, different parameter mappings for different base methods bound to the same role method |
| can be defined if separate callin bindings are used. |
| |
| </p> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.5"> |
| <h2 class="sect">§4.5 Lifting and lowering<a class="img" href="s4.html#s4.5" |
| title="PermaLink to §4.5 Lifting and lowering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <p>For basic definition see <a href="s2.html#s2.2" title="§2.2 Lowering" class="sect">§2.2</a> |
| and <a href="s2.html#s2.3" title="§2.3 Lifting" class="sect">§2.3</a>. |
| <br /> |
| (The following rules are reverse forms of those from <a href="s3.html#s3.3" title="§3.3 Lifting and lowering" |
| class="sect">§3.3</a>) |
| |
| </p> |
| <div class="subsect depth3" id="s4.5.a"> |
| <h4 class="subsect">(a) <span class="title">Call target translation</span><a class="img" href="s4.html#s4.5.a" |
| title="PermaLink to (a) Call target translation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Invoking a role method due to a callin binding first |
| <strong>lifts</strong> the base object to the role class of the |
| callin binding, in order to obtain the effective call target. |
| This is why callin bindings cannot be defined in roles that are <em>unliftable</em> |
| due to <em>potential binding ambiguity</em> (see <a href="#s4.1.b" title="§4.1.(b) Prerequisite: Class binding" |
| class="sect">§4.1.(b)</a> |
| above and <a href="s2.html#s2.3.4.a" title="§2.3.4.(a) Potential ambiguity" |
| class="sect">§2.3.4.(a)</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.5.b"> |
| <h4 class="subsect">(b) <span class="title">Parameter translation</span><a class="img" href="s4.html#s4.5.b" |
| title="PermaLink to (b) Parameter translation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>During callin execution, each parameter for which the role method expects a role |
| object is implicitly <strong>lifted</strong> to the declared role class. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.5.c"> |
| <h4 class="subsect">(c) <span class="title">Result translation</span><a class="img" href="s4.html#s4.5.c" |
| title="PermaLink to (c) Result translation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Returning a role object from a callin method implicitly |
| <strong>lowers</strong> this object. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.5.d"> |
| <h4 class="subsect">(d) <span class="title">Typing rules</span><a class="img" href="s4.html#s4.5.d" title="PermaLink to (d) Typing rules"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A parameter mapping (implicit by parameter position or explicit |
| by a <code>with</code> clause) is <strong>well typed</strong> if |
| the right hand side conforms to the left hand side, either by |
| |
| </p> |
| <ul> |
| <li>type equality</li> |
| <li>implicit primitive type conversion</li> |
| <li>subtype polymorphism</li> |
| <li>translation polymorphism, here: <em>lifting</em>;<br /> |
| however, within <code>replace</code> bindings step 1 of the smart lifting |
| algorithm (<a href="s2.html#s2.3.3.a" title="§2.3.3.(a) Static adjustment" |
| class="sect">§2.3.3.(a)</a>) is not applicable |
| </li> |
| <li><em>or</em> by a combination of the above. |
| </li> |
| </ul> |
| <p>A result mapping (implicit or explicit by a <code>with</code> clause) |
| is well typed, if the value at the left hand conforms to the |
| right hand side according to the rules given above, except that |
| translation polymorphism here applies <em>lowering</em> instead of |
| lifting. |
| </p> |
| <p> |
| These rules define <strong>translation polymorphism</strong> as introduced in |
| <a href="s2.html#s2.3" title="§2.3 Lifting" class="sect">§2.3</a>. |
| |
| </p> |
| <p>Additionally, in a <code>replace</code> callin binding compatibility of parameters and return |
| types must hold in both directions. |
| Thus, from the above list of conversions a replace binding cannot apply subtype polymorphism |
| nor primitive type conversion. |
| If more flexibility is desired, type parameters can be used as defined in <a href="#s4.10" title="§4.10 Generic callin bindings" class="sect">§4.10</a>. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.5.e"> |
| <h4 class="subsect">(e) <span class="title">Role arrays</span><a class="img" href="s4.html#s4.5.e" title="PermaLink to (e) Role arrays"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>For arrays of roles as parameters <a href="s2.html#s2.3.d" title="§2.3.(d) Lifting of arrays" |
| class="sect">§2.3.(d)</a> applies |
| accordingly. For arrays as return value <a href="s2.html#s2.2.e" title="§2.2.(e) Lowering of arrays" |
| class="sect">§2.2.(e)</a> applies. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.5.f"> |
| <h4 class="subsect">(f) <span class="title">Base calls</span><a class="img" href="s4.html#s4.5.f" title="PermaLink to (f) Base calls"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>For base calls these rules are reversed again, i.e., a |
| base call behaves like a callout binding. |
| |
| </p> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.6"> |
| <h2 class="sect">§4.6 Overriding access restrictions<a class="img" href="s4.html#s4.6" |
| title="PermaLink to §4.6 Overriding access restrictions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <p>Callin bindings may also mention inaccessible methods |
| (cf. decapsulation <a href="s3.html#s3.4" title="§3.4 Overriding access restrictions" |
| class="sect">§3.4</a>). |
| Due to the reverse call direction this is relevant only for base calls within |
| <code>callin</code> methods. |
| Base calls have unrestricted access to protected base methods. |
| Accessing a base method with private or default visibility is |
| also allowed, but signaled by a compiler warning. |
| |
| </p> |
| <div class="note"> |
| <h5>Comment:</h5> |
| A base call to an inaccessible base method is considered harmless, |
| since this is the originally intended method execution. |
| |
| </div> |
| <div class="subsect depth3" id="s4.6.a"> |
| <h4 class="subsect">(a) <span class="title">Private methods from super classes</span><a class="img" href="s4.html#s4.6.a" |
| title="PermaLink to (a) Private methods from super classes"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p><em>(Cf. <a href="s3.html#s3.4.d" |
| title="§3.4.(d) Private methods from super classes" |
| class="sect">§3.4.(d)</a>)</em> |
| If a callin binding shall bind to a private base method, that method |
| must be defined in the exact base class to which the current role |
| class is bound using <code>playedBy</code>. |
| <br /> |
| If a private base feature must indeed be callin-bound, a role class |
| must be defined that is played by the exact base class defining the |
| private feature. Another role bound to a sub-base-class can then |
| be defined as a sub class of the first role. It will inherit the |
| callin binding and through this it can access the desired feature. |
| |
| </p> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.7"> |
| <h2 class="sect">§4.7 Callin binding with static methods<a class="img" href="s4.html#s4.7" |
| title="PermaLink to §4.7 Callin binding with static methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <p>The normal case of callin bindings refers to non-static methods on both |
| sides (base and role). Furthermore, in Java inner classes can not define |
| static methods. Both restrictions are relaxed by the following rules: |
| |
| </p> |
| <div class="subsect depth3" id="s4.7.a"> |
| <h4 class="subsect">(a) <span class="title">Static role methods</span><a class="img" href="s4.html#s4.7.a" |
| title="PermaLink to (a) Static role methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A role class may define static methods (see also <a href="s1.html#s1.2.1.f" title="§1.2.1.(f) Static role methods" |
| class="sect">§1.2.1.(f)</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.7.b"> |
| <h4 class="subsect">(b) <span class="title">Binding static to static</span><a class="img" href="s4.html#s4.7.b" |
| title="PermaLink to (b) Binding static to static"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A callin binding may bind a static role method to one or more |
| static base methods. It is, however, an error to bind a static base method to a non-static role method, |
| because such binding would require to lift a base object that is not provided. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.7.c"> |
| <h4 class="subsect">(c) <span class="title">before/after</span><a class="img" href="s4.html#s4.7.c" title="PermaLink to (c) before/after"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>In addition to the above, <code>before</code> and <code>after</code> |
| callin bindings may also bind a static role method to non-static base methods. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.7.d"> |
| <h4 class="subsect">(d) <span class="title">replace</span><a class="img" href="s4.html#s4.7.d" title="PermaLink to (d) replace"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>In contrast to <a href="#s4.7.c" title="§4.7.(c) before/after" class="sect">§4.7.(c)</a> above, a <code>replace</code> callin binding |
| cannot bind a static role method to a non-static base method. |
| |
| </p> |
| </div> |
| <p>The following table summarizes the combinations defined above: |
| |
| </p> |
| <table border="2" class="sep" cellpadding="5" width="80%"> |
| <tr class="z1"> |
| <td colspan="2" rowspan="2" align="right"><strong><-</strong> |
| </td> |
| <td colspan="2" align="center" rowspan="1"><strong>base method</strong></td> |
| </tr> |
| <tr class="z1"> |
| <td align="center" rowspan="1" colspan="1">static</td> |
| <td align="center" rowspan="1" colspan="1">non-static</td> |
| </tr> |
| <tr class="z2"> |
| <td rowspan="2" align="center" class="z1" colspan="1"><strong>role<br />method</strong></td> |
| <td align="center" class="z1" rowspan="1" colspan="1">static</td> |
| <td rowspan="1" colspan="1"><span class="green">OK</span></td> |
| <td rowspan="1" colspan="1"><code>before/after</code>: <span class="green">OK</span><br /><code>replace</code>: <span class="error">illegal</span></td> |
| </tr> |
| <tr class="z2"> |
| <td align="center" class="z1" rowspan="1" colspan="1">non-static</td> |
| <td rowspan="1" colspan="1"><span class="error">illegal</span></td> |
| <td rowspan="1" colspan="1"><span class="green">OK</span></td> |
| </tr> |
| </table> |
| <div class="subsect depth3" id="s4.7.e"> |
| <h4 class="subsect">(e) <span class="title">No overriding</span><a class="img" href="s4.html#s4.7.e" title="PermaLink to (e) No overriding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Since static methods are not dynamically bound, <i>overriding</i> does not |
| apply in the normal semantics. Regarding callin bindings this has the |
| following consequences (assuming a role <code>RMid</code> played by |
| <code>BMid</code> plus its super-class <code>BSuper</code> and its sub-class |
| <code>BSub</code>. |
| |
| </p> |
| <ol> |
| <li>If a static base method <code>BMid.m</code> is bound by a callin binding |
| this has no effect on any method <code>m</code> in <code>BSub</code>. |
| </li> |
| <li>If a callin binding mentions a method <code>m</code> which is not present |
| in <code>BMid</code> but resolves to a static method in <code>BSuper</code> |
| the binding only affects invocations as <code>BMid.m()</code> but not |
| <code>BSuper.m()</code>. If the latter call should be affected, too, |
| the callin binding must appear in a role class bound to <code>BSuper</code>, |
| not <code>BMid</code>. |
| </li> |
| <li>In order to bind two static base methods with equal signatures, one being |
| defined in a sub-class of the other one, two roles have to be defined |
| where one role refines the <code>playedBy</code> clause of the other role |
| (say: <code>public class RSub extends RMid playedBy BSub</code>). |
| Now each role may bind to the static base method accessible in its direct |
| base-class. |
| </li> |
| </ol> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.8"> |
| <h2 class="sect">§4.8 Callin precedence<a class="img" href="s4.html#s4.8" |
| title="PermaLink to §4.8 Callin precedence"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <div class="syntaxlink"><a href="sA.html#sA.8" title="§A.8 Precedence declaration" |
| class="syntax">→ Syntax §A.8</a></div> |
| <p>If multiple callins from the same team refer to the same base method and also have the same |
| callin modifier (<code>before</code>, <code>after</code> or <code>replace</code>), the order in which |
| the callin bindings shall be triggered has to be declared using a precedence declaration. |
| |
| </p> |
| <div class="subsect depth3" id="s4.8.a"> |
| <h4 class="subsect">(a) <span class="title">Precedence declaration</span><a class="img" href="s4.html#s4.8.a" |
| title="PermaLink to (a) Precedence declaration"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A precedence declaration consists of the keyword <code>precedence</code> followed by |
| a list of names referring to callin bindings (see <a href="#s4.1.e" title="§4.1.(e) Named callin binding" class="sect">§4.1.(e)</a> for named callin bindings). |
| |
| </p> |
| <div class="listing plain"><pre><b>precedence</b> callinBinding1, callinBinding2;</pre></div> |
| <p> |
| A precedence declaration is only legal within a role or team class.<br /> |
| The order of elements in a precedence declaration determines their <strong>priority</strong> during dispatch, |
| similar to priorities based on activation of several team instances (<a href="s5.html#s5.1" title="§5.1 Effect of team activation" |
| class="sect">§5.1</a>). |
| This means that <code>before</code> and <code>replace</code> binding with highest priority trigger first, |
| whereas <code>after</code> bindings with highest priority trigger last. |
| For binding precedences (as opposed to class based precedence, see <a href="#s4.8.c" title="§4.8.(c) Class based precedence" |
| class="sect">§4.8.(c)</a> below) |
| which refer to <code>after</code> bindings, the precedence declaration must also use the <code>after</code> keyword |
| to remind the programmer that the execution order is inverse to the textual order. |
| |
| </p> |
| <div class="listing plain"><pre><b>precedence</b> <b>after</b> importantExecuteLast, lessImportantExecuteEarlier;</pre></div> |
| </div> |
| <div class="subsect depth3" id="s4.8.b"> |
| <h4 class="subsect">(b) <span class="title">Qualified and unqualified names</span><a class="img" href="s4.html#s4.8.b" |
| title="PermaLink to (b) Qualified and unqualified names"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Within a role class a callin binding may be referenced by its unqualified name. |
| A precedence declaration in a team class must qualify the callin name with the name of the declaring |
| role class. A team with nested teams may concat role class names. |
| Elements of a qualified callin name are separated by ".". |
| <br /> |
| The callin binding must be found in the role specified by the qualifying prefix or |
| in the enclosing role for unqualified names, or any super class of this role |
| (including implicit super classes <a href="s1.html#s1.3.1" |
| title="§1.3.1 Acquisition and implicit inheritance of role classes" |
| class="sect">§1.3.1</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.8.c"> |
| <h4 class="subsect">(c) <span class="title">Class based precedence</span><a class="img" href="s4.html#s4.8.c" |
| title="PermaLink to (c) Class based precedence"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>At the team level a precedence declaration may contain role class names without |
| explicitly mentioning callin bindings in order to refer to all callin bindings of the role. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.8.d"> |
| <h4 class="subsect">(d) <span class="title">Multiple precedence statements</span><a class="img" href="s4.html#s4.8.d" |
| title="PermaLink to (d) Multiple precedence statements"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>All precedence statements are collected at the outer-most team. At that level all |
| precedence declarations involving the same base method are merged using the |
| C3 algorithm <a href="#fn3-c3-algorithm" class="int">[3]</a>. |
| When merging precendence declarations more deeply nested declarations have higher priority |
| than outer declarations. |
| For several declarations at the same nesting level the lexical ordering determines the priority. |
| |
| </p> |
| <p> |
| At any point the C3 algorithm will ensure that the resulting order after merging is consistent |
| with each individual precedence declaration. |
| It is an error to declare |
| incompatible precedence lists that cannot be merged by the C3 algorithm. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.8.e"> |
| <h4 class="subsect">(e) <span class="title">Binding overriding</span><a class="img" href="s4.html#s4.8.e" |
| title="PermaLink to (e) Binding overriding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Precedence declarations may conflict with overriding of callin bindings |
| (see <a href="#s4.1.e" title="§4.1.(e) Named callin binding" class="sect">§4.1.(e)</a>): For each pair of callin bindings of which one |
| callin binding overrides the other one, precedence declarations are not applicable, |
| since dynamic binding will already select exactly one callin binding. |
| <br /> |
| It is an error to <i>explicitly mention</i> such a pair of overriding callin bindings in a precedence declaration. |
| <br /> |
| When a class-based precedence declaration <i>implicitly refers to</i> a callin binding that is overridden by, |
| or overrides any other callin binding within the same precedence declaration, this does not affect the fact, |
| that the most specific callin binding overrides less specific ones. |
| |
| </p> |
| </div> |
| <h5 class="listing">Callin binding example</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>public</b> <b>class</b> LogLog<b>in</b> <b>playedBy</b> Data<b>base</b> {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>callin</b> <b>void</b> log (String what) {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> System.out.println("enter " + what);</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> base.log(what.toLowerCase());</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> System.out.println("leave " + what);</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> <b>void</b> log(String what) <b><-</b> <b>replace</b> <b>void</b> login(String uid, String passwd) </pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> <b>with</b> { what <b><-</b> uid }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre>}</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre>(<b>new</b> Database()).login("Admin", "Passwd");</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Effects:</h5> |
| <p>Provided the callin bindings are active (cf. <a href="s5.html" title="§5 Team Activation" class="sect">§5</a>) then: |
| |
| </p> |
| <ul> |
| <li>the call in line 10 is intercepted by method <code>log</code> |
| of role <code>LogLogin</code>. |
| </li> |
| <li>the call target of <code>log</code> is a role of type |
| <code>LogLogin</code> which is created by lifting the |
| original call target (of type <code>Database</code>) to |
| <code>LogLogin</code>. |
| </li> |
| <li>only parameter <code>uid</code> is passed to <code>log</code> |
| (bound to formal parameter <code>what</code>). |
| </li> |
| <li>within method <code>log</code> the base call (line 4) invokes |
| the original method passing a modified uid (converted to lower case, cf. line 4) |
| and the unmodified password, which is hidden from the callin method due to the |
| parameter mapping in line 8. |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.9"> |
| <h2 class="sect">§4.9 Callin inheritance<a class="img" href="s4.html#s4.9" |
| title="PermaLink to §4.9 Callin inheritance"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <p> |
| This section defines how callin bindings and callin methods relate to inheritance. |
| |
| </p> |
| <div class="sect depth3" id="s4.9.1"> |
| <h3 class="sect">§4.9.1 Base side inheritance<a class="img" href="s4.html#s4.9.1" |
| title="PermaLink to §4.9.1 Base side inheritance"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#s4.9">↑ §4.9</a></span></h3> |
| <p> |
| Generally, a callin binding affects all sub-types of its bound base. |
| Specifically, if a role type <code>R</code> bound to a base type <code>B</code> |
| defines a callin binding <code>rm <- <i>callin_modifier</i> bm</code>, |
| the following rules apply: |
| |
| </p> |
| <div class="subsect depth4" id="s4.9.1.a"> |
| <h4 class="subsect">(a) <span class="title">Effect on sub-classes</span><a class="img" href="s4.html#s4.9.1.a" |
| title="PermaLink to (a) Effect on sub-classes"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The callin binding also effects instances of any type <code>BSub</code> |
| that is a sub-type of <code>B</code>. |
| If <code>BSub</code> overrides the bound base method <code>bm</code>, |
| the overridden version is generally affected, too. |
| However, if <code>bm</code> covariantly redefines the return type from its |
| super version, the callin binding has to explicitly specify if the covariant |
| sub-class version should be affected, too (see <a href="#s4.9.3.b" |
| title="§4.9.3.(b) Capturing covariant base methods" |
| class="sect">§4.9.3.(b)</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth4" id="s4.9.1.b"> |
| <h4 class="subsect">(b) <span class="title">No effect on super-classes</span><a class="img" href="s4.html#s4.9.1.b" |
| title="PermaLink to (b) No effect on super-classes"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The binding never affects an instance of any super-type of <code>B</code> |
| even if the method <code>bm</code> is inherited from a super-class |
| or overrides an inherited method. |
| This ensures that dispatching to a role method due to a callin binding |
| always provides a base instance that has at least the type declared in the role's |
| <code>playedBy</code> clause. |
| |
| </p> |
| </div> |
| <p> |
| For corresponding definitions regarding static methods see <a href="#s4.7.e" title="§4.7.(e) No overriding" class="sect">§4.7.(e)</a>. |
| |
| </p> |
| </div> |
| <div class="sect depth3" id="s4.9.2"> |
| <h3 class="sect">§4.9.2 Role side inheritance<a class="img" href="s4.html#s4.9.2" |
| title="PermaLink to §4.9.2 Role side inheritance"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#s4.9">↑ §4.9</a></span></h3> |
| <p> |
| Any sub-type of <code>R</code> inherits the given callin binding |
| (for overriding of bindings see <a href="#s4.8.e" title="§4.8.(e) Binding overriding" class="sect">§4.8.(e)</a>). |
| If the sub-role overrides the role method <code>rm</code> this will be considered |
| for dynamic dispatch when the callin binding is triggered. |
| |
| </p> |
| </div> |
| <div class="sect depth3" id="s4.9.3"> |
| <h3 class="sect">§4.9.3 Covariant return types<a class="img" href="s4.html#s4.9.3" |
| title="PermaLink to §4.9.3 Covariant return types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#s4.9">↑ §4.9</a></span></h3> |
| <p> |
| Since version 5, Java supports the covariant redefinition of a method's return type |
| (see <a href="http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.4.5" |
| class="ext">JLS 8.4.5</a>). |
| This is <em>not</em> supported for <code>callin</code> methods (<a href="#s4.9.3.a" title="§4.9.3.(a) No covariant callin methods" |
| class="sect">§4.9.3.(a)</a>). |
| If base methods with covariant redefinition of the return type are to be bound by a callin binding |
| the subsequent rules ensure that type safety is preserved. |
| Two <em>constraints</em> have to be considered: |
| |
| </p> |
| <ol> |
| <li> |
| When a callin method issues a base-call or calls its tsuper version, |
| this call must produce a value whose type is compatible to the |
| enclosing method's declared return type. |
| |
| </li> |
| <li> |
| If a replace-bound role method returns a value that is not the result of a base-call, |
| it must be ensured that the return value actually satisfies the declared signature of |
| the bound base method. |
| |
| </li> |
| </ol> |
| <div class="subsect depth4" id="s4.9.3.a"> |
| <h4 class="subsect">(a) <span class="title">No covariant callin methods</span><a class="img" href="s4.html#s4.9.3.a" |
| title="PermaLink to (a) No covariant callin methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| A method declared with the <code>callin</code> modifier that overrides an inherited method |
| must not redefine the return type with respect to the inherited method. |
| This reflects that fact that an inherited callin binding should remain type-safe |
| while binding to the new, overriding role method. |
| Binding a covariant role method to the original base method would break constraint (1) above. |
| |
| </p> |
| </div> |
| <div class="subsect depth4" id="s4.9.3.b"> |
| <h4 class="subsect">(b) <span class="title">Capturing covariant base methods</span><a class="img" href="s4.html#s4.9.3.b" |
| title="PermaLink to (b) Capturing covariant base methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| If a callin binding should indeed affect not only the specified base method |
| but also overriding versions which covariantly redefine the return type, |
| the binding must specify the base method's return type with a "+" appended |
| to the type name as in |
| </p> |
| <div class="listing plain"><pre><b>void</b> rm() <b><-</b> <b>before</b> <em>RT+</em> bm();</pre></div> |
| <p>Without the "+" sign the binding would only capture base methods whose |
| return type is exactly <code>RT</code>; |
| by appending "+" also sub-types of <code>RT</code> |
| are accepted as the declared return type. |
| |
| </p> |
| </div> |
| <div class="subsect depth4" id="s4.9.3.c"> |
| <h4 class="subsect">(c) <span class="title">Covariant replace binding</span><a class="img" href="s4.html#s4.9.3.c" |
| title="PermaLink to (c) Covariant replace binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| When using the syntax of <a href="#s4.9.3.b" |
| title="§4.9.3.(b) Capturing covariant base methods" |
| class="sect">§4.9.3.(b)</a> to capture base methods with |
| covariant return types in a callin binding with the <code>replace</code> modifier, |
| the role method must be specified using a free type parameter as follows: |
| </p> |
| <div class="listing plain"><pre><em><E <b>extends</b> RT> E</em> rm() <b><-</b> <b>replace</b> RT+ bm();</pre></div> |
| <p>The role method <code>rm</code> referenced by this callin binding must use the same style |
| of return type using a type parameter. |
| The only possible non-null value of type <code>E</code> |
| to be returned from such method is the value provided by a base-call or a tsuper-call.<br /> |
| This rule enforces the constraint (2) above.<br /> |
| Note that this rule is further generalized in <a href="#s4.10" title="§4.10 Generic callin bindings" class="sect">§4.10</a>. |
| |
| </p> |
| <h5 class="listing">Binding a parametric role method</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>public</b> <b>class</b> SuperBase {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> SuperBase foo() { <b>return</b> this; }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>void</b> check() { System.out.print("OK"); }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre>}</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre><b>public</b> <b>class</b> SubBase <b>extends</b> SuperBase {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> @Override</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> SubBase foo() { <b>return</b> this; }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> <b>void</b> print() { System.out.print("SubBase"); }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> String test() { </pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre> this.foo().print(); <span class="comment">// print() requires a SubBase</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">11</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">12</td> |
| <td><pre>}</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">13</td> |
| <td><pre></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">14</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> MyTeam {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">15</td> |
| <td><pre> <b>protected</b> <b>class</b> R <b>playedBy</b> SuperBase {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">16</td> |
| <td><pre> <b>callin</b> <E <b>extends</b> SuperBase> E ci() {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">17</td> |
| <td><pre> E result= base.ci();</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">18</td> |
| <td><pre> result.check(); <span class="comment">// check() is available on E via type bound SuperBase</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">19</td> |
| <td><pre> <b>return</b> result;</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">20</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">21</td> |
| <td><pre> <E <b>extends</b> SuperBase> E ci() <b><-</b> <b>replace</b> SuperBase+ foo();</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">22</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">23</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Explanation:</h5> |
| <ul> |
| <li> |
| Method <code>SubBase.foo</code> in line 7 redefines the return type |
| from <code>SuperBase</code> (inherited version) to <code>SubBase</code>, |
| thus clients like the method call in line 10 must be safe to assume |
| that the return value will always conform to <code>SubBase</code>. |
| |
| </li> |
| <li> |
| The callin binding in line 21 explicitly captures both versions of <code>foo</code> |
| by specifying <code>SuperBase+</code> as the expected return type. |
| Thus, if an instance of <code>MyTeam</code> is active at the method call |
| in line 10, this call to <code>foo</code> will indeed be intercepted |
| even though this call is statically known to return a value of type <code>SubBase</code>. |
| |
| </li> |
| <li> |
| The callin method in lines 16-20 has a return type which is not known statically, |
| but the return type is represented by the type variable <code>E</code>. |
| Since the base call is known to have the exact same signature as its enclosing |
| method, the value provided by the base call is of the same type <code>E</code> |
| and thus can be safely returned from <code>ci</code>. |
| <em>Note,</em> that no other non-null value is known to have the type <code>E</code>. |
| |
| </li> |
| <li> |
| By specifying <code>SuperBase</code> as an upper bound for the type <code>E</code> |
| the callin method <code>ci</code> may invoke |
| any method declared in type <code>SuperBase</code> |
| on any value of type <code>E</code>. For an example see the call to <code>check</code> |
| in line 18. |
| |
| </li> |
| </ul> |
| <p><em> |
| As an aside note that the above example uses type <code>SuperBase</code> |
| in an undisciplined way: within role <code>R</code> this type is bound |
| using <code>playedBy</code><strong> and</strong> the same type is also |
| used directly (as the upper bound for <code>E</code>). |
| This is considered bad style and it is prohibited if <code>SuperBase</code> |
| is imported using an base import (<a href="s2.html#s2.1.2.d" title="§2.1.2.(d) Base imports" |
| class="sect">§2.1.2.(d)</a>). |
| Here this rule is neglegted just for the purpose of keeping the example small. |
| </em></p> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect depth2" id="s4.10"> |
| <h2 class="sect">§4.10 Generic callin bindings<a class="img" href="s4.html#s4.10" |
| title="PermaLink to §4.10 Generic callin bindings"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §4</a></span></h2> |
| <p> |
| As mentioned in <a href="#s4.5.d" 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" 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.html#s4.10.a" |
| 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.html#s4.10.b" 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.html#s4.10.c" |
| 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.html#s4.10.d" |
| 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.html#s4.10.e" |
| 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> |
| <div class="aux" id="aux4.2"> |
| <h4 class="aux">Open issues:<span class="toplink"><a href="#s4">↑ §4</a></span></h4> |
| <p>The query language for specifying sets of base methods (<a href="#s4.1.d" title="§4.1.(d) Multiple base methods" |
| class="sect">§4.1.(d)</a>) |
| has not been finalized yet. In this version of the OTJLD <a href="s8.html" title="§8 Join Point Queries" class="sect">§8</a> |
| acts as a placeholder for the section that will define a join point query language in the future. |
| </p> |
| </div> |
| <div class="aux" id="aux4.1"> |
| <h4 class="aux">References:<span class="toplink"><a href="#s4">↑ §4</a></span></h4> |
| <p id="fn3-c3-algorithm"> |
| [3] Kim Barrett, Bob Cassels, Paul Haahr, David A. Moon, Keith Playford, P. Tucker Withington. <em>A monotonic superclass linearization for Dylan</em>. OOPSLA '96: Proceedings of the 11th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, |
| pages 69-82, 1996. |
| |
| </p> |
| </div> |
| </div> |
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