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          <div class="sect depth2" id="s5.4">
             <h2 class="sect">&sect;5.4&nbsp;Guard predicates<a class="img" href="s5.4.html"
                   title="PermaLink to &sect;5.4&nbsp;Guard predicates"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                        alt="" /></a></h2>
             <div class="syntaxlink"><a href="sA.7.html" title="&sect;A.7&nbsp;Guard predicates" class="syntax">&rarr;&nbsp;Syntax&nbsp;&sect;A.7</a></div>
             <p>The effect of callins can further be controlled using so called guard predicates.
                	Guards appear at four different levels:

             </p>
             <ul>
                <li>callin method binding</li>
                <li>role method</li>
                <li>role class</li>
                <li>team class</li>
             </ul>
             <p>Guards can be specified as <em>regular</em> guards or <code>base</code> guards,
                	which affects the exact point in the control flow, where the guard will be evaluated.

             </p>
             <div class="subsect depth3" id="s5.4.a">
                <h4 class="subsect">(a)&nbsp;<span class="title">General syntax for guards</span><a class="img" href="s5.4.a.html"
                      title="PermaLink to (a)&nbsp;General syntax for guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>A guard is declared using the keyword <code>when</code> followed by a
                   		boolean expression in parentheses:

                </p>
                <div class="listing plain"><pre><em><b>when</b> (</em><i>predicateExpression</i><em>)</em></pre></div>
                <p>Depending on the kind of guard different objects are in scope using
                   special identifiers like <code>this</code>, <code>base</code>.<br />
                   Any predicate expression that evaluates to <code>true</code> enables
                   the callin binding(s) to which it applies.
                   Evaluation to <code>false</code> disables the callin binding(s).

                </p>
             </div>
             <div class="subsect depth3" id="s5.4.b">
                <h4 class="subsect">(b)&nbsp;<span class="title">No side effects</span><a class="img" href="s5.4.b.html" title="PermaLink to (b)&nbsp;No side effects"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>A guard predicate should have no side effects.
                   			   A compiler should optionally check this condition, but inter-procedural analysis
                   			   actually depends on the availability of appropriate means to mark any method as side-effect free.

                </p>
             </div>
             <div class="subsect depth3" id="s5.4.c">
                <h4 class="subsect">(c)&nbsp;<span class="title">Exceptions</span><a class="img" href="s5.4.c.html" title="PermaLink to (c)&nbsp;Exceptions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>A guard predicate should not throw any exceptions.
                   			   Yet, any exception thrown within a guard predicate cause the guard to evaluate to <code>false</code>
                   			   rather than propagating the exception, meaning that the evaluation of a guard predicate will never
                   			   interrupt the current base behaviour.<br />
                   			   A compiler should flag any checked exception that is thrown within a guard.
                   			   Such diagnosis should by default be treated as an error, with the option of configuring
                   			   its severity to warning or ignore.

                </p>
             </div>
             <div class="sect depth3" id="s5.4.1">
                <h3 class="sect">&sect;5.4.1&nbsp;Regular guards<a class="img" href="s5.4.1.html"
                      title="PermaLink to &sect;5.4.1&nbsp;Regular guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a><span class="toplink"><a href="#top">&uarr;&nbsp;&sect;5.4</a></span></h3>
                <p>This group of guards evaluates within the context of a given role.
                   		These guards are evaluated <em>after</em> a callin target is lifted and
                   		<em>before</em> a callin bound role method is invoked.

                </p>
                <div class="subsect depth4" id="s5.4.1.a">
                   <h4 class="subsect">(a)&nbsp;<span class="title">Method binding guards</span><a class="img" href="s5.4.1.a.html"
                         title="PermaLink to (a)&nbsp;Method binding guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>A guard may be attached to a callin method binding as in:

                   </p>
                   <div class="listing plain"><pre><b>void</b> roleMethod(<b>int</b> ir) <b>&lt;-</b> <b>after</b> <b>void</b> baseMethod(<b>int</b> ib)
   <b>when</b> (ir &gt; MyTeam.this.threshold);</pre></div>
                   <p>Such a guard only affects the callin binding to which it is attached,
                      			i.e., this specific callin binding is only effective,
                      			if the predicate evaluates to <code>true</code>.
                      			<br />
                      			The following values are within the scope of the predicate expression,
                      			and thus can be used to express the condition:

                   </p>
                   <ul>
                      <li>The role instance denoted by <code>this</code>.<br />
                         				Features of the role instance can also be accessed relative to
                         				<code>this</code> with or without explicit qualifying <code>this</code>.

                      </li>
                      <li>The team instance denoted by a qualified this reference as in
                         				<code>MyTeam.this</code>.

                      </li>
                      <li>If the callin binding includes signatures (as in the example above):
                         				Parameters of the role method.<br />
                         				If parameter mappings are involved, they will be evaluated before evaluating the guard.

                      </li>
                   </ul>
                </div>
                <div class="subsect depth4" id="s5.4.1.b">
                   <h4 class="subsect">(b)&nbsp;<span class="title">Method guards</span><a class="img" href="s5.4.1.b.html" title="PermaLink to (b)&nbsp;Method guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>A method guard is similar to a method binding guard, but it applies
                      			to all callin method bindings of this method.<br />
                      			A method guard is declared between the method signature and the method body:

                   </p>
                   <div class="listing plain"><pre><b>void</b> roleMethod(<b>int</b> ir)
   <b>when</b> (ir &gt; MyTeam.this.threshold) { <i>body statements</i> }</pre></div>
                </div>
                <div class="subsect depth4" id="s5.4.1.c">
                   <h4 class="subsect">(c)&nbsp;<span class="title">Role level guards</span><a class="img" href="s5.4.1.c.html"
                         title="PermaLink to (c)&nbsp;Role level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>When a guard is specified at the role level, i.e., directly before the
                      			class body of a role class, it applies to all callin method bindings of
                      			the role class:

                   </p>
                   <div class="listing plain"><pre><b>protected</b> <b>class</b> MyRole
   <b>when</b> (value &gt; MyTeam.this.threshold)
 {
     <b>int</b> value;
     <i>other <b>class</b> body declarations</i>
 }</pre></div>
                   <p>The following values are within the scope of the predicate expression:

                   </p>
                   <ul>
                      <li>The role instance denoted by <code>this</code> (explicit or implicit, see above).
                         				Thus, in the example <code>value</code> will be interpreted as a field of the enclosing role.

                      </li>
                      <li>The team instance denoted by a qualified this reference as in <code>MyTeam.this</code></li>
                   </ul>
                </div>
                <div class="subsect depth4" id="s5.4.1.d">
                   <h4 class="subsect">(d)&nbsp;<span class="title">Team level guards</span><a class="img" href="s5.4.1.d.html"
                         title="PermaLink to (d)&nbsp;Team level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>A guard specified in the header of a team class may disable the callin
                      			bindings of all contained role classes. The syntax corresponds to the syntax
                      			of role level guards.<br />
                      			The only value directly available within team level guard is the
                      			team instance (denoted by <code>this</code>) and its features.

                   </p>
                </div>
                <p>Of course all guards can also access any visible static feature of a visible class.

                </p>
                <p>Even if a guard has no direct effect, because, e.g., a role class has no callin
                   		bindings (maybe not even a role-base binding), predicates at such abstract
                   		levels are useful, because all predicates are inherited by all sub classes
                   		(explicit and implicit).

                </p>
             </div>
             <div class="sect depth3" id="s5.4.2">
                <h3 class="sect">&sect;5.4.2&nbsp;Base guards<a class="img" href="s5.4.2.html"
                      title="PermaLink to &sect;5.4.2&nbsp;Base guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a><span class="toplink"><a href="#top">&uarr;&nbsp;&sect;5.4</a></span></h3>
                <p>The intention behind base guards is to prevent lifting of a callin-target
                   		if a guard evaluates to <code>false</code> and thus refuses to invoke the
                   		callin bound role method. Using base guards it is easier to prevent any
                   		side-effects caused by a callin binding, because lifting could cause side-effects
                   		at two levels:

                </p>
                <ul>
                   <li>Creating a role on-demand already is a side-effect (observable e.g.
                      			by the reflective function <code><a href="s6.1.html" title="&sect;6.1&nbsp;Reflection" class="sect">hasRole (&sect;6.1)</a></code>)

                   </li>
                   <li>Role creation triggers execution of a role constructor
                      			(see <a href="s2.3.1.c.html"
                         title="&sect;2.3.1.(c)&nbsp;Custom lifting constructor"
                         class="sect">custom lifting constructor (&sect;2.3.1.(c))</a>)
                      			which could produce arbitrary side-effects.

                   </li>
                </ul>
                <p>Both kinds of side-effects can be avoided using a base guard which prevents
                   		unnecessary lifting.

                </p>
                <p>Any guard (5.4.1 (b)-(e)) can be turned into a base guard by adding
                   		the modifier <code>base</code> as in:

                </p>
                <div class="listing plain"><pre><b>protected</b> <b>class</b> MyRole <b>playedBy</b> MyBase
   <em><b>base</b> when</em> (base.value &gt; MyTeam.this.threshold)
 {
     <i><b>class</b> body declarations</i>
 }</pre></div>
                <p>However, different scoping rules apply for the identifiers
                   		that can be used in a base guard:

                </p>
                <div class="subsect depth4" id="s5.4.2.a">
                   <h4 class="subsect">(a)&nbsp;<span class="title">Base object reference</span><a class="img" href="s5.4.2.a.html"
                         title="PermaLink to (a)&nbsp;Base object reference"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>In all base guard predicates the special identifier <code>base</code>
                      			can be used to denote the base object that is about to be lifted.

                   </p>
                </div>
                <div class="subsect depth4" id="s5.4.2.b">
                   <h4 class="subsect">(b)&nbsp;<span class="title">Method binding guards</span><a class="img" href="s5.4.2.b.html"
                         title="PermaLink to (b)&nbsp;Method binding guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>A base method binding guard may access parameters as passed to the
                      			base method. Parameter mappings are not considered.<br />
                      Additionally, for <code>after</code> callin bindings, the identifier <code>result</code>
                      may be used to refer to the result of the base method (if any).

                   </p>
                   <div class="note">
                      <h5>Note:</h5>
                      <p>In order to achieve the same effect of accessing
                         	the base method's result, a regular binding guard (not a base guard)
                         	must use a suitable parameter mapping (see <a href="s4.4.c.html"
                            title="&sect;4.4.(c)&nbsp;Mapping the result of a base method"
                            class="sect">&sect;4.4.(c)</a>).

                      </p>
                   </div>
                </div>
                <div class="subsect depth4" id="s5.4.2.c">
                   <h4 class="subsect">(c)&nbsp;<span class="title">Method guards</span><a class="img" href="s5.4.2.c.html" title="PermaLink to (c)&nbsp;Method guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>In contrast to regular method guards, a <em>base</em> guard attached to a role method
                      			cannot access any method parameters. See the next item (d) for values that are actually in scope.

                   </p>
                </div>
                <div class="subsect depth4" id="s5.4.2.d">
                   <h4 class="subsect">(d)&nbsp;<span class="title">Role level guards</span><a class="img" href="s5.4.2.d.html"
                         title="PermaLink to (d)&nbsp;Role level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>Role level base guards may use these values:

                   </p>
                   <ul>
                      <li>The base instance using the special identifier <code>base</code>.
                      </li>
                      <li>The team instance using a qualified this references (<code>MyTeam.this</code>).
                      </li>
                   </ul>
                </div>
                <div class="subsect depth4" id="s5.4.2.e">
                   <h4 class="subsect">(e)&nbsp;<span class="title">Team level guards</span><a class="img" href="s5.4.2.e.html"
                         title="PermaLink to (e)&nbsp;Team level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>Team level base guards have the same scope as role level base guards (d).
                      			However, the type of the role instance is not known here, i.e., here <code>base</code>
                      			has the static type <code>java.lang.Object</code>.

                   </p>
                </div>
                <div class="subsect depth4" id="s5.4.2.f">
                   <h4 class="subsect">(f)&nbsp;<span class="title">Unbound roles</span><a class="img" href="s5.4.2.f.html" title="PermaLink to (f)&nbsp;Unbound roles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                              alt="" /></a></h4>
                   <p>In contrast to regular guards, base guards cannot be attached to
                      	unbound role classes nor to their methods.<br />
                      	Only team level base guards are independent of role binding.

                   </p>
                </div><img src="../images/guards.png" alt="Overview: Guard predicates" /></div>
             <div class="sect depth3" id="s5.4.3">
                <h3 class="sect">&sect;5.4.3&nbsp;Multiple guards<a class="img" href="s5.4.3.html"
                      title="PermaLink to &sect;5.4.3&nbsp;Multiple guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a><span class="toplink"><a href="#top">&uarr;&nbsp;&sect;5.4</a></span></h3>
                <p>Due to the different ranges of applicability different guards may affect the same method binding.
                   		In that case all applicable guards are conjoined using a logical <code>and</code>.<br />
                   Any guard is interpreted as the conjunction of these predicates (if present):

                </p>
                <ul>
                   <li>The direct predicate expression of the guard.</li>
                   <li>The next outer guard along the chain
                      			<em>method binding -&gt; method -&gt; role level -&gt; team level</em></li>
                   <li>The guard at the same level that is inherited from the <em>implicit</em> super role.
                   </li>
                   <li>The guard at the same level that is inherited from the <em>explicit</em> super role.
                   </li>
                </ul>
                <h5 class="listing">Example code (Guard Predicates):</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> ATM {</pre></td>
                      </tr>
                      <tr class="line even">
                         <td class="ln">2</td>
                         <td><pre>  <b>private</b> Bank myBank;</pre></td>
                      </tr>
                      <tr class="line odd">
                         <td class="ln">3</td>
                         <td><pre>  <b>public</b> <b>class</b> ForeignAccount <b>playedBy</b> Account</pre></td>
                      </tr>
                      <tr class="line even">
                         <td class="ln">4</td>
                         <td><pre>    <em><b>base</b> when</em> (!ATM.this.myBank.equals(<em>base</em>.getBank()))</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>    <b>callin</b> <b>void</b> debitWithFee(<b>int</b> amount) {</pre></td>
                      </tr>
                      <tr class="line odd">
                         <td class="ln">7</td>
                         <td><pre>      base.debitWithFee(fee+amount);</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>    <b>void</b> debitWithFee(<b>int</b> i) <b>&lt;-</b> <b>replace</b> <b>void</b> debit(<b>int</b> amount)</pre></td>
                      </tr>
                      <tr class="line even">
                         <td class="ln">10</td>
                         <td><pre>      <em><b>base</b> when</em> (amount &lt; 1000);</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>
                   </table>
                </div>
                <div class="codecomment">
                   <h5>Effects:</h5>
                   				The team in this example causes that an additional fee has to be payed while debiting
                   				less than 1000 Euros from a "foreign" account.

                   <ul>
                      <li>The base guard in line 4 ensures that <code>Account</code> objects only get
                         				<code>ForeignAccount</code> roles, if they belong to a different bank than the
                         				surrounding <code>ATM</code> team.<br />
                         				It accesses the bank of the base via the <code>base</code> identifier.

                      </li>
                      <li>The method binding guard in line 10 restricts the callin to
                         				<code>debitWithFee</code> to calls where the base method argument
                         				<code>amount</code> is lower than 1000.

                      </li>
                      <li>A call to <code>Account.debit</code> causes a replace callin to
                         				<code>debitWithFee</code><em> only</em> if <em>both</em> predicates evaluate to true.

                      </li>
                   </ul>
                </div>
             </div>
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          			OT/J version 1.3 &mdash; last modified: 2011-05-15
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	<div class="sect depth2" id="s5.4">
	<h2 class="sect">§5.4 Guard predicates<a class="img" href="s5.4.html"
	title="PermaLink to §5.4 Guard predicates"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h2>
	<div class="syntaxlink"><a href="sA.7.html" title="§A.7 Guard predicates" class="syntax">→ Syntax §A.7</a></div>
	<p>The effect of callins can further be controlled using so called guard predicates.
	Guards appear at four different levels:

	</p>
	<ul>
	<li>callin method binding</li>
	<li>role method</li>
	<li>role class</li>
	<li>team class</li>
	</ul>
	<p>Guards can be specified as <em>regular</em> guards or <code>base</code> guards,
	which affects the exact point in the control flow, where the guard will be evaluated.

	</p>
	<div class="subsect depth3" id="s5.4.a">
	<h4 class="subsect">(a) <span class="title">General syntax for guards</span><a class="img" href="s5.4.a.html"
	title="PermaLink to (a) General syntax for guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A guard is declared using the keyword <code>when</code> followed by a
	boolean expression in parentheses:

	</p>
	<div class="listing plain"><pre><em><b>when</b> (</em><i>predicateExpression</i><em>)</em></pre></div>
	<p>Depending on the kind of guard different objects are in scope using
	special identifiers like <code>this</code>, <code>base</code>.<br />
	Any predicate expression that evaluates to <code>true</code> enables
	the callin binding(s) to which it applies.
	Evaluation to <code>false</code> disables the callin binding(s).

	</p>
	</div>
	<div class="subsect depth3" id="s5.4.b">
	<h4 class="subsect">(b) <span class="title">No side effects</span><a class="img" href="s5.4.b.html" title="PermaLink to (b) No side effects"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A guard predicate should have no side effects.
	A compiler should optionally check this condition, but inter-procedural analysis
	actually depends on the availability of appropriate means to mark any method as side-effect free.

	</p>
	</div>
	<div class="subsect depth3" id="s5.4.c">
	<h4 class="subsect">(c) <span class="title">Exceptions</span><a class="img" href="s5.4.c.html" title="PermaLink to (c) Exceptions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A guard predicate should not throw any exceptions.
	Yet, any exception thrown within a guard predicate cause the guard to evaluate to <code>false</code>
	rather than propagating the exception, meaning that the evaluation of a guard predicate will never
	interrupt the current base behaviour.<br />
	A compiler should flag any checked exception that is thrown within a guard.
	Such diagnosis should by default be treated as an error, with the option of configuring
	its severity to warning or ignore.

	</p>
	</div>
	<div class="sect depth3" id="s5.4.1">
	<h3 class="sect">§5.4.1 Regular guards<a class="img" href="s5.4.1.html"
	title="PermaLink to §5.4.1 Regular guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#top">↑ §5.4</a></span></h3>
	<p>This group of guards evaluates within the context of a given role.
	These guards are evaluated <em>after</em> a callin target is lifted and
	<em>before</em> a callin bound role method is invoked.

	</p>
	<div class="subsect depth4" id="s5.4.1.a">
	<h4 class="subsect">(a) <span class="title">Method binding guards</span><a class="img" href="s5.4.1.a.html"
	title="PermaLink to (a) Method binding guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A guard may be attached to a callin method binding as in:

	</p>
	<div class="listing plain"><pre><b>void</b> roleMethod(<b>int</b> ir) <b><-</b> <b>after</b> <b>void</b> baseMethod(<b>int</b> ib)
	<b>when</b> (ir > MyTeam.this.threshold);</pre></div>
	<p>Such a guard only affects the callin binding to which it is attached,
	i.e., this specific callin binding is only effective,
	if the predicate evaluates to <code>true</code>.
	<br />
	The following values are within the scope of the predicate expression,
	and thus can be used to express the condition:

	</p>
	<ul>
	<li>The role instance denoted by <code>this</code>.<br />
	Features of the role instance can also be accessed relative to
	<code>this</code> with or without explicit qualifying <code>this</code>.

	</li>
	<li>The team instance denoted by a qualified this reference as in
	<code>MyTeam.this</code>.

	</li>
	<li>If the callin binding includes signatures (as in the example above):
	Parameters of the role method.<br />
	If parameter mappings are involved, they will be evaluated before evaluating the guard.

	</li>
	</ul>
	</div>
	<div class="subsect depth4" id="s5.4.1.b">
	<h4 class="subsect">(b) <span class="title">Method guards</span><a class="img" href="s5.4.1.b.html" title="PermaLink to (b) Method guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A method guard is similar to a method binding guard, but it applies
	to all callin method bindings of this method.<br />
	A method guard is declared between the method signature and the method body:

	</p>
	<div class="listing plain"><pre><b>void</b> roleMethod(<b>int</b> ir)
	<b>when</b> (ir > MyTeam.this.threshold) { <i>body statements</i> }</pre></div>
	</div>
	<div class="subsect depth4" id="s5.4.1.c">
	<h4 class="subsect">(c) <span class="title">Role level guards</span><a class="img" href="s5.4.1.c.html"
	title="PermaLink to (c) Role level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>When a guard is specified at the role level, i.e., directly before the
	class body of a role class, it applies to all callin method bindings of
	the role class:

	</p>
	<div class="listing plain"><pre><b>protected</b> <b>class</b> MyRole
	<b>when</b> (value > MyTeam.this.threshold)
	{
	<b>int</b> value;
	<i>other <b>class</b> body declarations</i>
	}</pre></div>
	<p>The following values are within the scope of the predicate expression:

	</p>
	<ul>
	<li>The role instance denoted by <code>this</code> (explicit or implicit, see above).
	Thus, in the example <code>value</code> will be interpreted as a field of the enclosing role.

	</li>
	<li>The team instance denoted by a qualified this reference as in <code>MyTeam.this</code></li>
	</ul>
	</div>
	<div class="subsect depth4" id="s5.4.1.d">
	<h4 class="subsect">(d) <span class="title">Team level guards</span><a class="img" href="s5.4.1.d.html"
	title="PermaLink to (d) Team level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A guard specified in the header of a team class may disable the callin
	bindings of all contained role classes. The syntax corresponds to the syntax
	of role level guards.<br />
	The only value directly available within team level guard is the
	team instance (denoted by <code>this</code>) and its features.

	</p>
	</div>
	<p>Of course all guards can also access any visible static feature of a visible class.

	</p>
	<p>Even if a guard has no direct effect, because, e.g., a role class has no callin
	bindings (maybe not even a role-base binding), predicates at such abstract
	levels are useful, because all predicates are inherited by all sub classes
	(explicit and implicit).

	</p>
	</div>
	<div class="sect depth3" id="s5.4.2">
	<h3 class="sect">§5.4.2 Base guards<a class="img" href="s5.4.2.html"
	title="PermaLink to §5.4.2 Base guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#top">↑ §5.4</a></span></h3>
	<p>The intention behind base guards is to prevent lifting of a callin-target
	if a guard evaluates to <code>false</code> and thus refuses to invoke the
	callin bound role method. Using base guards it is easier to prevent any
	side-effects caused by a callin binding, because lifting could cause side-effects
	at two levels:

	</p>
	<ul>
	<li>Creating a role on-demand already is a side-effect (observable e.g.
	by the reflective function <code><a href="s6.1.html" title="§6.1 Reflection" class="sect">hasRole (§6.1)</a></code>)

	</li>
	<li>Role creation triggers execution of a role constructor
	(see <a href="s2.3.1.c.html"
	title="§2.3.1.(c) Custom lifting constructor"
	class="sect">custom lifting constructor (§2.3.1.(c))</a>)
	which could produce arbitrary side-effects.

	</li>
	</ul>
	<p>Both kinds of side-effects can be avoided using a base guard which prevents
	unnecessary lifting.

	</p>
	<p>Any guard (5.4.1 (b)-(e)) can be turned into a base guard by adding
	the modifier <code>base</code> as in:

	</p>
	<div class="listing plain"><pre><b>protected</b> <b>class</b> MyRole <b>playedBy</b> MyBase
	<em><b>base</b> when</em> (base.value > MyTeam.this.threshold)
	{
	<i><b>class</b> body declarations</i>
	}</pre></div>
	<p>However, different scoping rules apply for the identifiers
	that can be used in a base guard:

	</p>
	<div class="subsect depth4" id="s5.4.2.a">
	<h4 class="subsect">(a) <span class="title">Base object reference</span><a class="img" href="s5.4.2.a.html"
	title="PermaLink to (a) Base object reference"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>In all base guard predicates the special identifier <code>base</code>
	can be used to denote the base object that is about to be lifted.

	</p>
	</div>
	<div class="subsect depth4" id="s5.4.2.b">
	<h4 class="subsect">(b) <span class="title">Method binding guards</span><a class="img" href="s5.4.2.b.html"
	title="PermaLink to (b) Method binding guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A base method binding guard may access parameters as passed to the
	base method. Parameter mappings are not considered.<br />
	Additionally, for <code>after</code> callin bindings, the identifier <code>result</code>
	may be used to refer to the result of the base method (if any).

	</p>
	<div class="note">
	<h5>Note:</h5>
	<p>In order to achieve the same effect of accessing
	the base method's result, a regular binding guard (not a base guard)
	must use a suitable parameter mapping (see <a href="s4.4.c.html"
	title="§4.4.(c) Mapping the result of a base method"
	class="sect">§4.4.(c)</a>).

	</p>
	</div>
	</div>
	<div class="subsect depth4" id="s5.4.2.c">
	<h4 class="subsect">(c) <span class="title">Method guards</span><a class="img" href="s5.4.2.c.html" title="PermaLink to (c) Method guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>In contrast to regular method guards, a <em>base</em> guard attached to a role method
	cannot access any method parameters. See the next item (d) for values that are actually in scope.

	</p>
	</div>
	<div class="subsect depth4" id="s5.4.2.d">
	<h4 class="subsect">(d) <span class="title">Role level guards</span><a class="img" href="s5.4.2.d.html"
	title="PermaLink to (d) Role level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Role level base guards may use these values:

	</p>
	<ul>
	<li>The base instance using the special identifier <code>base</code>.
	</li>
	<li>The team instance using a qualified this references (<code>MyTeam.this</code>).
	</li>
	</ul>
	</div>
	<div class="subsect depth4" id="s5.4.2.e">
	<h4 class="subsect">(e) <span class="title">Team level guards</span><a class="img" href="s5.4.2.e.html"
	title="PermaLink to (e) Team level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Team level base guards have the same scope as role level base guards (d).
	However, the type of the role instance is not known here, i.e., here <code>base</code>
	has the static type <code>java.lang.Object</code>.

	</p>
	</div>
	<div class="subsect depth4" id="s5.4.2.f">
	<h4 class="subsect">(f) <span class="title">Unbound roles</span><a class="img" href="s5.4.2.f.html" title="PermaLink to (f) Unbound roles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>In contrast to regular guards, base guards cannot be attached to
	unbound role classes nor to their methods.<br />
	Only team level base guards are independent of role binding.

	</p>
	</div><img src="../images/guards.png" alt="Overview: Guard predicates" /></div>
	<div class="sect depth3" id="s5.4.3">
	<h3 class="sect">§5.4.3 Multiple guards<a class="img" href="s5.4.3.html"
	title="PermaLink to §5.4.3 Multiple guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a><span class="toplink"><a href="#top">↑ §5.4</a></span></h3>
	<p>Due to the different ranges of applicability different guards may affect the same method binding.
	In that case all applicable guards are conjoined using a logical <code>and</code>.<br />
	Any guard is interpreted as the conjunction of these predicates (if present):

	</p>
	<ul>
	<li>The direct predicate expression of the guard.</li>
	<li>The next outer guard along the chain
	<em>method binding -> method -> role level -> team level</em></li>
	<li>The guard at the same level that is inherited from the <em>implicit</em> super role.
	</li>
	<li>The guard at the same level that is inherited from the <em>explicit</em> super role.
	</li>
	</ul>
	<h5 class="listing">Example code (Guard Predicates):</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> ATM {</pre></td>
	</tr>
	<tr class="line even">
	<td class="ln">2</td>
	<td><pre> <b>private</b> Bank myBank;</pre></td>
	</tr>
	<tr class="line odd">
	<td class="ln">3</td>
	<td><pre> <b>public</b> <b>class</b> ForeignAccount <b>playedBy</b> Account</pre></td>
	</tr>
	<tr class="line even">
	<td class="ln">4</td>
	<td><pre> <em><b>base</b> when</em> (!ATM.this.myBank.equals(<em>base</em>.getBank()))</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> <b>callin</b> <b>void</b> debitWithFee(<b>int</b> amount) {</pre></td>
	</tr>
	<tr class="line odd">
	<td class="ln">7</td>
	<td><pre> base.debitWithFee(fee+amount);</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> <b>void</b> debitWithFee(<b>int</b> i) <b><-</b> <b>replace</b> <b>void</b> debit(<b>int</b> amount)</pre></td>
	</tr>
	<tr class="line even">
	<td class="ln">10</td>
	<td><pre> <em><b>base</b> when</em> (amount < 1000);</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>
	</table>
	</div>
	<div class="codecomment">
	<h5>Effects:</h5>
	The team in this example causes that an additional fee has to be payed while debiting
	less than 1000 Euros from a "foreign" account.

	<ul>
	<li>The base guard in line 4 ensures that <code>Account</code> objects only get
	<code>ForeignAccount</code> roles, if they belong to a different bank than the
	surrounding <code>ATM</code> team.<br />
	It accesses the bank of the base via the <code>base</code> identifier.

	</li>
	<li>The method binding guard in line 10 restricts the callin to
	<code>debitWithFee</code> to calls where the base method argument
	<code>amount</code> is lower than 1000.

	</li>
	<li>A call to <code>Account.debit</code> causes a replace callin to
	<code>debitWithFee</code><em> only</em> if <em>both</em> predicates evaluate to true.

	</li>
	</ul>
	</div>
	</div>
	</div>
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