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Text in Composite pages, (URLs starting with 'routine' or 'syntax') can be edited by clicking on the link labeled 'in context', and editing the text there.</p> <p>Exit this popup by pressing <Escape>, or clicking on X or on the background.</p> </div> </div> <button class="modal-close is-large" aria-label="close"></button> </div> <div id="left-column" class="tile is-parent is-2 is-hidden"> <div id="left-col-inner"> <div class="tabs" id="tabs"> <ul> <li class="is-active" id="toc-tab"> <a>Table of Contents</a> </li> <li id="index-tab"> <a>Index</a> </li> </ul> </div> <div class="field"> <div class="control has-icons-right"> <input id="toc-filter" class="input" type="text" placeholder="Filter"> <span class="icon is-right has-text-grey"> <i class="fas fa-search is-medium"></i> </span> </div> </div> <div class="raku-sidebar"> <aside id="toc-menu" class="menu"> <ul class="menu-list"> <li><a href="#(Operators)_infix_andthen">In Operators</a></li> </ul> </aside> <aside id="index-menu" class="menu is-hidden"> </aside> </div> </div> </div> <div id="main-column" class="tile is-parent" style="overflow-x: hidden;"> <div id="main-col-inner"> <section class="raku page-header"> <div class="container px-4"> <div class="raku page-title has-text-centered"> andthen </div> <div class="raku page-subtitle has-text-centered"> Combined from primary sources listed below. </div> </div> </section> <section class="raku page-content"><div class="container px-4"><div class="columns one-col"> <h1 id="(Operators)_infix_andthen" class="raku-h1"><a href="#___top" title="go to top of document">In Operators<a class="raku-anchor" title="direct link" href="#(Operators)_infix_andthen">§</a></a></h1> <p> See primary documentation <a href="/language/operators#infix_andthen">in context</a> for <b>infix andthen</b>. </p><p>The <code>andthen</code> operator returns <a href="/type/Slip#constant_Empty"><code>Empty</code></a> upon encountering the first <a href="/routine/defined">undefined</a> argument, otherwise the last argument. Last argument is returned as-is, without being checked for definedness at all. Short-circuits. The result of the left side is bound to <code>$_</code> for the right side, or passed as arguments if the right side is a <a href="/type/Callable"><code>Callable</code></a>, whose <a href="/routine/count">count</a> must be <code>0</code> or <code>1</code>.</p><p>A handy use of this operator is to alias a routine's return value to <code>$_</code> and to do additional manipulation with it, such as printing or returning it to caller. Since the <code>andthen</code> operator short-circuits, statements on the right-hand side won't get executed, unless left-hand side is defined (tip: <a href="/type/Failure"><code>Failure</code></a>s are never defined, so you can handle them with this operator).</p> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"><i class="far fa-clipboard"></i></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"><span class="highlite-KEYWORD">sub</span> load-data { <span class="highlite-ROUTINE">rand</span> <span class="highlite-OPERATOR">></span> .5 <span class="highlite-OPERATOR">or</span> <span class="highlite-KEYWORD">return</span>; <span class="highlite-COMMENT"># simulated load data failure; return Nil </span> (<span class="highlite-ROUTINE">rand</span> <span class="highlite-OPERATOR">></span> .3 <span class="highlite-OPERATOR">??</span> <span class="highlite-STRING_DELIMITER">'</span><span class="highlite-STRING">error</span><span class="highlite-STRING_DELIMITER">'</span> <span class="highlite-OPERATOR">!!</span> <span class="highlite-STRING_DELIMITER">'</span><span class="highlite-STRING">good data</span><span class="highlite-STRING_DELIMITER">'</span>) <span class="highlite-OPERATOR">xx</span> 10 <span class="highlite-COMMENT"># our loaded data </span>} load-data.first: <span class="highlite-REGEX_DELIMITER">/</span><span class="highlite-REGEX_LITERAL">good</span><span class="highlite-REGEX_DELIMITER">/</span> <span class="highlite-OPERATOR">andthen</span> <span class="highlite-ROUTINE">say</span> <span class="highlite-STRING_DELIMITER">"</span><span class="highlite-NAME_SCALAR">$_</span><span class="highlite-STRING"> is good</span><span class="highlite-STRING_DELIMITER">"</span>; <span class="highlite-COMMENT"># OUTPUT: «(good data is good)» </span> load-data() <span class="highlite-OPERATOR">andthen</span> .<span class="highlite-KEYWORD">return</span>; <span class="highlite-COMMENT"># return loaded data, if it's defined </span><span class="highlite-ROUTINE">die</span> <span class="highlite-STRING_DELIMITER">"</span><span class="highlite-STRING">Failed to load data!!</span><span class="highlite-STRING_DELIMITER">"</span>;</pre> </div> </div> <p>The above example will print <code>good data is good</code> only if the subroutine returned any items that match <code>/good/</code> and will die unless loading data returned a defined value. The aliasing behavior lets us pipe the values across the operator.</p><p>The <code>andthen</code> operator is a close relative of the <a href="/language/control#with_orwith_without"><code>with</code> statement modifier</a>, and some compilers compile <code>with</code> to <code>andthen</code>, meaning these two lines have equivalent behavior:</p> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"><i class="far fa-clipboard"></i></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">.<span class="highlite-ROUTINE">say</span> <span class="highlite-KEYWORD">with</span> 42; 42 <span class="highlite-OPERATOR">andthen</span> .<span class="highlite-ROUTINE">say</span>;</pre> </div> </div></div></div></section> </div> </div> </div> <footer class="footer main-footer"> <div class="container px-4"> <nav class="level"> <div class="level-left"> <div class="level-item"> <a href="/about">About</a> </div> <div class="level-item"> <a id="toggle-theme">Toggle theme</a> </div> </div> <div class="level-right"> <div class="level-item"> <a href="/license">License</a> </div> </div> </nav> </div> </footer> </body> </html>