wiki/GEP-4.html

<!DOCTYPE html>     <html class="no-js"> <head> <meta charset='utf-8'/><meta http-equiv='X-UA-Compatible' content='IE=edge'/><meta name='viewport' content='width=device-width, initial-scale=1'/><title>The Apache Groovy programming language - Developer docs - GEP-4</title><link href='../img/favicon.ico' type='image/x-ico' rel='icon'/><script src='../js/matomo.js'></script><link rel='stylesheet' type='text/css' href='../css/bootstrap.css'/><link rel='stylesheet' type='text/css' href='../css/font-awesome.min.css'/><link rel='stylesheet' type='text/css' href='../css/style.css'/><link rel='stylesheet' type='text/css' href='../css/../css/prettify.min.css'/> </head><body> <div id='fork-me'> <a href='https://github.com/apache/groovy'> <img style='position: fixed; top: 20px; right: -58px; border: 0; z-index: 100; transform: rotate(45deg);' src='../img/horizontal-github-ribbon.png'/> </a> </div><div id='st-container' class='st-container st-effect-9'> <nav class='st-menu st-effect-9' 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</ul> </nav><div class='st-pusher'> <div class='st-content'> <div class='st-content-inner'> <div><div class='navbar navbar-default navbar-static-top' role='navigation'> <div class='container'> <div class='navbar-header'> <button type='button' class='navbar-toggle' data-toggle='collapse' data-target='.navbar-collapse'> <span class='sr-only'></span><span class='icon-bar'></span><span class='icon-bar'></span><span class='icon-bar'></span> </button><a class='navbar-brand' href='../index.html'> <i class='fa fa-star'></i> Apache Groovy™ </a> </div><div class='navbar-collapse collapse'> <ul class='nav navbar-nav navbar-right'> <li class=''><a href='https://groovy-lang.org/learn.html'>Learn</a></li><li class=''><a href='https://groovy-lang.org/documentation.html'>Documentation</a></li><li class=''><a href='/download.html'>Download</a></li><li class=''><a href='https://groovy-lang.org/support.html'>Support</a></li><li class=''><a href='/'>Contribute</a></li><li class=''><a 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class="content"> <div class="title">Metadata</div> <div class="hdlist"> <table> <tr> <td class="hdlist1"> <strong>Number</strong> </td> <td class="hdlist2"> <p>GEP-4</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Title</strong> </td> <td class="hdlist2"> <p>AstBuilder AST Templates</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Version</strong> </td> <td class="hdlist2"> <p>2</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Type</strong> </td> <td class="hdlist2"> <p>Feature</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Status</strong> </td> <td class="hdlist2"> <p>Rejected</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Comment</strong> </td> <td class="hdlist2"> <p>The approach used in Groovy macros was pursued instead</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Leader</strong> </td> <td class="hdlist2"> <p>Hamlet D’Arcy</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Created</strong> </td> <td class="hdlist2"> <p>2010-02-16</p> </td> </tr> <tr> <td class="hdlist1"> <strong>Last modification</strong>  </td> <td class="hdlist2"> <p>2018-10-12</p> </td> </tr> </table> </div> </div> </div> </div> </div> <div class="sect1"> <h2 id="_abstract">Abstract</h2> <div class="sectionbody"> <div class="paragraph"> <p>The AstBuilder was introduced in Groovy 1.7 as a way to ease the task of writing AST transformations. The "fromCode" approach allows you to generate the AST from a piece of code. Currently, the AstBuilder fromCode API accepts a closure as a parameter, and the code within the closure it transformed to AST. A limitation of the AstBuilder is that the closure parameter does not allow variables in the surrounding context to be referenced: there is no way to pass a parameter into the code block. It was initially discussed but the idea was deemed too much effort to fit into 1.7.</p> </div> <div class="paragraph"> <p>This AST Template feature allows you to pass parameters into the AstBuilder. Similar to how a $ works in GString interpolation, there needs to be a way to bind a variable from the enclosing scope into an AstBuilder closure. The syntax proposed is to use oxford brackets (see below). An AstBuilder closure containing GEP 4 - AstBuilder AST Templates will have the variable within the brackets bound in.</p> </div> </div> </div> <div class="sect1"> <h2 id="_approach">Approach</h2> <div class="sectionbody"> <div class="paragraph"> <p>The oxford brackets are used to bind, or quasi-quote, variables from the enclosing scope into an AstBuilder parameter.</p> </div> <div class="paragraph"> <p>Example 1: String concatenation</p> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight"><code>def constant = new ConstantExpression("World") List<ASTNode> greeting = AstBuilder.buildFromCode { "Hello" + [ | constant | ] }</code></pre> </div> </div> <div class="paragraph"> <p>Produces the AST</p> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight"><code>BlockStatement -> ExpressionStatement -> BinaryExpression + -> ConstantExpresssion - Hello -> ConstantExpression - World</code></pre> </div> </div> <div class="paragraph"> <p>Example 2: Producing a println</p> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight"><code>new AstBuilder().buildFromCode { println([ | message | ]) }[0]</code></pre> </div> </div> <div class="paragraph"> <p>is equivalent to invoking this method:</p> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight"><code>private Statement createPrintlnAst(String message) { return new ExpressionStatement( new MethodCallExpression( new VariableExpression("this"), new ConstantExpression("println"), new ArgumentListExpression( new ConstantExpression(message) ) ) ) }</code></pre> </div> </div> <div class="paragraph"> <p>Example 3: Memoization of a method</p> </div> <div class="paragraph"> <p>Presumably, you would have the existing method AST in a variable called "methodNode":</p> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight"><code>def methodNode = ... def parameters = methodNode.parameters def newMethod = new AstBuilder().buildFromCode { if (cache.contains([ | parameters | ])) { return cache.get([ | parameters | ]) } def result = [ | methodNode.code | ] cache.put([ | parameters | ]) return result } methodNode.code = newMethod[0]</code></pre> </div> </div> <div class="sect2"> <h3 id="_alternatives">Alternatives</h3> <div class="paragraph"> <p>There are many syntax alternatives:</p> </div> <div class="ulist"> <ul> <li> <p>Lisp uses ´ and ,</p> </li> <li> <p>Template Haskell uses something similar to oxford brackets but with slightly different semantics</p> </li> <li> <p>Boo uses GEP 4 - AstBuilder AST Templates and $ but with slightly different semantics</p> </li> </ul> </div> <div class="paragraph"> <p>A $ was considered but cannot be used as it is already a meaningful identifier. Boo Metamethod Comparison The Boo Metamethod feature overlaps with the Groovy AST Transformation features. Instead of an @AstTransformation interface with a separately defined AstTransformation subclass, Boo offers Meta methods, where the AST Transformation is called and generated at compile time.</p> </div> <div class="paragraph"> <p>While Groovy offers AstBuilder to turn code into AST, Boo uses the oxford brackets. And while GEP-4 proposed oxford brackets to signify an AST variable templating, Boo uses the $. Consider a simple println AST in Groovy</p> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight"><code>new AstBuilder().buildFromCode { println([ | message | ]) }[0]</code></pre> </div> </div> <div class="paragraph"> <p>Compared to a similar construct in Boo:</p> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight"><code>[Meta] static def methodname(expr as Expression): [| println($expr) |]</code></pre> </div> </div> <div class="paragraph"> <p>GEP-4 proposes the opposite syntax from Boo: oxford brackets for AST Templating instead of the $. The $ is already a character in Java, and can be part of a variable name.</p> </div> <div class="paragraph"> <p>There are no real drawbacks to having an opposite syntax. The number of Boo AST Transformation writers migrating to Groovy is most likely quite low.</p> </div> </div> </div> </div> <div class="sect1"> <h2 id="_references_and_useful_links">References and useful links</h2> <div class="sectionbody"> <div class="paragraph"> <p>Mailing List Discussions</p> </div> <div class="ulist"> <ul> <li> <p>groovy-dev: <a href="https://marc.info/?l=groovy-dev&m=126632536126812&w=2">Groovy AstBuilder AST Templates</a></p> </li> </ul> </div> </div> </div> <div class="sect1"> <h2 id="_update_history">Update history</h2> <div class="sectionbody"> <div class="dlist"> <dl> <dt class="hdlist1">1 (2010-02-16)</dt> <dd> <p>Version as extracted from Codehaus wiki</p> </dd> <dt class="hdlist1">2 (2018-10-14)</dt> <dd> <p>Numerous minor tweaks</p> </dd> </dl> </div> </div> </div></div></div></div></div><footer id='footer'> <div class='row'> <div class='colset-3-footer'> <div class='col-1'> <h1>Groovy</h1><ul> <li><a href='https://groovy-lang.org/learn.html'>Learn</a></li><li><a href='https://groovy-lang.org/documentation.html'>Documentation</a></li><li><a href='/download.html'>Download</a></li><li><a href='https://groovy-lang.org/support.html'>Support</a></li><li><a href='/'>Contribute</a></li><li><a href='https://groovy-lang.org/ecosystem.html'>Ecosystem</a></li><li><a href='/blog'>Blog posts</a></li><li><a href='https://groovy.apache.org/events.html'></a></li> </ul> </div><div class='col-2'> <h1>About</h1><ul> <li><a href='https://github.com/apache/groovy'>Source code</a></li><li><a href='https://groovy-lang.org/security.html'>Security</a></li><li><a href='https://groovy-lang.org/learn.html#books'>Books</a></li><li><a href='https://groovy-lang.org/thanks.html'>Thanks</a></li><li><a href='http://www.apache.org/foundation/sponsorship.html'>Sponsorship</a></li><li><a href='https://groovy-lang.org/faq.html'>FAQ</a></li><li><a href='https://groovy-lang.org/search.html'>Search</a></li> </ul> </div><div class='col-3'> <h1>Socialize</h1><ul> <li><a href='https://groovy-lang.org/mailing-lists.html'>Discuss on the mailing-list</a></li><li><a href='https://twitter.com/ApacheGroovy'>Groovy on Twitter</a></li><li><a href='https://groovy-lang.org/events.html'>Events and conferences</a></li><li><a href='https://github.com/apache/groovy'>Source code on GitHub</a></li><li><a href='https://groovy-lang.org/reporting-issues.html'>Report issues in Jira</a></li><li><a href='http://stackoverflow.com/questions/tagged/groovy'>Stack Overflow questions</a></li><li><a href='http://www.groovycommunity.com/'>Slack Community</a></li> </ul> </div><div class='col-right'> <p> The Groovy programming language is supported by the <a href='http://www.apache.org'>Apache Software Foundation</a> and the Groovy community. </p><div text-align='right'> <img src='../img/asf_logo.png' title='The Apache Software Foundation' alt='The Apache Software Foundation' style='width:60%'/> </div><p>Apache, Apache Groovy, Groovy, and the Apache feather logo are either registered trademarks or trademarks of The Apache Software Foundation.</p> </div> </div><div class='clearfix'>© 2003-2025 the Apache Groovy project — Groovy is Open Source: <a href='http://www.apache.org/licenses/LICENSE-2.0.html' alt='Apache 2 License'>license</a>, <a href='https://privacy.apache.org/policies/privacy-policy-public.html'>privacy policy</a>.</div> </div> </footer></div> </div> </div> </div> </div><script src='../js/vendor/jquery-1.10.2.min.js' defer></script><script src='../js/vendor/classie.js' defer></script><script src='../js/vendor/bootstrap.js' defer></script><script src='../js/vendor/sidebarEffects.js' defer></script><script src='../js/vendor/modernizr-2.6.2.min.js' defer></script><script src='../js/plugins.js' defer></script><script src='../js/vendor/prettify.min.js'></script><script>document.addEventListener('DOMContentLoaded',prettyPrint)</script> </body></html>

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