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| </div><div id='content' class='page-1'><div class='row'><div class='row-fluid'><div class='col-lg-3'><ul class='nav-sidebar'><li class='active'><a href='#doc'>GEP-4</a></li><li><a href='#_abstract' class='anchor-link'>Abstract</a></li><li><a href='#_approach' class='anchor-link'>Approach</a></li><li><a href='#_references_and_useful_links' class='anchor-link'>References and useful links</a></li><li><a href='#_update_history' class='anchor-link'>Update history</a></li></ul></div><div class='col-lg-8 col-lg-pull-0'><a name='doc'></a><h1>GEP-4</h1><p>Author: <i/></p><hr/><div id="preamble"> |
| <div class="sectionbody"> |
| <div class="sidebarblock"> |
| <div 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 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 ยด and ,</p> |
| </li> |
| <li> |
| <p>Template Haskell uses something similar to oxford brackets but with slightly different semantics</p> |
| </li> |
| <li> |
| <p>Boo uses 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 Meta Method Comparison |
| The Boo Metamethod feature overlaps with the Groovy AST Transformation features. Instead of an @AstTransformation interface with a seperately 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://groovy.markmail.org/thread/mgo3ze3uvn2jalz4">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> |
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