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Posted to commits@groovy.apache.org by pa...@apache.org on 2021/07/27 06:00:27 UTC
[groovy-dev-site] branch asf-site updated: added GEP-13
This is an automated email from the ASF dual-hosted git repository.
paulk pushed a commit to branch asf-site
in repository https://gitbox.apache.org/repos/asf/groovy-dev-site.git
The following commit(s) were added to refs/heads/asf-site by this push:
new c9e3f17 added GEP-13
c9e3f17 is described below
commit c9e3f178e5ec134e25a6cd117cee5592be6649b2
Author: Paul King <pa...@asert.com.au>
AuthorDate: Tue Jul 27 15:59:08 2021 +1000
added GEP-13
---
wiki/GEP-13.html | 326 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 326 insertions(+)
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+ <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-13</title><link href='../img/favicon.ico' type='image/x-ico' rel='icon'/><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 [...]
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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-13</a></li><li><a href='#_abstract_sealed_classes' class='anchor-link'>Abstract: Sealed classes</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- [...]
+<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-13</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Title</strong>
+</td>
+<td class="hdlist2">
+<p>Sealed classes</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Version</strong>
+</td>
+<td class="hdlist2">
+<p>1</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>Draft</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Leader</strong>
+</td>
+<td class="hdlist2">
+<p>Paul King</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Created</strong>
+</td>
+<td class="hdlist2">
+<p>2021-07-22</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Last modification</strong> 
+</td>
+<td class="hdlist2">
+<p>2021-07-22</p>
+</td>
+</tr>
+</table>
+</div>
+</div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_abstract_sealed_classes">Abstract: Sealed classes</h2>
+<div class="sectionbody">
+<div class="paragraph">
+<p>Sealed classes and interfaces restrict which other classes or interfaces may extend or implement them.
+By supporting sealed classes and interfaces, the Groovy programming language
+can offer an additional mechanism for controlling class hierarchy construction.</p>
+</div>
+<div class="sect2">
+<h3 id="_motivation">Motivation</h3>
+<div class="paragraph">
+<p>Inheritance is a powerful mechanism for creating hierarchies of related class and interfaces.
+Sometimes, it is desirable to restrict the definition of children in such hierarchies.
+Modifiers already provide some mechanisms:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>If all of our classes and interfaces are public, this indicates that we want
+maximum reuse.</p>
+</li>
+<li>
+<p>The <code>final</code> modifier offers one mechanism for restricting further inheritance at the method or class level.
+It effectively limits all further extension and indicates no further code reuse is desired.</p>
+</li>
+<li>
+<p>By making a base class package-private we can limit extension to only classes within
+the same package. If an abstract <code>Shape</code> class is package-private, I could have
+public classes <code>Square</code> and <code>Circle</code> in the same package. This indicates that we want
+code reuse to occur only within the package. While it does limit creation of
+new shapes outside the original package, it offers no abstraction for a shape which
+could be either a square or circle since <code>Shape</code> is not public.</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>Sealed classes or interfaces can be public but have an associated list of allowed children.
+Classes or interfaces which are not in that list cannot inherit from those sealed types.
+This indicates that we want code reuse within the hierarchy but not beyond.
+Parent classes in the hierarchy can be made <em>accessible</em>, without also making them <em>extensible</em>.
+This allows hierarchies to be created with maximum reuse within but without having
+to defensively code for arbitrary extensions added at a later time.</p>
+</div>
+<div class="paragraph">
+<p>Such classes are useful in defining Algebraic Data Types (ADTs) and in scenarios where
+we might want to reason about whether we have accounted for all possible types, e.g. the
+static compiler may wish to give a warning if a switch block doesn’t exhaustively
+cover all possible types by respective case branches.</p>
+</div>
+<div class="sect3">
+<h4 id="_initial_implementation">Initial implementation</h4>
+<div class="ulist">
+<ul>
+<li>
+<p>Provide a <code>@Sealed</code> marker annotation or AST transform which allows a list of
+permitted children to be defined. Use of this annotation will be an incubating
+feature subject to change. Explicit use may eventually be discouraged and instead
+a keyword, e.g. <code>sealed</code> would be encouraged instead. However, the annotation
+could be retained to offer support for this feature on earlier JVMs or versions
+of Groovy prior to any grammar changes.</p>
+</li>
+<li>
+<p>Prohibit extension of JDK16+ sealed classes or annotated <code>@Sealed</code> classes.
+Likewise for interfaces. This also applies for anonymous inner classes and traits.</p>
+</li>
+<li>
+<p>Provide checks in other places where such extension might occur implicitly, e.g.: with <code>@Delegate</code>,
+when using type coercion, etc.</p>
+</li>
+</ul>
+</div>
+</div>
+<div class="sect3">
+<h4 id="_potential_extensions">Potential extensions</h4>
+<div class="paragraph">
+<p>The following potential extensions are possibly all desirable but
+are non-goals for the first implementation:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>Introduce the <code>sealed</code> modifier and <code>permits</code> clause in the grammar.</p>
+</li>
+<li>
+<p>Support <code>non-sealed</code> or <code>unsealed</code> sub-hierarchies.</p>
+</li>
+<li>
+<p>Require that all classes within a sealed hierarchy be compiled at the same time.</p>
+</li>
+<li>
+<p>Require that all classes within a sealed hierarchy belong to the same module.</p>
+</li>
+<li>
+<p>Add warnings to the static compiler if a switch is used for a sealed hierarchy
+and not all types are exhaustively covered.</p>
+</li>
+<li>
+<p>When running on JDK16+, also add sealed class information into the bytecode.</p>
+</li>
+</ul>
+</div>
+</div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_references_and_useful_links">References and useful links</h2>
+<div class="sectionbody">
+<div class="ulist">
+<ul>
+<li>
+<p><a href="https://openjdk.java.net/jeps/360">JEP 360: Sealed Classes (Preview)</a></p>
+</li>
+<li>
+<p><a href="https://openjdk.java.net/jeps/397">JEP 397: Sealed Classes (Second Preview)</a></p>
+</li>
+<li>
+<p><a href="https://openjdk.java.net/jeps/409">JEP 409: Sealed Classes</a></p>
+</li>
+<li>
+<p><a href="https://kotlinlang.org/docs/sealed-classes.html">Sealed Classes</a> in Kotlin</p>
+</li>
+<li>
+<p><a href="https://docs.scala-lang.org/sips/sealed-types.html">Sealed Classes</a> in Scala</p>
+</li>
+</ul>
+</div>
+<div class="sect2">
+<h3 id="_reference_implementation">Reference implementation</h3>
+<div class="ulist">
+<ul>
+<li>
+<p>TBD</p>
+</li>
+</ul>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_jira_issues">JIRA issues</h3>
+<div class="ulist">
+<ul>
+<li>
+<p><a href="https://issues.apache.org/jira/browse/GROOVY-10148">GROOVY-10148: Groovy should not allow classes to extend sealed Java classes</a></p>
+</li>
+</ul>
+</div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_update_history">Update history</h2>
+<div class="sectionbody">
+<div class="paragraph">
+<p>1 (2021-07-22) Initial draft</p>
+</div>
+</div>
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