http://dev.clojure.org/jira This file is an XML representation of an issue en-us 4.4 649 25-07-2011 http://dev.clojure.org/jira/browse/CLJ-890 Clojure <p>See <a href="http://dev.clojure.org/display/design/Tagged+Literals">http://dev.clojure.org/display/design/Tagged+Literals</a></p> CLJ-890

Tagged literals in reader

Enhancement Major Closed Completed Stuart Sierra Stuart Sierra Fri, 2 Dec 2011 17:15:43 -0600 Fri, 1 Mar 2013 12:47:02 -0600 Fri, 3 Feb 2012 09:12:01 -0600 0 0 <p>Rough draft in <a href="http://dev.clojure.org/jira/browse/CLJ-890" title="Tagged literals in reader"><del>CLJ-890</del></a>-001.patch:</p> <p>This adds reader tags like <tt>#user/foo</tt>. If the symbol following the <tt>#</tt> is namespace-qualified, or if it is one of a predefined set of built-in tags (currently empty) it is interpreted as a tag. Otherwise, it is interpreted as a class name, as in defrecord literals.</p> <p>The dynamic Var <tt>&#42;data-readers&#42;</tt> is a map from tags (symbols) to reader functions/Vars. Each invocation of <tt>Compiler.load</tt> creates a new thread-local binding, as does the REPL. At the top of a source file, use <tt>(set! &#42;data-readers&#42; ...)</tt> to define the tags just for that file. For runtime invocations of the reader, use <tt>binding</tt>.</p> <p>When reading a tagged literal, if the tag is <b>not</b> defined in <tt>&#42;data-readers&#42;</tt> it is added as <tt>:data</tt> metadata on the following form. If the following form does not support metadata, the tag is ignored. This causes a problem when an undefined tag is used in source code, because the compiler tries to evaluate it:</p> <div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent"> <pre class="code-java">user=&gt; #foo.bar/bar {:a 1} CompilerException java.lang.RuntimeException: No such <span class="code-keyword">var</span>: foo.bar/bar, compiling:(NO_SOURCE_PATH:0) user=&gt; (quote #foo.bar/bar {:a 1}) {:a 1} user=&gt; (meta *1) {:data foo.bar/bar}</pre> </div></div> <p>New file <a href="http://dev.clojure.org/jira/browse/CLJ-890" title="Tagged literals in reader"><del>CLJ-890</del></a>-002.patch:</p> <ul> <li>tag definitions loaded from <tt>/data_readers.clj</tt> files on classpath</li> <li>read as a series of symbol-symbol pairs</li> <li>"target" symbol interpreted as a Var</li> <li>creates namespaces and interns Vars as necessary</li> <li>may be more than one file</li> <li>throws exception on conflict</li> <li>sets root binding of <tt>&#42;data-readers&#42;</tt></li> </ul> <p>Patch <a href="http://dev.clojure.org/jira/browse/CLJ-890" title="Tagged literals in reader"><del>CLJ-890</del></a>-002.patch applied following discussion with Rich. Will be included in release 1.4.0-alpha4</p> <p>It would be a small extension to allow overriding of readers for untagged forms as well.</p> <p>For example, MapReader currently calls RT.map(a) when it could call RT.get(data_readers, MAP_TAG).invoke(a)</p> <p>This would allow consumers of the reader to supply their own implementations for any basic data structure. Not that anyone is likely to have a <b>better</b> map/set/etc, but I could see it being useful for using a different RegExp engine or something like that. The primary use case I have in mind, however, is the ClojureScript compiler. The compiler needs to know the <b>order</b> of the key-value-pairs in a map or the elements in a set. See <a href="http://dev.clojure.org/jira/browse/CLJS-288" title="Compilation of unordered collections ">CLJS-288</a></p> Approval Vetted Global Rank Patch Code