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<h1>Motivation</h1><p>To add extensibility to the Clojure reader in a composable way.</p><h1>General Goals</h1><ul><li>new syntax for &quot;tagged&quot; literals in the reader</li><li>tags can be namespaced</li><li>non-namespace-qualified tags are reserved for Clojure</li><li>when the reader encounters a tagged literal, it invokes a function associated with that tag</li><li>users may define new tags and tag-reader functions</li><li>users may override functions for built-in tags</li></ul><h3>Non-goals</h3><ul><li>Common Lisp-style reader macros</li><li>Custom parsing of the character stream<ul><li>Reader will parse tagged literals as data and pass that to the tag function</li></ul></li></ul><h1>Open Questions</h1><ul><li>What is the syntax for tags?<ul><li>Keywords, like <code>#:tag form</code><ul><li>One character longer</li><li>Slightly easier to parse</li></ul></li><li>Symbols, like <code>#tag form</code><ul><li>One character shorter</li><li>Slightly harder to parse</li><li>Ambiguous with record constructors?<ul><li>Not if we assume that: <ul><li>1) record names always contain a dot<ul><li>Which they must, because deftypes occur in namespaces</li></ul></li><li>2) user-defined tags are always namespace-qualified</li><li>3) built-in tags do not contain a dot</li></ul></li></ul></li></ul></li></ul></li><li>What happens when the reader encounters a tag for which no function is defined?<ul><li>Error (YES)</li><li>Return form unchanged?</li><li>Return form with extra metadata?<ul><li>Only possible if form supports metadata, ignore otherwise?</li></ul></li></ul></li></ul><ul><li>How to associate tags with reader functions?<ul><li>file containing mapping from tags to Vars<ul><li>&quot;magic&quot; file name gets loaded before other files</li><li>doesn't load any code directly</li><li>If 2 JARs on classpath have a magic file, who wins?<ul><li>throw an exception</li></ul></li></ul></li><li>map stored in dynamic var<ul><li>thread-local binding for each source file<ul><li><p>created in <code>Compiler.load and Compiler.compile</code></p></li></ul></li><li>can be set! within a source file</li><li>can be rebound when invoking the reader at runtime</li></ul></li></ul></li></ul><h1>References / See Also</h1><ul><li><a href="http://dev.clojure.org/jira/browse/CLJ-890">CLJ-890</a></li><li><a href="/pages/viewpage.action?pageId=950382" data-linked-resource-id="950382" data-linked-resource-type="page" data-linked-resource-default-alias="Clojure Instant Literal &amp;amp; Library" data-base-url="http://dev.clojure.org">Clojure Instant Literal &amp; Library</a></li></ul>