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<h1>Synopsis</h1><p>The ClojureScript compiler currently directly prints JavaScript source code strings. It is desirable to instead produce a JavaScript Abstract Syntax Tree to simplify code generation, emit source maps, and to enable higher level optimizations.</p><h1>Problems</h1><ul><li>Compiler should be more functionally pure<ul><li>Code generation currently emits strings as a side effect</li><li>Enhancing the compiler is difficult because printing forces ordering and limits higher order composition</li><li>It's not safe to interleave multiple passes of analysis, transformations, and code generation</li></ul></li><li>emit complects Code Generation and Code Printing<br /><ul><li>requires simultaneous consideration of JavaScript's abstract structure and particulars of syntax</li><li>code generation is trivially functionally pure; code printing could be, but wouldn't benefit much</li></ul></li><li>SourceMaps! Strings lack structure to associate mappings<ul><li>If you currently are printing &quot;foo(bar)&quot;, you might need to assign different source lines to both foo and bar</li><li>Adding source mappings to printing would give very low mapping resolution for the current ad-hoc strings</li><li>Increasing the resolution would yield something that looks very much like an AST</li></ul></li></ul><h1>Goals</h1><ul><li>Decouple code generation from code printing</li><li>Simplify compiler.clj</li><li>Include source mappings on outputted AST</li><li>Preserve compiler and generated code performance</li></ul><h1>Approach</h1><ul><li>Target the Google Closure AST<ul><li>Advantages<ul><li>Keeps us out of the JS AST design business</li><li>Existing code printer</li><li>Includes facilities for Closure Compiler type system</li><li>Perf? Can skip roundtrip to JS source and to disk</li></ul></li><li>Disadvantages<ul><li>Closure AST nodes are stateful<ul><li>Each Node has an explicit parent reference</li><li>Cloning policy is necessary to reuse subtrees (wasn't an issue in my port so far, see below)</li></ul></li><li>Increases our dependence on Closure<ul><li>Closure AST evolves more quickly than the JS spec</li><li>Recent changes: <a href="http://code.google.com/p/closure-compiler/source/list?path=/trunk/src/com/google/javascript/jscomp/parsing/IRFactory.java&amp;start=2295">http://code.google.com/p/closure-compiler/source/list?path=/trunk/src/com/google/javascript/jscomp/parsing/IRFactory.java&amp;start=2295</a></li></ul></li></ul></li><li>Alternative: Intermediate JavaScript AST as Clojure data-structures<ul><li>Requires an additional translation step</li><li>GWT and Dart use a different AST; See: <a href="http://code.google.com/p/google-web-toolkit/source/browse/trunk/dev/core/src/com/google/gwt/dev/js/ClosureJsAstTranslator.java?r=10778">http://code.google.com/p/google-web-toolkit/source/browse/trunk/dev/core/src/com/google/gwt/dev/js/ClosureJsAstTranslator.java?r=10778</a></li></ul></li><li>Closure Compiler Discussion: <a href="https://groups.google.com/forum/?fromgroups=#!topic/closure-compiler-discuss/OEwLWkw4Kug">https://groups.google.com/forum/?fromgroups=#!topic/closure-compiler-discuss/OEwLWkw4Kug</a></li></ul></li><li>Thin JS-AST DSL<ul><li>Recursively transpiles arguments</li><li>Can be extended to automatically attach source mappings from dynamic bindings</li><li>Does a Hiccup like seq expansion</li><li>Eliminate js* form, replaces it with DSL invokes: (js* &quot;foo({0})&quot; x) becomes (js*/call 'foo x)</li></ul></li></ul><h1>Preliminary Progress</h1><ul><li>Fork: <a href="https://github.com/brandonbloom/clojurescript/tree/js-ast">https://github.com/brandonbloom/clojurescript/tree/js-ast</a><ul><li>Interesting files: <a href="https://github.com/brandonbloom/clojurescript/blob/js-ast/src/clj/cljs/compiler.clj">compiler.clj</a> and <a href="https://github.com/brandonbloom/clojurescript/blob/js-ast/src/clj/cljs/js.clj">js.clj</a></li></ul></li><li>As of Nov 9<ul><li>Compiles all the CLJS in the ClojureScript tree</li><li>Bugged in advanced compilation mode</li><li>Does not generate source mappings</li><li>Does not bypass the extra round-trip to JS source and to disk</li><li>Takes 3X as long as naive string printing; 10 vs 30 seconds for (compile-root &quot;src/cljs/cljs&quot;) on my machine</li><li>core.clj and core.cljs are fully ported from (js* &quot;...&quot; ...) to (js*/... ...) </li></ul></li></ul>