The Parenscript Common Lisp to JavaScript compiler

The Parenscript Common Lisp to JavaScript compiler Vladimir Sedach <vsedach@gmail.com>

What Parenscript is not  Not a full ANSI INCITS 226-1994 (the standard formerly known as X3J13) implementation in JavaScript  Not a Lisp interpreter in JavaScript  Not a framework, toolkit, UI or AJAX library

What Parenscript doesn't do  No dependencies or run-time library  Any JS code produced runs as-is  No new data types  No weird calling convention  Any PS code can call any JS code directly and vice- versa  No whole program compilation  JS code can be produced incrementally

Where is Parenscript used?  Web frameworks: BKNR, UCW, TPD2, Weblocks  Libraries: cl-closure-template, Suave, css-lite, clouchdb, uri-template  Many startups and commercial projects

Parenscript history  March 2005, Manuel Odendahl announces initial release  Swallowed up by the bese project in 2006  Skysheet developers spin Parenscript out as separate project again in 2007, take over development, do lots of PR

Similar projects  Not many options for Common Lisp:  Jason Kantz's JSGEN (2006, s-exp over JS)  Peter Seibel's Lispscript (2006?, s-exp over JS)  Many more options for Scheme:  Scheme2JS (2005?)  Feeley and Gaudron's JSS  Many others  Moritz Heidkamp's survey of Lisp-in-JavaScript implementations lists 28!

Use cases  Lisp at runtime  No Lisp at runtime

Lisp at runtime  Compile to inline JavaScript in HTML page  Stream client-specific JavaScript to browser via XMLHttpRequest or page loads  SWANK-based development environment

No Lisp at runtime  Compile to static files sent to browser  If done with a runtime, can use all of CL features for a build/deploy/cache system  Compile to library to be used by other JS code  Compile to JS client- and server-side (node.js) code at the same time  Deploy JS only  Parenscript used as static compiler for whole web stack

What should JS code look like?  Write CL, but debug JS  Generated code has to be short, readable, idiomatic  ”Optimization” in the context of this talk means ”more readable code”  What about performance?  Can only assume that JS compilers try to optimize for code written by humans (ie – readable and idiomatic code)

Statements vs Expressions  In JavaScript, can transform any statement to an expression by wrapping it in a lambda  Except for BREAK, CONTINUE, and RETURN  Many optimizations to avoid lambda-wrapping possible, depending on combination of special forms

Function argument lists  Full Common Lisp lambda lists in JavaScript  In a way completely compatible with other JavaScript code  ARGUMENTS is great  NULL (argument provided) vs UNDEFINED (argument not provided)  Keywords: f(”key2”, value2, ”key1”, value1);  CL keywords and JS strings both self-evaluating

Macros  Share the same representation (macro- function) in CL and PS compilers  But have different namespaces  Share same code between client (CL) and server (JS) even if implementation details completely different (ex – IO, graphics)  Without modifying CL code  PS macros can shadow CL functions

Data structures  No car/cdr  Many Common Lisp forms are sequence oriented (DOLIST, LOOP, etc.)  Most idiomatic Common Lisp code uses these forms  Most code turns out to care about sequences, not linked lists vs arrays, and works as-is with JS arrays

Namespace (Lisp-1 and Lisp-N)  JavaScript is a Lisp-1  Common Lisp is a Lisp-4  With macros, Lisp-N  Parenscript tries to be a Lisp-2 (separate variable and function namespaces)  Code can be output unambiguously for lexically bound names (via α-renaming), but free variables referring to globals might cause conflicts  No thought given to hygiene

Namespace (package system)  All Parenscript symbols are CL symbols  Packages can be given prefixes to namespace in JS:  foo::bar => foo_bar  Package system also used for obfuscation and/or minification  Exports list used to produce JS libraries with minified internals and prefixed API names

Namespace (case and escape)  CL readtable case-insensitive by default, but can be set to preserve case  Parenscript recognizes mixed-case symbols and outputs them correctly  Simple translation for case-insensitive ones:  foo-bar => fooBar  JS reserved chars escaped:  foo* => foostar

JavaScript dot operator  Causes confusion  Some people think it's ok to use it for namespacing  foo.bar is really (getprop foo 'bar)  What about foo.bar.baz(foobar)?  (funcall (getprop (getprop foo 'bar) 'baz) foobar)  Macros to the rescue:  (funcall (@ foo bar baz) foobar)

JavaScript dot operator 2  Why not (foo.bar.baz foobar)?  Parenscript 1 did it in compilation step  Turns out this doesn't work well with package system, breaks symbol macros  Symbol macros used for a lot of things in Parenscript 2 compiler  Possible to do it in the reader  But ”.” is already reserved for dotted lists  Dotted lists are evil; remove them from Lisp

Lexical scoping  Done by using α-renaming to declare variables in nearest function scope: (lambda () function () { (let ((x 1)) var x = 1; (let ((x 2)) var x1 = 2; x))) return x1; }

Lexical scoping: loops  For variables in loops, JS creates a shared binding for all iterations, breaking closures  WITH trick from Scheme2JS: (dotimes (i 10) for (var i = 0; i < 10; i += 1) { (lambda () (+ i 1))) with ({i : i}) { function () { return i + 1; };};};

Dynamic scoping  TRY/FINALLY to set a global variable (let ((*foo* 1)) var FOO_TMPSTACK7; (+ *foo* 2)) try { FOO_TMPSTACK7 = FOO; FOO = 1; return FOO + 2; } finally { FOO = FOO_TMPSTACK7; };

Multiple value return  ARGUMENTS has 'callee' property (function)  'callee' has 'caller' property (function)  Functions are JS objects, can attach arbitrary properties to them  To return MV:  arguments.callee.caller.mv = [val2, ...]  To receive MV:  others_vals = arguments.callee.mv

Multiple value return 2  Actually requires more cleanup code than this.  arguments.callee works even when calling to/from functions that don't know about Parenscript (a global ”return value arg” wouldn't)  Non-lexical returns (see next slide) with multiple values easier to implement (but aren't yet)

Control transfer  JavaScript only supports RETURNing from innermost function, BREAKing out of innermost block or loop  Can implement CL's BLOCK/RETURN-FROM and CATCH/THROW using TRY-CATCH- FINALLY in general case  Optimizations possible depending on special forms, whether a return value is wanted

CLOS  Red Daly's PSOS library provides CLOS and Common Lisp's condition and resumable exception system  Not part of core Parenscript because it requires runtime library and introduces new types  Possible to build up a full Common Lisp in JavaScript this way using Parenscript as a cross-compiler

Development environments  SWANK-based  slime-proxy  Non-SWANK  Numen (V8 only, to be released)

Future directions  JS implementations have mostly caught on speed-wise  But have not, and most likely will not, catch up in efficient memory consumption  Would like a full Common Lisp implementation in JS on the server (V8, Rhino, etc.) and web browser (Firefox, Chrome, etc.)

The Parenscript Common Lisp to JavaScript compiler

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