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Compiler that compiles (a subset of?) Clojure to Javascript


Because js is the only universally available client target

and can’t be supplanted due to its browser integration and installed base

yet isn’t very good, expressive, concise, or robust

but JS engines continue to get lots of optimization love, and are now quite capable perf-wise

Use same skillset and libs for client and server

only other similar options are:

JS native client, JS (e.g. node) server
node still much less powerful than JVM, and might be a mere fad
Java (GWT) client, Java native server
GWT has lots of baggage due to semantic mismatches etc
but familiar tooling if already a Java dev
esoteric, open question as to skills and libs:
Clamato Smalltalk
Moby Scheme

Clojure semantics can fit well on JS

i.e. defn/type/protocol won’t fight with js core model

ClojureJS arguably becomes most powerful lang on client

Robust, simple Clojure model

Macros etc

Might be best way to run Clojure on mobile

js a possible path to delivering Clojure libs to C-linkage clients

via embedded Google V8 js engine

somewhat speculative, but considering for M

V8 wrappers exist for Python, Ruby, PHP etc


Compiler written in Clojure, generates (readable?) JS

Optionally run that JS through Google Closure js->js compiler

for minification, dead code elimination etc

Use Google Closure js library where needed for implementation support

e.g. goog.math has Long arithmetic

module system

use gclosure annotations for stronger type checking or better code gen?

dependency system

Macros written in Clojure proper, run by the compiler

stick to subset in macros if eval supported

Any runtime support written completely in itself

make deftype and protocols work early


complete Clojure

feel free to subset, especially at first

but try to make anything present in both work identically

compiling e.g. core.clj as-is

don’t want to touch Clojure itself for this

bootstrap will differ anyway

Ancillary benefits

Analysis component of compiler might serve Clojure-in-Clojure, or other tooling

maybe - we’ll need far less analysis support in js than we do in Java

Boost for Clojure adoption due to increased reach

Power tool for exploring next-gen client approach







variable arity

arity overloading

closures shouldn’t map directly to js closures? - no, they should

they capture entire surrounding environment
hearsay, V8 already better
premature optimization to avoid that? - yes
shouldn’t js engines do that for us? - yes
try goog.partial? - not for this

variable arity how?

switch on arguments.length


need to match Clojure semantics (nil/false)

must deal with undefined (group with nil/false?)


must fix local scoping problem

nested fns or renaming?

let* semantics


as with Java, not an expression

doFn(…) -> returns last arg

must alloc array for arguments?


use ‘var for this?

already Clojure special op
but wrong semantics, (var x) is ns-relative
no true unqualified globals in Clojure


to loop

to fn head

can’t do in single pass




(ns my.ns (:require your.ns …) (:macros a-clojure-ns …))



make a clojure ns?
goog.provide(‘my.ns’); goog.require(‘your.ns’);


maps to prototype/contructor combo

deftype macro is deftype* + extend-type

extend-type with ::fields meta on param vectors


yes, for one-off protocol impls

no ctor created, just put impls on object

can share code with putting impls on prototype?


not primitive in Clojure proper

extend, when given ctor, modifies prototype with slot

slot is ns-qualified
what about core prototypes - Object, Array et al (String, Number, Boolean, Function)?
poor citizenship to modify these?
Object different in that it is used as map
nested window scope issues?

protocol fns just turn (my.ns/foo x a b c) into x[“my.ns/foo”](a, b ,c)

foo(x, a, b, c)
must pass target
better -$ns$foo(a, b ,c)
can be minified



any way to get (:foo x) =>

beware GClojure renaming


what to do? ordinary invoke works fine

new could be aliased, not special form then
not ordinary - first arg not evaluated
but should be in JS since new is an operator on a function, not a name

new itself shouldn’t be evaluated, won’t pass fnOf

(my.ns.Blah. x y z) - just macroexpander stuff

(Blah. x y z) - requires import and registry

class aliases a bigger issue, will there be more conflicts?
any interpretation will fit only one ns strategy (e.g. gclosure’s, and thus ClojureScript’s)
start without this


field/zero-arg-method distinguished how?

not, just support scoped var and be done

set! (assign)

same binding rules?


or just allow assign to scoped ‘vars’?

name munging

special chars

js reserved words

(js* code-string)

with name escaping





won’t have exception type



Evaluated collections





callable non-function types?

seems not possible portably

could do with __proto__ (non-standard, all but IE support, even IE9 doesn’t)

how would Clojure feel without callable collections and keywords?

could do with conditional every invocation:

(f instanceof Function?f:f.cljs_lang_invoke)(args)
but where to put f (in expr context)?
needs helper fn
function fnOf(x){return (f instanceof Function?f:f.cljs_lang_invoke);}
i.e. every call is 2 calls
tracing jit will inline?


(def x 42)[‘x’] = 42Following gclosure module systemNo vars? Compilation-time representation of ns? = 42only this one will get minifiedbut this precludes special chars in names
def returns var in Clojure, no var here
(fn [x y] …)(function (x, y) {…})never do named function, use anon + defUse for closures too?
(fn [x y] … (recur…)rewrite as fn + nested looprequire analysis to transmit recur fact up
rewrite when?
block always in return contextaccess to this for methods?
(if test then else)(test ? then : else)
(do e1 e2 e3)cljs.dofn(e1,e2,e3)dofn returns last arg, allocs array?requires js bootstrap file?
no, forces all to be exprsno fn needed when not expr context
(function () {e1;e2;return e3;})()
expr context becomes return except when
single expr
(let [x 1 y 2] …)(function [x,y] {…})(1, 2)need to create nested functions for let*how to detect ref to earlier?
var x__42 = 1;var y__43 = 2; …var numberingstatement/expr dichotomy if inline?
(function []could wrap in no-arg function alwaysneeded for expr anyhow
{var x = 1; var y = 2; …})()if always wrapped, don’t need numbers?can we do var x = 42; var x = 43?
might still when nestedyes, but not var x = 42 …nesting… var x = x
expr always becomes return context
(. x y)x.y or x.y()?no type info to distinguishbigger problem, both calling and retrieving
fn in slot are viable, Clojure says method wins
(. x y …)x.y(…)
(: x y) ?x.ymake all calls, add special field accessor
x.yx.y. not used for classes in JSso not global, but scoped?
can’t test from Clojurebut would want resolution of first segment to locals
what do macros use?
(. x (y))already defined for this casewasn’t going to carry this into cljs, butno arg == field, penalize no-arg methods?
((. x y))more correct, it’s a slotrationale, it’s not a method, just a slot,
(-> (. x y) ())doesn’t currently work, couldbut then why do the arg-taking ones work?
(set! (. x y) 42)x.y = 42whither vars and binding??
(set! 42) = 42
(loop [bindings]while(true){wrap in function? depends on context
… (recur))… rebind-continue
(deftype Foo [a b c])my.ns.Foo = function(a,b,c)turn inline defs into explicit extends?deftype inline methods split out arities
{this.a = a;…this.c=c;}can’t access this and fields.
in locals map, bind a to this.a etc
(new Foo 1 2 3)(new Foo(1,2,3))
(defprotocol = function(obj args)How to extend built-ins, default, nil, undefined
(foo [args])){obj[‘’](obj, args);}can’t minify$ns$foo(obj, args)
P.ns = ‘my.ns’this only compile-time need, but compiler
not in js world, can’t see it
Require fully qualified protocol names?
(extend Foo my.ns.Pfor each fn in map:if no reified protocols, extend can’t beor use Object.defineProperty to add method to
{:foo (fn [foo]…)}Foo.prototype[‘’] = fna function, unless protocol quotedprototype? can then set enumerable to false$ns$foo = fnor string
if extend is a macro or special, could
still evaluate fn map, but then can’t be
evaluated extend requires maps, keywords
high bar for bootstrap if protocols
at bottom - extend* unevaluated?
make extend-type primitive instead? YES
“foo”, true, false, 42.0same
‘foosymbol ctor
:foo?how to do keyword interning?
don’t want intern every reference
(ns my.ns
(:require [ :as fb]…)
(:macros [my.macros :as mm]…)):require-macros?


persistent data structures?

make base literals create JS base literals? (array, object-as-map)

seems a big waste not to leverage js optimization of dynamic properties
or, that’s what deftype is about, maps have always added overhead
we care more about accessors than assignment/modification
i.e. we will superimpose copy-on-write
string/keyword problem
can make {:a 1 :b 2 :c 3} => {a: 1, b: 2, c: 3}
but (keys that) => [“a” “b” “c”]
could use internal array of keys trick
keys used as strings in property mapkept intact in internal arrays, which is what is returned by keys fnmeans keys must be string distinct - ick

promote only on conj?

or on size as well?


equality and hashing


turn into nil?

can’t catch everywhere


def should create slots in global ns objects?

what var semantics matter?

keywords and symbols

make separate object types?

not many symbols make it into runtime use, but keywords do

need to make sure {:key val} and (:key obj) are fast

native maps can have only string keys


just claim a slot?


tie into gclosure module system?

compile-time enumerability?


runtime compiler?

would let you develop in the browser, repl etc

means compiler must self-host

and needs runtime reader
and syntax-quote

no macros?

can’t do without, as so many basic things are macros

won’t have google closure compiler there

ok, shouldn’t rely on that


not a great fit

GC probably not as good

unlikely to be working with bigger-than-memory

non-lazy mapping/filtering or mapv, filterv

can make it back into Clojure



or just use safe lib fns to avoid
lets you use base types
no final on which to base it anyway
would need fancy encapsulation techniques
how fancy?
correct (non-assigning) code does the same thing, but incorrect not caught
fair compromise?

gclosure compiler can do some enforcement

given some const hints in comment-based annotations

Interactive development


Clojure read -> cljs analyze -> cljs emits -> embedded Rhino eval+print

Incompatible constructs

for host interop
preclude development in Clojure

Missing JS things

e.g. DOM etc
headless JS environment with DOM mocks?

Namespaces and macros

Macro problem?

syntax quote in the reader uses Clojure namespaces

hardwired to Compiler.currentNS().getMapping() Compiler.resolveSymbol(), isSpecial() etc

::keyword resolution uses Compiler.currentNS(), namespaceFor()

if it expands to calls to other macros, those need to be in clojure-land

maybe - they need to be the cljs-compatible versions

argument for calling core clojure.core in cljs too?
but we can’t have 2 clojure.core namespaces during compilation
translate/consider clojure.core/xxx => cljs.core/xxx during cljs compilation?
doesn’t work if we have separate cljs.macros
some core things will be in cljs.core, some in cljs.macrosput core macros on cljs/core.cljother core code in cljs/core.cljsboth contain ns declarations - ok?

but expansions destined for cljs compilation need to be resolved in cljs-land

dummy vars in dummy namespaces?

no - doesn’t cover external nses, cljs aliases
just fully qualify everything non-core in macroexpansions

different specials set a problem

e.g. global, ns, defprotocol* not specials
could use var for global
could make ns a special?
probably notbut what macro would emit that?
install all cljs specials in dummy nses?
no, doesn’t help macros file

inline defmacro in cljs?

calls Clojure eval of defmacro call

can expand to calls to enclosing cljs ns

but expander can’t call code in enclosing cljs ns
convention - all macro helper fns are local fns in macro

gets icky for load/require purposes, as must be loaded with cljs compiler

better to keep public macros separate - can use ordinary Clojure require then
local macros would be ok for same-file consumption though

convention - portable libs that expose macros package them separately

Want some equivalent of refer clojure.core

else practically everything will be qualified

e.g. core/defn - ick

but fewer things brought in by default?

requires selectivity control, or just a smaller core.cljs?

this is equivalent to a ‘use’, which we otherwise aren’t supporting

unfair or don’t care?

Any ‘use’ equivalent (e.g. refer core) means compile-time disambiguation of unqualified references

if names a referred thing, that thing, else

like current namespaces

but if refers are limited to (entirety of) core, just look there first

so double lookup instead of copying core vars into name table

Some core things defined in js

where we don’t want to otherwise expose things needed for their impl

e.g. ==, ===, math ops, instanceOf typeof etc

how to reserve names?

declare in core.cljs?

if in actual .js file, separate ns for deps purposes?

i.e. it will be a different file than that produced by compiling core.cljs

or just a wad of js injected into core.cljs?

include a (js code-string) primitive for this purpose?
yes, much better than js files
accept only at top level? - no
using in local scope means knowing how locals are represented
some sort of escaping construct for getting (local and other) names resolved

Are we doing forward reference detection here?

requires listing of contents of current ns

like namespaces

Are we doing extern-ns name validation?

could do for cljs names, but not others

e.g. goog.whatever not enumerable in cljs

can we discern this situation?

probably not, when compiling from files
since ‘require’ doesn’t load code at compile time

another reason we can’t support ‘use’

we do want to be able to (:require
but not a compile-time enumerable ns
or especially: (:require [ :as gfoo])
means alias map, like namespaces

Macros written in separate Clojure files

Clojure code, in regular namespaces

Means core split into core.cljs, and core-macros.clj

both need to be auto-referred

if no use/only for macro ns, then can only get as succinct as (alias/macro …)

could allow explicit aliasing of vars instead of use

extend alias for this?

not really extending, alias will do this due to how nses are just vars
but need not be used in that pat of resolution

goog.provide throws called-twice exception

intended to prevent providing the same ns in more than one file

actually prevents reloading same file? - aargh

can’t wrap, since deps checkers look for it at top level

will we need to track at compilation-time?

will we still need compile-file notion?

Compilation needs

current ns

cljs-ns ?

is this a Clojure ns?

not a fit
map is sym->Var or Class
aliases are sym->Namespace

ns has:

cljs-namespaces - {name->ns}

{:name “my.ns” :defs {sym qualified.sym} :deps {alias Namepsace-or-qualified.sym}}


just set of names? no map
or map to fully qualified self?


can’t merge macros and cljs ns in deps
same ns might map to both
i.e. cljs.core will
is this a separate mapping vs macros and requires?
if not, fn alias can mask out ns aliasthat can’t happen in Clojure
macro nses
map of sym->Namespaces?
require an alias?(:macros {mm my.macros, ym your.macros})
aliases for these same as others?
required libs must have aliases too?
(:require [goog.math.Long :as gml])
or new (:require {gml goog.math.Long})

lookup ‘foo - no ns, no dots

if special - done

if local - done

if found in cljs.macros Namespace, the macro cljs.macros/foo

if found in cljs.core ns,

whatever ‘foo maps to in (-> env :ns :requires)

no use of deps

lookup ‘ - no ns, dot(s)

if foo is a local,

if foo has a mapping in (:ns env) - - no

really? covered by alias/whatever
more idiomatic for goog.stuff than goog.stuff/foo
but no :as there
leave out for now


lookup ‘foo/bar - ns with no dots

get what ‘foo maps to in (:ns env) deps

if nothing - error “no alias foo”

if maps to Namespace, the macro ‘bar in that ns

else a symbol, e.g. ‘fred.ethel =>

lookup fully.qualified/foo - ns with dots

would only use this if local shadowed (and no alias)?

what doesn’t have alias?

cljs.core, cljs.macros
could use for former
always interpret as macro ns?
or check deps vals for Namespace, else not
if Namespace, the macro foo in Namespace

everything might have alias, but macros/syntax-quote need to emit full expansions

how to refer to true globals?

e.g. Object, String, goog

JS Globals

(var Name)?

that doesn’t match Clojure, where (var x) means ‘whatever x means in current ns’
there are no unqualified globals in Clojure

means (extend (var Object) …) needs to work

ok if extend evaluates first arg
it does in Clojure, as it is a function



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