Module system strawpersons

Kam Kasravi kamkasravi at
Fri Jan 15 17:04:51 PST 2010

I guess the problem with examples is their analysis may not surface a particular use case. I agree at runtime this example may allow the interpreter to not over import.
The original question about how an interpreter may be 
non-deterministic is whether the rules that determine whether the interpreter is lazy or eager have not been defined yet. The example was an attempt on my part to 
have the developer drive how aggressive the interpreter is in prefetching resources. To summarize, there should be some rules that make each vendors interpreter behave like the next one, and there should be a way for the developer to make the interpreter prefetch even if the interpreter sees no reason to do so.  

On Jan 15, 2010, at 4:11 PM, Mike Samuel <mikesamuel at> wrote:

2010/1/15 Kam Kasravi <kamkasravi at>:
comments below...
From: Mike Samuel <mikesamuel at>
To: Kam Kasravi <kamkasravi at>
Cc: kkasravi <kkasravi at>; es-discuss <es-discuss at>
Sent: Fri, January 15, 2010 1:45:52 PM
Subject: Re: Module system strawpersons

2010/1/15 Kam Kasravi <kamkasravi at>:
Ihab, Mike - Thanks for the comments.
Yes, my examples confused definition with declaration, thanks for the
I think I still have some unanswered questions which I'll try and clarify
1. I'm still not clear what the module identifier represents.
    Something like 'a/b/c' could be a URI or associated with a URI.
    Commonjs implicitly adds a '.js' which suggests this maps to a REST
    I think it would be helpful to clarify the above literal, the,
    any relation to javascript namespace and a mapping to server resources
since there is
    such an overlap with package management.

2. If the eagerness of the interpreter is determined by its implementation
    we could have lots of indeterminism across interpreters. There should

Which sources of indeterminism do you foresee?
Is this indeterminism as apparent to a network observer or as apparent
to code running in the same interpreter?
[Kam] the network observer. A given interpreter may end up making lots of
or a few depending when it decided to resolve the import's.

    probably be another construct that defines when resources are fetched.
    I'm not sure if the interpreter shouldn't be influenced by the

Could you rephrase this last sentence, please?
[Kam]  as an example. lets say a module contains 3 adapters:
one that uses jQuery, one that uses Ext and one that uses YUI.
A eager interpreter may fetch all three when evaluating the module.
Yet if the choice of what framework to use (jQuery, Ext or YUI) was
determined by the developer/user early on, then the interpreter would
fetch 2 frameworks without needing to. If the developer had a
way to indicating to the interpreter that 2 of the frameworks weren't
needed then the behavior of the interpreter would be predictable.

If the module needs all 3, then it needs to fetch all 3.

But if only one adapter is needed, then there is presumably some code
somewhere which chooses which adapter.  Let's call it the
bootstrapper.  Then the module relationship looks like:

   bootstrapper imports YUI-adapter
   YUI adapter imports YUI
   bootstrapper passes adapter to framework as a module parameter

Since modules are first class, and are parameterizable, dependencies
can be injected without requiring over-import.
By not importing the adapters that are not needed, the other libraries
are not imported.

Brendan, do you recall es4 where the concept of a unit was introduced?
There was also some discussion where management and security
where implicitly mapped to the '.' between identifier parts. I cannot find
that reference.

From: Mike Samuel <mikesamuel at>
To: kkasravi <kkasravi at>
Cc: es-discuss <es-discuss at>
Sent: Fri, January 15, 2010 11:20:48 AM
Subject: Re: Module system strawpersons

2010/1/15 kkasravi <kkasravi at>:
Hi Mike, Ihab:

One factor that would influence eager or lazy fetching is where the
import keyword may appear within the ecmascript grammar.

The proposal suggests it could appear anywhere
an identifier could appear. For example:

I believe it cannot appear where a LeftHandSideExpression is expected
because a function cannot be assigned to.
So no replacing = baz
  (import ...) = baz

1)  function find(name) {
      var name =; // identifier ''

Replacing with import would yield

2) function find(name) {
     var name = (import 'foo/bar').Baz.find(name);

Provisioning is separate from importing.  The above is looking up Baz
on a function instance which normally would be the same as
  var name = Function.prototype.Baz.find(name)

I think normal usage would look like
    var name = (import ...)(module, instance, parameters).Baz.find(name)


Replacing with import would yield

3) function find(name) {
     var name = (import 'foo/bar/Baz').find(name);

I provided 2) and 3) because its not clear where a module definition
The presumption by naming convention is that can be found
a relative resource named ./foo/bar.js on some server. This metadata
be somehow made available to the javascript runtime. So in some ways
module's need package info. Let's assume the correct one is 2).

Looking at 2), current javascript behavior is always lazy fetching
the above function find

fetching can be eager, but evaluation happens whenever the ImportExpr
is reached, possibly not at all.

may not be called immediately or not at all.  The javascript runtime
not complain about
identifiers that are not immediately accessed or not accessed at all.

Doing eager fetching means that we would need to fetch all imports prior
to eval.
Impacts are:
1. We fetch more than necessary and immediately. User perception is the
new js runtime is much slower.

Eager fetching can be done, or can be delayed, or multiple module
definitions could be fetched in the same transaction.  The degree of
eagerness is up to the interpreter.

2. We forces all import expressions to be statically defined.

If import can appear in place of an identifier, there is no choice but to
do lazy fetching. Lazy fetching without CPS (continuation passing style)
support means you could not do (import foo/bar').Baz.find(name);


On Jan 14, 2010, at 2:29 PM, Mike Samuel wrote:

Ok, so let me try to make sure I understand the special form in
strawman:modules_emaker_style, by defining a mini-strawman and
knocking it down.

Consider a special form
  (f, definer) = defineDelayedFunction();
where f is a function that will block on first call until definer has
been called with an arguments array and a function body upon which
point it will behave as if it were defined via new Function(...) with
those arguments and function body.  definer raises an Error on second
and subsequent calls.

defineDelayedFunction could be used with an async channel like
XMLHttpRequest to define a module system, but all module definitions
would happen at execute time, so it cannot be used to define a module
system that resolves definitions prior to body evaluation.

In modules_emaker_style, imports are statically determinable, so the
set of dependencies is fully satisfied before the Program is
evaluated, and this can be done recursively to make full use of
sockets.  A browser could optimistically find import definitions and
start downloading before the full script body is loaded.

defineDelayedFunction would impose lazy fetching, but
modules_emaker_style allows eager fetching.

2010/1/14  <ihab.awad at>:
Hi Mike,

On Thu, Jan 14, 2010 at 1:29 PM, Mike Samuel <mikesamuel at>
Are these proposals mutually exclusive or complementary.
If (1) and (2) are exclusive, is there a place to collect use cases
for a module-off?

For my part, I think (1) and (2) are counterproposals, but there may
exist a bunch of cross-pollination. For example, I will probably end
up stealing Kris's "import with" syntax sometime.

I don't see the word module anywhere in the packages proposal besides
What is the relationship between packages and modules.

The packages proposal was written with module proposal (1) in mind,
but is compatible in the broad sense with either. It does say in the
beginning that it "... is intended to satisfy the Uniform location and
retrieval goal of modules_emaker_style". There are more details under
"Use in import" (where I just updated the syntax a little bit).


     Module proposal, by Ihab, focusing on EMaker-style invocation

"First class objects. To the extent possible, module system components
must be represented as first-class ECMAScript objects."
Does object exclude primitives such as strings, or is the goal to
exclude abstractions like LexicalScopes and References?

The point is that module system components are made to appear, to the
extent possible, as first-class objects rather than unmentionable
"static" magic. E.g., contrast Java where, if Foo is a class, the
expression "Foo" by itself is not an object; generic class parameters
are not objects; etc. -- which makes higher-order programming somewhat

"import" is added to the reserved keyword list?

It should be reserved now, and is for sure reserved according to this

"return {
 getX: function() { return x; },
 getY: function() { return y; },"

Fixed, tx.

"In the module’s code, this is initialized to a new object that is
instanceof the module function"
Does this mean that access to a module instance conveys authority to
create new module instances?

Yes. Just like any other ES function where it's return value is
instanceof that function.

"An import expression evaluates immediately to a module function
despite the fact that the specified module may have been fetched by
asynchronous means (e.g., nonblocking network operations). This is
done by taking advantage of the fact that import is a special form."
Does this require "import" to be a special form instead of a
desugaring?  Or can the special form be boiled down to a function
which when called, delays until another function is invoked,
presumably with a valid FunctionExpr string as its sole argument?

Would that require a continuation-passing transform of the code after
the desugared "import"? If so, that may not be semantics preserving.


Ihab A.B. Awad, Palo Alto, CA

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