Module isolation

Brendan Eich brendan at
Mon Jan 18 12:14:30 PST 2010

On Jan 18, 2010, at 11:58 AM, Mark S. Miller wrote:

> On Mon, Jan 18, 2010 at 10:25 AM, Brendan Eich <brendan at>  
> wrote:
> On Jan 18, 2010, at 10:20 AM, Mark S. Miller wrote:
>> I simply don't understand what you mean by the phrase "migrate into  
>> a module". What does this mean?
> Use on the inside of a module. I have code I want to put in a  
> module. It uses Prototype. That's all.
> How would you express "putting code in a module"? I just don't have  
> a concrete idea what you mean. Could you show a program fragment?

Copy and paste. I copy prototype- into mybigfatmodule.js,  
add my special sauce which makes good use of Prototype's extensions to  
primordials such as Array.prototype, and then purvey it to the world.

> Why is this hard to understand? Are you assuming modules are small,
> Or at least, that module boundaries coincide with boundaries between  
> separately written source files, as is the case with module systems  
> I'm familiar with from other languages. For example, Java classes-as- 
> modules.

Nothing prevents people from inline-expanding Prototype into their  
code, crashing and carrying whatever they want. The codesearch hits I  
linked earlier shows instances, including v8's raytrace.js benchmark.

Would a raytracer with inline-expanded, possibly hand-minimized  
Prototype make a good module? Maybe, maybe not; I'm not judging, just  
noticing that people do this sort of thing today on the Web. Modules  
might reform all bad habits, but probably not -- and developers don't  
all agree on your definition of "bad".

> and/or consisting of new code only?
> Or at least a new revision of old code, in order to repackage the  
> code to do module-based import/export linkage, rather than the  
> current practice of implicit linkage through shared global variables.

Precedent and developer conversations I've had strongly suggest that  
some code wants mutable primordials on the inside of a module that can  
be consumed without the mutations affecting the importer's primordials.

> Since I do not yet understand what you are trying to say, I may be  
> the wrong person to guess why I'm finding it hard to understand ;).  
> Altogether, my best guess right now is that what you are calling a  
> module is what I would call a sandbox.

Could be. Is there a hard difference? It's not obvious.

Users definitely want isolated frame-like "modules". Some browsers  
even support such things, with postMessage for safe inter-"module"  
communication. It's a bit heavyweight, but to get back to the topic of  
this sub-thread: is the only alternative frozen primordials? I think  

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