How to solve this basic ES6-module circular dependency problem?
/#!/JoePea
joe at trusktr.io
Wed Aug 10 19:55:42 UTC 2016
Isiah, here's the [rollup.js result](http://goo.gl/jl1B8H) using my setup
functions technique. When I paste the result in my console it complains
that A is undefined inside the `setUpA` function, which seems odd. Here's
the [result of my original code](http://goo.gl/cbjVOi) (similar to your
example), and as you can see it will evaluate B first in which case C will
be undefined and throw an error.
Bradley, true, but C is also child of A, so it can also make sense to
evaluate C before A. They are children of each other. In that case, what is
the correct order of evaluation?
*/#!/*JoePea
On Wed, Aug 10, 2016 at 12:47 PM, Bradley Meck <bradley.meck at gmail.com>
wrote:
> Please note that in https://tc39.github.io/ecma262/#sec-moduleevaluation
> , Modules evaluate their children prior to evaluating
> themselves (15.2.1.16.5.6.c) , C should never be evaluate before A or B
> in this dep graph.
>
> On Wed, Aug 10, 2016 at 2:41 PM, /#!/JoePea <joe at trusktr.io> wrote:
>
>> Oh! And although I think that my `setUpA` and `setUpB` technique should
>> work due to the fact that Webpack and Meteor load the modules in the exact
>> same order where the C module is executed last, this may in fact fail in
>> some other ES6 environment that happens to execute the C module first, in
>> which case `setUpA` and `setUpB` will be undefined when C is evaluated.
>>
>> So, I don't know if my solution is good. I am wondering if there's
>> something in the spec that guarantees that the C module evaluates last?
>>
>> */#!/*JoePea
>>
>> On Wed, Aug 10, 2016 at 12:38 PM, /#!/JoePea <joe at trusktr.io> wrote:
>>
>>> Isaiah, also note that
>>>
>>> ```js
>>> export default class Foo {}
>>> ```
>>>
>>> does not create a live binding that can be modified at a later point in
>>> time, which is the feature that my `setUpA` and `setUpB` functions are
>>> theoretically relying on (and which I believe the Meteor and Webpack
>>> environments don't handle properly if I understand live bindings correctly).
>>>
>>> */#!/*JoePea
>>>
>>> On Wed, Aug 10, 2016 at 12:31 PM, /#!/JoePea <joe at trusktr.io> wrote:
>>>
>>>> In your module B, `class B` should `extends C` instead of A, so both
>>>> classes `A` and `B` extend from `C`.
>>>>
>>>> I made a reproduction that you can run (assuming you have Meteor
>>>> installed). See the following with instructions:
>>>> https://github.com/meteor/meteor/issues/7621#issuecomment-238923360
>>>>
>>>> But, anyways, the example you just gave is almost identical to my
>>>> [original example](https://esdiscuss.org/topic/how-to-solve-this-basic
>>>> -es6-module-circular-dependency-problem#content-0) (except for my `B`
>>>> class extends from the `C` class).
>>>>
>>>> I can make a reproduction of that too if you want, but what happens is
>>>> that the environment will try to execute module A before executing module
>>>> C, at which point `C` is `undefined` inside of module `A`. Basically, the
>>>> result would be the same as writing:
>>>>
>>>> ```js
>>>> class A extends undefined {} // throws an Error
>>>> ```
>>>>
>>>> I noticed that both Meteor and Webpack will try and execute modules A
>>>> and B *first*, then finally C, so I thought I could export the setup
>>>> functions andrun them after defining the C class so that even if modules A
>>>> and B run first the actual class definitions would run after the C class
>>>> definition. You can see that I'm trying to take advantage of "live
>>>> bindings" in order for this to work (but it didn't, hence I have to pass C
>>>> into the setup functions).
>>>>
>>>> I have a feeling that both Meteor's and Webpack's implementations
>>>> aren't up-to-spec as far as live bindings with circular dependencies, but I
>>>> could be wrong.
>>>>
>>>> > You shouldn't need a `setUpA` export, especially called by one of its
>>>> dependencies. Just declare and initialize that crap when it's being
>>>> declared.
>>>>
>>>> That's what I thought at first too, but it's not the case, and I'm
>>>> trying to find a solution.
>>>>
>>>> */#!/*JoePea
>>>>
>>>> On Wed, Aug 10, 2016 at 2:41 AM, Isiah Meadows <isiahmeadows at gmail.com>
>>>> wrote:
>>>>
>>>>> First of all, I'll point out that even if it's an internal API, you
>>>>> should just initialize them immediately. You already have an otherwise
>>>>> fully initialized C, so you should just add them whenever it comes. You
>>>>> shouldn't need a `setUpA` export, especially called by one of its
>>>>> dependencies. Just declare and initialize that crap when it's being
>>>>> declared.
>>>>>
>>>>> ```js
>>>>> /* index.js */
>>>>> import A from './app/A'
>>>>> console.log('Entrypoint', A)
>>>>> ```
>>>>>
>>>>> ```js
>>>>> /* app/A.js */
>>>>> import C from './C'
>>>>>
>>>>> export default class A eclxtends C {
>>>>> // ...
>>>>> }
>>>>>
>>>>> // set up A here
>>>>> console.log('Module A')
>>>>> ```
>>>>>
>>>>> ```js
>>>>> /* app/B.js */
>>>>> import C from './C'
>>>>>
>>>>> export default class B extends A {
>>>>> // ...
>>>>> }
>>>>>
>>>>> // set up B here
>>>>> console.log('Module B')
>>>>> ```
>>>>>
>>>>> ```js
>>>>> /* app/C.js */
>>>>> import A from './A'
>>>>> import B from './B'
>>>>>
>>>>> export default class C {
>>>>> constructor() {
>>>>> // this may run later, after all three modules are evaluated,
>>>>> or
>>>>> // possibly never.
>>>>> console.log(A)
>>>>> console.log(B)
>>>>> }
>>>>> }
>>>>>
>>>>> // set up C
>>>>> console.log('Module C')
>>>>> ```
>>>>>
>>>>> What's your full output, anyways? That would help me best explain
>>>>> what's going on, though.
>>>>>
>>>>> On Wed, Aug 10, 2016, 02:47 /#!/JoePea <joe at trusktr.io> wrote:
>>>>>
>>>>>> When I try this same code with Webpack, I get the *exact same
>>>>>> results*: the `console.log` statements in the exact same order, where the
>>>>>> last output shows that `A` in the entry point is `undefined`).
>>>>>>
>>>>>> Am I misunderstanding something about live bindings? Is there some
>>>>>> guaranteed order in which these modules should be evaluated?
>>>>>>
>>>>>> The reason why I'm after a solution for the circular dependency is
>>>>>> because in my real-world case I need to use `instanceof A` and `intanceof
>>>>>> B` within the `C` superclass defined in module C. This is a case of the
>>>>>> Fragile Base Class Problem where a class should usually *not* have
>>>>>> knowledge of it's subclasses, but the base class in my case is intended to
>>>>>> be internal only, not a part of the public API that end users will extend
>>>>>> from.
>>>>>>
>>>>>> */#!/*JoePea
>>>>>>
>>>>>> On Tue, Aug 9, 2016 at 11:12 PM, /#!/JoePea <joe at trusktr.io> wrote:
>>>>>>
>>>>>>> I can get the whole thing to work if I pass the C dependency into
>>>>>>> the `setUpA` and `setUpB` functions as follows, but oddly `A` is
>>>>>>> `undefined` in the Entrypoint module at the `console.log` statement, which
>>>>>>> makes it seem to me like live bindings aren't working the I was expecting.
>>>>>>>
>>>>>>> ```js
>>>>>>> // --- Entrypoint
>>>>>>>
>>>>>>> import A from './app/A'
>>>>>>> console.log('Entrypoint', A) // HERE, output: "Entrypoint undefined"
>>>>>>> ```
>>>>>>>
>>>>>>> ```js
>>>>>>> // --- Module A
>>>>>>>
>>>>>>> import C from './C'
>>>>>>>
>>>>>>> let A
>>>>>>>
>>>>>>> export
>>>>>>> function setUpA(C) {
>>>>>>>
>>>>>>> console.log('setUpA')
>>>>>>> console.log(C)
>>>>>>>
>>>>>>> A = class A extends C {
>>>>>>> // ...
>>>>>>> }
>>>>>>>
>>>>>>> }
>>>>>>>
>>>>>>> console.log('Module A', C, setUpA)
>>>>>>>
>>>>>>> export {A as default}
>>>>>>> ```
>>>>>>>
>>>>>>> ```js
>>>>>>> // --- Module B
>>>>>>>
>>>>>>> import C from './C'
>>>>>>>
>>>>>>> let B
>>>>>>>
>>>>>>> export
>>>>>>> function setUpB(C) {
>>>>>>>
>>>>>>> console.log('setUpB', C)
>>>>>>>
>>>>>>> B = class B extends C {
>>>>>>> // ...
>>>>>>> }
>>>>>>>
>>>>>>> }
>>>>>>>
>>>>>>> console.log('Module B', C, setUpB)
>>>>>>>
>>>>>>> export {B as default}
>>>>>>> ```
>>>>>>>
>>>>>>> ```js
>>>>>>> // --- Module C
>>>>>>>
>>>>>>> import A, {setUpA} from './A'
>>>>>>> import B, {setUpB} from './B'
>>>>>>>
>>>>>>> let C = class C {
>>>>>>> constructor() {
>>>>>>> // this may run later, after all three modules are
>>>>>>> evaluated, or
>>>>>>> // possibly never.
>>>>>>> console.log(A)
>>>>>>> console.log(B)
>>>>>>> }
>>>>>>> }
>>>>>>>
>>>>>>> setUpA(C)
>>>>>>> console.log('Module C', A)
>>>>>>>
>>>>>>> setUpB(C)
>>>>>>> console.log('Module C', B)
>>>>>>>
>>>>>>> export {C as default}
>>>>>>> ```
>>>>>>>
>>>>>>> */#!/*JoePea
>>>>>>>
>>>>>>> On Tue, Aug 9, 2016 at 9:59 PM, /#!/JoePea <joe at trusktr.io> wrote:
>>>>>>>
>>>>>>>> It seems that the environment I'm in (Meteor uses [reify](
>>>>>>>> https://github.com/benjamn/reify)) tries to evaluate A and B
>>>>>>>> first, so I thought I could take advantage of "live bindings" by changing
>>>>>>>> my modules to the following:
>>>>>>>>
>>>>>>>> ```js
>>>>>>>> // --- Entrypoint
>>>>>>>>
>>>>>>>> import A from './app/A'
>>>>>>>> console.log('Entrypoint', A)
>>>>>>>> ```
>>>>>>>>
>>>>>>>> ```js
>>>>>>>> // --- Module A
>>>>>>>>
>>>>>>>> import C from './C'
>>>>>>>>
>>>>>>>> let A
>>>>>>>>
>>>>>>>> export
>>>>>>>> function setUpA() {
>>>>>>>>
>>>>>>>> console.log('setUpA')
>>>>>>>> console.log(C)
>>>>>>>>
>>>>>>>> A = class A extends C {
>>>>>>>> // ...
>>>>>>>> }
>>>>>>>>
>>>>>>>> }
>>>>>>>>
>>>>>>>> console.log('Module A', C, setUpA)
>>>>>>>>
>>>>>>>> export {A as default}
>>>>>>>> ```
>>>>>>>>
>>>>>>>> ```js
>>>>>>>> // --- Module B
>>>>>>>>
>>>>>>>> import C from './C'
>>>>>>>>
>>>>>>>> let B
>>>>>>>>
>>>>>>>> export
>>>>>>>> function setUpB() {
>>>>>>>>
>>>>>>>> console.log('setUpB', C)
>>>>>>>>
>>>>>>>> B = class B extends C {
>>>>>>>> // ...
>>>>>>>> }
>>>>>>>>
>>>>>>>> }
>>>>>>>>
>>>>>>>> console.log('Module B', C, setUpB)
>>>>>>>>
>>>>>>>> export {B as default}
>>>>>>>> ```
>>>>>>>>
>>>>>>>> ```js
>>>>>>>> // --- Module C
>>>>>>>>
>>>>>>>> import A, {setUpA} from './A'
>>>>>>>> import B, {setUpB} from './B'
>>>>>>>>
>>>>>>>> let C = class C {
>>>>>>>> constructor() {
>>>>>>>> // this may run later, after all three modules are
>>>>>>>> evaluated, or
>>>>>>>> // possibly never.
>>>>>>>> console.log(A)
>>>>>>>> console.log(B)
>>>>>>>> }
>>>>>>>> }
>>>>>>>>
>>>>>>>> setUpA()
>>>>>>>> console.log('Module C', A)
>>>>>>>>
>>>>>>>> setUpB()
>>>>>>>> console.log('Module C', B)
>>>>>>>>
>>>>>>>> export {C as default}
>>>>>>>> ```
>>>>>>>>
>>>>>>>> As you can see, modules A and B simply export the code that should
>>>>>>>> be evaluated (note the live bindings). Then finally, the C module is
>>>>>>>> evaluated last. At the end of the C module, you see that it calls `setUpA`
>>>>>>>> and `setUpB`. When it fires `setUpA`, an error is thrown on the second
>>>>>>>> `console.log` that `C` is undefined (or, specifically, `C.default` is
>>>>>>>> `undefined` because the ES6 modules are compiled into CommonJS form).
>>>>>>>>
>>>>>>>> I thought that if `C` was a live binding, then it should be ready
>>>>>>>> by the time the `setUpA` function is called. Should this in fact be the
>>>>>>>> case?
>>>>>>>>
>>>>>>>> */#!/*JoePea
>>>>>>>>
>>>>>>>> On Tue, Aug 9, 2016 at 5:36 PM, John Lenz <concavelenz at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Without a way to load "later" (aka "soft") dependencies, ES6
>>>>>>>>> module will continue to be more or less broken for circular dependencies.
>>>>>>>>>
>>>>>>>>> On Tue, Aug 9, 2016 at 4:11 PM, Tab Atkins Jr. <
>>>>>>>>> jackalmage at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> On Tue, Aug 9, 2016 at 4:00 PM, /#!/JoePea <joe at trusktr.io>
>>>>>>>>>> wrote:
>>>>>>>>>> > True, and so that's why I'm wondering if the module system can
>>>>>>>>>> see that it
>>>>>>>>>> > can satisfy all module requirements if it simply evaluates
>>>>>>>>>> module C first,
>>>>>>>>>> > followed by A or B in any order. It is easy for us humans to
>>>>>>>>>> see that. It
>>>>>>>>>> > would be nice for the module system to see that as well (I'm
>>>>>>>>>> not sure if
>>>>>>>>>> > that is spec'd or not).
>>>>>>>>>>
>>>>>>>>>> That knowledge requires, at minimum, evaluating the rest of each
>>>>>>>>>> module, beyond what is expressed in the `import` statements.
>>>>>>>>>> That's
>>>>>>>>>> assuming there's no dynamic trickery going on that would
>>>>>>>>>> invalidate
>>>>>>>>>> whatever assumptions it can draw from surface-level analysis.
>>>>>>>>>>
>>>>>>>>>> Because of this, only the `import` statements are declaratively
>>>>>>>>>> available to the module system to work with. Based on that, it
>>>>>>>>>> definitely can't make any ordering assumptions; all it knows is
>>>>>>>>>> that A
>>>>>>>>>> imports C, B imports C, and C imports both A and B, making a
>>>>>>>>>> circular
>>>>>>>>>> import.
>>>>>>>>>>
>>>>>>>>>> ~TJ
>>>>>>>>>> _______________________________________________
>>>>>>>>>> es-discuss mailing list
>>>>>>>>>> es-discuss at mozilla.org
>>>>>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>> _______________________________________________
>>>>>> es-discuss mailing list
>>>>>> es-discuss at mozilla.org
>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>>>
>>>>>
>>>>
>>>
>>
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>>
>
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