Loader Hooks
Calvin, I don't fully understand what you mean by "manipulate the exports and imports". I assume you're talking about native modules since you already have full control over the dynamic modules workflow. Maybe Guy Bedford can provide more details on how he implemented this process in es6-module-loader.
Say you wanted to add a hook which automatically added an export named filepath to a module is the path to the file, or add an import just for the side effects (say a shim or something). The relevant code in es6 module loader is ModuleLoader/es6-module-loader/blob/master/lib/loader.js#L181
Guy Bedford's es6-module-loader follows the Loader spec closely. I just
refactored it to create a function call parse(load) -> deps:
ModuleLoader/es6-module-loader/blob/master/lib/loader.js#L389
This location in the pipeline is pretty much where you suggest. I think I've convinced Guy to let me add it as a hook. Maybe your request will push him over the edge ;-)
Note that as far as I can tell, this function call provides a neat interface between the Loader and parser.
No no, neither of those are real use-cases for the hooks IMO. Native modules are immutable, and you're not suppose to add new named exports dynamically because you loose the ability to statically verify them. In the case of the "path to file" (we call it address), you will have access to it thru the module metas (API pending to be defined). In the case of shims, you can do that by implementing a loader extension that uses a dynamic module to shim the access to a native one, the way we do this today is by hooking into the normalization process for dependencies, and replace the dep with the shim dep, e.g.:
- "foo" imports "bar"
- "bar-shim" imports "bar"
- "bar-shim" wraps all exports of "bar"
- the loader extension wire "foo" to consume "bar-shim" instead of "bar".
The same principle applies for alias, buckets, etc.
sorry I by shims I meant something like the es5-shim ( es-shims/es5-shim) module as an example of something to import that only had side effects, as for exports examples a hook that looks for default exports of objects and added the objects properties as named exports might be an example (aka a loader hook to do what is being discussed right now on the other thread) or a hook to allow you to make a mutable default export
You are 100% correct that this screws up static analysis so it might be better to think in terms of creating dynamic modules using the native parser function then in terms of mutating modules, i.e. more like my third suggestion then my second.
We're also discussing ways of making these platform / environment dependencies work nicely.
The hooks are asynchronous, so it is actually possible to import a module from within a hook like the locate hook in this case.
Some combination of meta information and the above seems like it would work.
On Tuesday, June 24, 2014 11:38 AM, Calvin Metcalf <calvin.metcalf at gmail.com> wrote:
Say you wanted to add a hook which automatically added an export named filepath to a module is the path to the file
For this use case and other similar ones it'd be nice if the metadata property of load records would be exposed as part of the modules metadata in this or Reflect.currentModule().
or add an import just for the side effects (say a shim or something) We've been discussing conditional loading for shims and other similar stuff at systemjs/systemjs#9.
Another helpful place for a hook might be an executeDependencies type thing, currently the list of dependencies returned by instantiate are all loaded and ensureEvaluated is run on them, this makes sense for ES6 modules which always evaluate all dependencies before the module that depends on them, but it means you can't do an alternative evaluation strategy like running ensureEvaluated when the module is imported in the code (when using loading a module from some other system that works that way obviously), currently it's impossible to do that without loading your dependencies through some side channel which would likely break static checking of those dependencies.
I've been doing work with the loader hooks and one gap that stands out is that there is no hook to let you manipulate the exports and imports of an module without parsing it yourself, in other words if you want to add, remove, or modify exports or imports of a module you have to write your own parsing function because the default instantiate function returns undefined.
Ideas: