System.register module name update

[guybedford]

Since posting #6097, we've decided to alter the format of System.register (while we still can!), to rather use a module context object as the second argument:

So that:

export let name = __moduleName;

Becomes:

System.register([], function($export, $moduleContext) {
  return {
    setters: [],
    execute: function() {
      $export('name', $moduleContext.id);
    }
  };
});

instead of:

System.register([], function($export, __moduleName) {
  return {
    setters: [],
    execute: function() {
      $export('name', __moduleName);
    }
  };
});

This is so that this context argument can potentially be amended in future with additional metadata if and when this is specified at any point in future in the language.

My knowledge of TypeScript internals is quite limited, so I wasn't able to quickly get together a transformer to replace __moduleName with $moduleContext.id, but this should be relatively straightforward to do.

In the mean time, it may be worth reverting #6097 to avoid unnecessary breaking changes here.

This corresponds to Traceur PR google/traceur-compiler#2051 and Babel PR babel/babel#3166.

Apologies for any confusion here!

[mhegazy]

@guybedford feel free to send a PR when the design has stabilized on the SystemJs side.

[guybedford]

@mhegazy many thanks for understanding! We have the context object implementations merged into Traceur and Babel now (google/traceur-compiler#2051 and babel/babel#3166), so it would be great to get this in TypeScript. This would involve replacing instances of __moduleName with a _context.id member expression. I can try and give this a go again, but any tips for how to put such a transformation together would be very much appreciated too.

[mhegazy]

@guybedford any news about this?

[guybedford]

@mhegazy I'm still not sure what the best way would be to implement __moduleName identifier replacement within the existing TypeScript implementation. If someone can give me some pointers to how to go about this, I'll gladly dive in.

[vladima]

@guybedford out of curiosity, will it be simpler if instead of replacing __moduleName with _context.id we'll inject var __moduleName = _context.id at the very beginning of the rewritten module?

[guybedford]

@vladima yes that would work! In the Babel and Traceur implementations, we only do this rewriting when needed to keep the output simple, but it works just as well.

[guybedford]

I've implemented the above suggestion in #6688.