WIT should have the same expressivity power as the component model

[fitzgen]

My mental model has been that WIT is "just" syntax sugar¹ on top of the component model: anything expressible in a raw component model interface should also be expressible in WIT.

But this is not actually the case today. I strongly believe it should be.

The biggest counterexample I am aware of is #287.

My hopes in filing this issue are two-fold:

1. Build consensus that "anything expressible in a raw component model interface should also be expressible in WIT" is a highly-desirable property. Does anyone disagree? Can we formalize this into one of the design docs in this repo or something?

2. We can do a collective survey/audit of CM features and exhaustively enumerate WIT's expressivity gaps. Is anyone aware of any other counterexamples?

Footnotes

1. Curly brackets instead of S-expressions, stuff that helps bindings generators but don't affect semantics like doc comments, a package system to spread definitions across files, etc... ↩

[lukewagner]

I totally agree that full co-expressivity is a goal for WIT and think it does belong on the "todo list" for 1.0. Because this is a purely WIT-level feature, unless someone is motivated to work on it sooner (which would be great!), it would naturally go near the end, after other runtime-impacting features.

FWIW, I don't believe #287 is an example of a feature gap: raw CM types have the same constraints as WIT (name uniqueness means only one import/export with the same interfacename).

There are however a number of other gaps to fill:

• expressing non-default resource-type-sharing behavior (Split 'use' inside worlds into 'use import' and 'use export' #308) (this is the big one, and felt)
• expressing nested plain-named instance types (Nested interfaces #372)
• exposing "dependency" and "URL" names directly in WIT (Unlocked deps #393)
• importing/exporting types directly (not wrapped in an interface)

I expect there are others too. An easy way to find missing cases is to harvest every component type we can find in the WAST tests and roundtrip them through WIT.