Document proposed WIT to enable component Worlds

This PR sketches out the initial format for WebAssembly profiles,
i.e. world files.

Signed-off-by: Brian H <[email protected]>

design/mvp/WORLD.md

@@ -0,0 +1,125 @@
+# WIT + `world`
+
+As it exists today [WIT](https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md) is a format for describing the interface of a singular [instance](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#instance-definitions). Toolchains like [wit-bindgen](https://github.com/bytecodealliance/wit-bindgen) consume `*.wit` files as *imports* or *exports* to generate *guest* or *host* components implying **4** ways to consume an interface:
+
+| {guest,host}⋅{export, import} | export | import |
+| -------- | -------- | -------- |
+| **guest**     | _guest export_     | _guest import_    |
+| **host**     | _host export_     | _host import_     |
+
+Furthermore, a single component may refer to 0..N interfaces in its imports and exports.
+
+The `world` syntax described below allows collecting *N* _imports_ and *N* _exports_ into a single definition reducing this number of perspectives to **2**:
+1. As a *guest* **targeting** the `world`
+
+    From the perspective of a guest the *imports* declared in a `world` describe the **maximum** set of imports that the component may `import`, whereas the *exports* describe the **minimum** set of imports that the component must `export`. 
+
+2. A a *host* **supporting** the `world`
+
+   From the perspective of a host, the *imports* declared in a `world` describe the **minimum** set of functions that must be supported by the host and supplied as imports to components running on that host, whereas the exports declared describe the **maximum** set of functions that the host may call in response to host events. 
+
+Thus, there are inherently two opposing perspectives on each `world`: the **host** and **guest**.
+
+> NOTE: As a developer, assuming the *guest* or *host* perspective is inherent in whether we are embedding a WASM runtime or compiling to WASM. so a world effectively gives us 1 thing to say, and it implies all the rest, i.e. (imports and exports).
+> 
+> Because the [Component Model](https://github.com/WebAssembly/component-model) allows one component to [instantiate]() and link other components, components can technically take either the host or guest perspective for a given `world` (although it will be far more common for components to take the guest perspective). Native host runtimes traditionally only take the host perspective, but in theory a native runtime could implement a component that is imported and instantiated by a wasm component, with the native runtime thereby taking the guest perspective.
+
+In the examples shown below, because *import* and *export* in the syntax go in the same direction as they are defined in a guest component, we can say that a `world` is **written** from the *guest* perspective. 
+
+
+---
+
+The lexical structure of the following syntax is identical to that of [WIT](https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md#lexical-structure) with the following additions:
+
+- keywords ```world import export```
+- a new token `quoted-string`
+
+## `world`
+A `world` declaration defines a set of imports and exports. From a component-model POV a `world` is syntax for a [componenttype](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#type-definitions), e.g.:
+
+```
+// wasi:http/proxy 
+world proxy {
+    import frontend: "wasi:http/handler"
+    export backend: "wasi:http/handler"
+}
+```
+
+By way of metaphor, a `world` is like a function type:
+   * `world` imports and exports are like parameters and results
+   * `world` hosts are like the callers of a given function type
+   * `world` guests are like functions that have a given function type
+   * The rules for when a given guest is compatible with a given host are like traditional function subtyping
+
+> Given any component `.wasm`, one should be able to derive a `WIT` file containing a `world` that represents the syntax for that `.wasm`'s component type. 
+
+
+Conceretely, the structure of a `world` declaration is:
+
+```
+world-item ::= 'world' id '{' world-items* '}' .
+
+world-items ::= import-item | export-item .
+```
+
+### `import`
+
+An `import` declaration states that a component type depends on an externally provided [function](https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md#item-func), [value](https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md#types), instance of an interface type, or component type.
+
+For example, consider the following `import` declarations:
+
+```
+world {
+    // Import instance of "wasi:filesystem" interface and give it name "fs"
+    import fs: "wasi:filesystem"
+
+    // Import function with functype and give it name "some-func"
+    import some-func: func(arg: string) -> result<_, _>
+
+    // Import the inlined interface and give it name "foobar"
+    import foobar: interface {
+        foo: func()
+        bar: func()
+    }
+}
+
+// ...
+```
+
+Concretely, the structure of an `import` declaration is:
+
+```
+import-item ::= 'import' id ':' import-type .
+import-type ::= id | extern-type .
+extern-type ::= wit-url | func-type | value-type | instance-type | component-type .
+
+instance-type ::= 'interface' '{' interface-items* '}' .
+
+component-type ::= 'world' '{' world-items* '}' .
+```
+
+> NOTE: `wit-url` is lexically a `quoted-string` describing a [URL](https://url.spec.whatwg.org/) resolving to a `*.wit` document.
+
+### `export`
+
+An `export` declaration states that a component type can provide a instance of a [function](https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md#item-func), [value](https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md#types), instance of an interface type, or component type.
+
+For example:
+
+```
+world {
+    // Export an instance of the "wasi:http/handler" interface.
+    export frontend: "wasi:http/handler"
+}
+
+// ...
+```
+
+Concretely, the structure of an `export` statement is:
+
+```
+export-item ::= `export` id `:` export-type .
+export-type ::= id | extern-type .
+```
+
+> NOTE: `extern-type` as specified in `import`.