Component Model & WIT IDL

RFCs/component-model-and-wit.md

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+# RFC: Component Model & WIT Guest/Host Support
+
+## Overview
+
+- I propose the compatibility of component model and wit for moonbit.
+- Using component model in Moonbit's build.
+- Moonbit as a wit host.
+
+This might be a too large proposition. Depending on the requirements, it might be better to generate and divide host, guest, and wit idl.
+
+## Why
+
+Moonbit is a language with wonderful expressive power, but the built `.wasm` only has a numerical interface in accordance with the specification of the simple WebAssembly interface.
+
+Especially for the interoperability between JavaScript/TypeScript and Moonbit, I thought it would be nice to have a correspondence with the component model.
+
+## About WebAssembly Component Model & WIT
+
+(This section is unnecessary for those who already have knowledge)
+
+The component model is a specification for the interoperability of wasm binaries, and wit is an IDL for the component model.
+
+```wit
+package component:my-wit;
+world example {
+    export add: func(a: s32, b: s32) -> s32;
+}
+```
+
+Below is an example of implementing Rust's Guest with cargo component and running JS as Host with jco.
+
+```bash
+$ cargo component new --reactor my-wit && cd my-wit
+$ cargo component build --target wasm32-unknown-unknown --release
+# Omitting the implementation of add
+$ npx jco transpile target/wasm32-unknown-unknown/release/my_wit.wasm -o out
+$ deno eval 'import("./out/my_wit.js").then(({add})=>console.log(add(1,2)))'
+3
+```
+
+Please refer to the following for details
+
+https://component-model.bytecodealliance.org/language-support/rust.html
+https://component-model.bytecodealliance.org/language-support/javascript.html
+
+## Proposal: moonbit as wit guest
+
+This is an implementation for moonbit to be run from other environments.
+
+It is necessary to embed the definition of the component model in the `.wasm` binary generated by moonbit.
+
+https://github.com/WebAssembly/component-model/blob/main/design/mvp/Binary.md#component-definitions
+https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md
+
+Wat Example
+
+```
+(component
+  (core module (;0;)
+    (type (;0;) (func))
+    (type (;1;) (func (param i32 i32) (result i32)))
+    (type (;2;) (func (param i32 i32 i32 i32) (result i32)))
+```
+
+There are two choices for implementation.
+
+1. `pub fn` or `pub enum` `pub struct` generates component model and wit from moonc
+2. (Like cargo component), first define world.wit, generate `trait` code from there, and the user implements to meet the trait
+
+Personally, I prefer 1, but if the expressive power of moonbit and wit do not mesh well, there may be problems. The generation of a wit file is not mandatory, but the component-model written in wasm has low readability and needs to be read out by other tools.
+
+I assume that the component model-compatible `.wasm` will be used as follows.
+
+```js
+$ moon build --target wasm-gc
+$ npx jco transpile target/wasm-gc/release/build/mylib/mylib.wasm -o out
+```
+
+Here is an image of the actual output wit.
+
+```
+// lib.mbt
+struct Point {
+  x: Int
+  y: Int
+}
+pub fn new_point(x: Int, y: Int) -> Point {
+  Point::{x, y}
+}
+```
+
+Generated wit
+
+```wit
+package component:mylib;
+world example {
+    record Point {
+        x: s32,
+        y: s32,
+    }
+    new_point: func(a: s32, b: s32) -> Point;
+}
+```
+
+The generated type should be a subset of moonbit's expressive power. Depending on the case, it may be necessary to put some constraints.
+
+## Proposal: moonbit as wit host
+
+This is an implementation to load other `.wasm` implemented with the component model.
+
+You need to parse the component-model or WIT IDL and generate glue code for moonbit.
+
+This is the glue code `out/my_wit.js` generated by the previous cargo component + jco.
+
+```js
+// Excerpt from out/my_wit.js
+function id(arg0) {
+  var ptr0 = utf8Encode(arg0, realloc0, memory0);
+  var len0 = utf8EncodedLen;
+  const ret = exports0.id(ptr0, len0);
+  var ptr1 = dataView(memory0).getInt32(ret + 0, true);
+  var len1 = dataView(memory0).getInt32(ret + 4, true);
+  var result1 = utf8Decoder.decode(new Uint8Array(memory0.buffer, ptr1, len1));
+  postReturn0(ret);
+  return result1;
+}
+// ...
+const $init = (async() => {
+  const module0 = fetchCompile(new URL('./my_wit.core.wasm', import.meta.url));
+  ({ exports: exports0 } = await instantiateCore(await module0));
+  memory0 = exports0.memory;
+  realloc0 = exports0.cabi_realloc;
+  postReturn0 = exports0.cabi_post_id;
+})();
+```