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fix: async examples #522

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lukewagner merged 1 commit into from
Jun 2, 2025
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fix: async examples #522

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37 changes: 25 additions & 12 deletions design/mvp/Async.md
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Original file line number Diff line number Diff line change
Expand Up @@ -729,8 +729,11 @@ the core wasm toolchain) `$retp` must be a 4-byte-aligned pointer into linear
memory from which the 8-byte (pointer, length) of the string result can be
loaded.

The async export ABI provides two flavors: stackful and stackless. The stackful
ABI is currently gated by the 🚟 feature.
The async export ABI provides two flavors: stackful and stackless.

##### Stackful Async Exports

The stackful ABI is currently gated by the 🚟 feature.

The async stackful export function signature is:
```wat
Expand All @@ -749,6 +752,8 @@ WIT parameter type of a synchronous function. For example, if more than 16
core parameters would be needed, a single `i32` pointer into linear memory is
used.

##### Stackless Async Exports

The async stackless export function signature is:
```wat
;; async, callback
Expand Down Expand Up @@ -789,7 +794,9 @@ This rest of this section sketches the shape of a component that uses `async`
to lift and lower its imports and exports with both the stackful and stackless
ABI options.

Starting with the stackless ABI, the meat of this example component is replaced
### Stackful ABI example

Starting with the stackful ABI, the meat of this example component is replaced
with `...` to focus on the overall flow of function calls:
```wat
(component
Expand All @@ -800,8 +807,8 @@ with `...` to focus on the overall flow of function calls:
...
)
(core module $Main
(import "libc" "mem" (memory 1))
(import "libc" "realloc" (func (param i32 i32 i32 i32) (result i32)))
(import "" "mem" (memory 1))
(import "" "realloc" (func (param i32 i32 i32 i32) (result i32)))
(import "" "fetch" (func $fetch (param i32 i32 i32) (result i32)))
(import "" "waitable-set.new" (func $new_waitable_set (result i32)))
(import "" "waitable-set.wait" (func $wait (param i32 i32) (result i32)))
Expand Down Expand Up @@ -876,13 +883,19 @@ call to `waitable-set.wait` blocks, the runtime will suspend its callstack
entry point and store it in context-local storage (via `context.set`) instead
of simply using a `global`, as in a synchronous function.

This same example can be re-written to use the `callback` immediate (thereby
avoiding the need for fibers) as follows. Note that the internal structure of
this component is almost the same as the previous one (other than that
`summarize` is now lifted from *two* core wasm functions instead of one) and
the public signature of this component is the exact same. Thus, the difference
is just about whether the stack is cleared by the core wasm code between events,
not externally-visible behavior.
### Stackless ABI example

The stackful example can be re-written to use the `callback` immediate (thereby
avoiding the need for fibers) as follows.

Note that the internal structure of this component is almost the same as the
previous one (other than that `summarize` is now lifted from *two* core wasm
functions instead of one) and the public signature of this component is
the exact same.

Thus, the difference is just about whether the stack is cleared by the
core wasm code between events, not externally-visible behavior.

```wat
(component
(import "fetch" (func $fetch (param "url" string) (result (list u8))))
Expand Down