Draft of spec for jspi #37
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document/js-api/index.bs
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| 1. If [=list/size=] of |ret| is 1 and [$IsPromise$](|ret|.\[[Value]][0]): | ||
| 1. Let |promise| be |ret|.\[[Value]][0]. | ||
| 1. Suspend |async_context|, removing it from the ambient [=Execution Context Stack=]. | ||
| 1. If |async_context| is not performing the steps in [=run a Promising function=] then trigger a WebAssembly trap. |
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Is there precedent for this kind of introspection on performed steps?
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Yes. This is based on a similar operation in ecmascript: https://tc39.es/ecma262/#await
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I don't know this precedent. Which step are you referring to in Await?
document/js-api/index.bs
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| 1. Let |rejected| be an [=AbstractClosure=] with parameters (|e|) that captures |async_context| and performs the following steps when called: | ||
| 1. Converts |e| to the WebAssembly exception |we|. | ||
| 1. Push |async_context| on to the [=ExecutionContext=] stack. | ||
| 1. Resume the suspended evaluation of asyncContext using throw |we| as the result of the operation that suspended it. |
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| 1. Resume the suspended evaluation of asyncContext using throw |we| as the result of the operation that suspended it. | |
| 1. Resume the suspended evaluation of |async_context| using throw |we| as the result of the operation that suspended it. |
Is this the correct behavior in the presence of WebAssembly exception handling? I would expect that when EH is available, a rejected promise would resume the Wasm with a thrown JS exception, not just skip the Wasm completely.
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Agreed. Plan to use language similar to that in EH js API text.
document/js-api/index.bs
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| 1. If [=list/size=] of |ret| is 1 and [$IsPromise$](|ret|.\[[Value]][0]): | ||
| 1. Let |promise| be |ret|.\[[Value]][0]. | ||
| 1. Suspend |async_context|, removing it from the ambient [=Execution Context Stack=]. | ||
| 1. If |async_context| is not performing the steps in [=run a Promising function=] then trigger a WebAssembly trap. |
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I don't know this precedent. Which step are you referring to in Await?
document/js-api/index.bs
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| 1. If |ret|.\[[Type]] is <emu-const>throw</emu-const>, then trigger a WebAssembly trap, and propagate |ret|.\[[Value]] to the enclosing JavaScript. | ||
| 1. If [=list/size=] of |ret| is 1 and [$IsPromise$](|ret|.\[[Value]][0]): | ||
| 1. Let |promise| be |ret|.\[[Value]][0]. | ||
| 1. Suspend |async_context|, removing it from the ambient [=Execution Context Stack=]. |
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I am deeply uncomfortable with this. I understand where @fgmccabe is coming from, and that the powers of the Suspend verb is underspecified in 262 itself. But I am not comfortable with the heavy lifting it's doing that it's able to effectively pause any embedder spec of ecma262. The current usage of Suspend in ecma262 itself is only thinks about suspension of 262 steps. If it had general suspension powers, then it would also be able to suspend in the middle of e.g. web API functions. That is not an implementable thing today.
This is a draft of a specification for jspi.