address comment (#1609)

kainino0x · web-flow · commit 7b729db62ee6 · 2021-04-08T14:35:11.000-07:00
diff --git a/explainer/index.bs b/explainer/index.bs
@@ -234,10 +234,21 @@ that occur. To solve this, WebGPU uses *Error Scopes*.
 
 ### Error Scopes ### {#errors-errorscopes}
 
+WebGL exposed errors using a `getError` function returning the first error the last `getError` call.
+This is simple, but has two problems.
+
+- It is synchronous, incurring a round-trip and requiring all previously issued work to be finished.
+    We solve this by returning errors asynchronously.
+- Its flat state model composes poorly: errors can leak to/from unrelated code, possibly in
+    libraries/middleware, browser extensions, etc. We solve this with a stack of error "scopes",
+    allowing each component to hermetically capture and handle its own errors.
+
 Each device<sup>1</sup> maintains a persistent "error scope" stack state.
 Initially, the device's error scope stack is empty.
 `GPUDevice.pushErrorScope('validation')` or `GPUDevice.pushErrorScope('out-of-memory')`
 begins an error scope and pushes it onto the stack.
+This scope captures only errors of a particular type depending on the type of error the application
+wants to detect.
 
 `GPUDevice.popErrorScope()` ends an error scope, popping it from the stack and returning a
 `Promise<GPUError?>`, which resolves once all enclosed fallible operations have completed and
@@ -246,11 +257,6 @@ It resolves to `null` if no errors were captured, and otherwise resolves to an o
 the first error that was captured by the scope - either a `GPUValidationError` or a
 `GPUOutOfMemoryError`.
 
-An error scope captures an error if its filter (`'validation'` or `'out-of-memory'`) matches the
-type of the error.
-This filter mechanism avoids, e.g., accidentally silencing validation errors when trying to
-implement fallible allocation.
-
 Any device-timeline error from an operation is passed to the top-most error scope on the stack at
 the time it was issued.
 
