Add 2025-09 slides

Author: mhofman

slides/2025-09-stage-1.md

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+title: Native promise predicate for Stage 1 or 2
+info: |
+  Proposal for exposing a native promise predicate
+class: text-center
+drawings:
+  persist: false
+# slide transition: https://sli.dev/guide/animations.html#slide-transitions
+transition: fade
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+  local: ["Consolas", "Fira Code", "monospace"]
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+---
+
+# Native promise predicate <br /> for Stage 1 (or 2)
+
+Mathieu Hofman (@mhofman)
+
+---
+
+# Motivation
+
+Native promises are recognized and don't use `.then` in some operations.
+
+<v-click>
+
+Native promises
+
+```js {1-4|*|*|*|5}
+const promise = Promise.resolve(42);
+typeof promise.then === 'function';
+
+const v = await promise; // does not call promise.then
+Promise.resolve(promise) === promise; // pass-through
+```
+
+</v-click>
+
+<div v-click="3">
+
+Thenables
+
+```js {1-3|*|4}{at:4}
+const thenable = { then(onFulfilled) { onFulfilled(42) } };
+
+const v = await thenable; // does call thenable.then
+Promise.resolve(thenable) !== thenable; // does not pass-through, calls thenable.then, allocates new promise
+```
+
+</div>
+
+<div v-click="5">
+
+--> Cannot build a side-effect-free predicate
+
+</div>
+
+---
+
+# Precedents
+
+<v-clicks>
+
+* `Error.isError`
+  * pure brand check
+  * implies some stack information is associated
+* `Array.isArray`
+  * pierces proxy
+  * indicates when object treated as array, e.g. in `JSON.serialize`
+
+</v-clicks>
+
+---
+
+# Proposal
+
+Predicate that cleanly exposes `IsPromise` AO
+
+<v-clicks>
+
+* `Promise.isPromise` ?
+
+* `Promise.isNativePromise` ?
+
+* Some other less ergonomic but clean brand check ?
+
+</v-clicks>
+
+<v-click>
+
+## Membrane transparency
+
+<v-clicks at="5">
+
+* Predicate does not pierce proxies
+  * Like `Error.isError`
+* It's already possible to brand check promises with side-effects
+* It's better for membranes to create a new native promise (pass by copy)
+  * Need a predicate to know which thenables are native promises
+
+</v-clicks>
+
+</v-click>
+
+---
+layout: section
+---
+# Stage 1?
+
+<br/>
+
+Predicate for native promises
+
+---
+layout: section
+---
+# Stage 2?
+
+<br/>
+
+`Promise.isXXX()`
+
+[Spec text](/)
+
+`isPromise` ? `isNativePromise` ?