add from() spec

spec.bs

@@ -26,6 +26,12 @@ WPT Display: open
 urlPrefix: https://tc39.es/ecma262/#; spec: ECMASCRIPT
   type: dfn
     text: current realm
+    text: IsPromise; url: sec-ispromise
+    text: GetMethod; url: sec-getmethod
+    text: GetIteratorFromMethod; url: sec-getiteratorfrommethod
+    text: IteratorStepValue; url: sec-iteratorstepvalue
+    text: normal completion; url: sec-normalcompletion
+    text: throw completion; url: sec-throwcompletion
 urlPrefix: https://dom.spec.whatwg.org; spec: DOM
   type: dfn
     for: event listener
@@ -37,6 +43,13 @@ urlPrefix: https://dom.spec.whatwg.org; spec: DOM
     for: AbortSignal
       text: dependent signals; url: abortsignal-dependent-signals
       text: signal abort; url:abortsignal-signal-abort
+urlPrefix: https://webidl.spec.whatwg.org; spec: WEBIDL
+  type: dfn
+    text: a promise rejected with
+  type: dfn
+    text: Upon fulfillment
+  type: dfn
+    text: Upon rejection
 </pre>
 
 <style>
@@ -354,7 +367,7 @@ interface Observable {
   //
   // takeUntil() can consume promises, iterables, async iterables, and other
   // observables.
-  Observable takeUntil(any notifier);
+  Observable takeUntil(any value);
   Observable map(Mapper mapper);
   Observable filter(Predicate predicate);
   Observable take(unsigned long long amount);
@@ -442,6 +455,130 @@ An <dfn>internal observer</dfn> is a [=struct=] with the following [=struct/item
   [[#promise-returning-operators]] that make use of this, for example.</p>
 </div>
 
+<div algorithm>
+  To <dfn for=Observable>convert to an Observable</dfn> given an {{any}} |value|, run these steps:
+
+  Note: We split this algorithm out from the Web IDL {{Observable/from()}} method, so that
+  spec prose can <a for=Observable lt="convert to an observable">convert</a> an {{Observable}}
+  without going through the Web IDL bindings.
+
+  Note: The resolution of value to its descrete types happens before
+  [=Observable/subscribe callback=] is called. This means mutations of values, such as adding
+  the iterable protocols to the object, will not take affect between the creation of the returned
+  observable, and when it is subscribed to.
+
+    1. If |value| is an {{Observable}}, then return |value|.
+
+    1. Let |asyncIteratorMethodRecord| be [=GetMethod=](|value|, %Symbol.asyncIterator%).
+
+    1. If |asyncIteratorMethodRecord| is a [=normal completion=] and 
+       |asyncIteratorMethodRecord|'s \[[Value]] is not undefined, then:
+
+       Note: [=GetMethod=] may return a [=normal completion=] with an undefined value when the object
+       simply has no asyncIterator method.
+
+       1. Let |nextAlgorithm| be the following steps, given |subscriber| and |iterator|:
+
+          1. If |iterator|'s \[[Done]] is true, then:
+
+             1. Run |subscriber|'s {{Subscriber/complete()}} method and abort these steps.
+
+          1. Let |nextRecord| be [=IteratorStepValue=](|iterator|).
+
+          1. Let |nextPromise| be undefined.
+
+          1.  If |nextRecord| is a [=throw completion=] then:
+
+              1. Set |nextPromise| to [=a promise rejected with=] |nextRecord|'s \[[Value]].
+
+          1. Otherwise, set |nextPromise| to |nextRecord|'s \[[Value]].
+
+          1. [=Upon fulfillment=] of |nextPromise|, run the following steps, given |resolution|:
+
+             1. Run |subscriber|'s {{Subscriber/next()}} method, given |resolution|.
+
+             1. Run |nextAlgorithm|, given |subscriber| and |iterator|.
+
+          1. [=Upon rejection=] of |nextPromise|, run the following steps, given |rejection|:
+
+             1. Run |subscriber|'s {{Subscriber/error()}} method, given |rejection|.
+
+       1. Return a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
+          algorithm that takes a {{Subscriber}} |subscriber| and does the following:
+
+          1. Let |iteratorRecord| be [=GetIteratorFromMethod=](|value|, %Symbol.asyncIterator%).
+
+          1. If |iteratorRecord| is a [=throw completion=] then:
+
+             1. [=queue a microtask=] to perform the following steps:
+
+                1. Run |subscriber|'s {{Subscriber/error()}} method, given |iteratorRecord|'s \[[Value]].
+
+          1. Otherwise, [=queue a microtask=] to perform the following steps:
+
+             1. Run |nextAlgorithm| given |subscriber| and |iteratorRecord|'s \[[Value]].
+
+          Note: It is important to [=queue a microtask=] in both branches here to guarantee that
+          coercing an AsyncIterable never stops the Subscription synchronously, thereby releasing
+          Zalgo.
+
+    1. Let |iteratorMethodRecord| be [=GetMethod=](|value|, %Symbol.iterator%).
+
+    1. If |iteratorMethodRecord| is a [=normal completion=] and 
+       |iteratorMethodRecord|'s \[[Value]] is not undefined, then:
+
+       Note: [=GetMethod=] may return a [=normal completion=] with an undefined value when the object
+       simply has no asyncIterator method.
+
+       1. Return a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
+          algorithm that takes a {{Subscriber}} |subscriber| and does the following:
+
+          1. Let |iteratorRecord| be [=GetIteratorFromMethod=](|value|, %Symbol.iterator%).
+
+          1. If |iteratorRecord| is a [=throw completion=] then:
+
+             1. Run |subscriber|'s {{Subscriber/error()}} method, given |iteratorRecord|'s \[[Value]].
+
+             1. Abort these steps.
+
+          1. Let |iterator| be |iteratorRecord|'s \[[Value]].
+
+          1. Repeat:
+
+             1. If |iterator|'s \[[Done]] is true, then:
+
+                1. Run |subscriber|'s {{Subscriber/complete()}} method and abort these steps.
+
+             1. Let |nextRecord| be [=IteratorStepValue=](|iterator|).
+
+             1. If |nextRecord| is a [=throw completion=] then:
+
+                1. Run |subscriber|'s {{Subscriber/error()}} method, given |nextRecord|'s \[[Value]].
+
+                1. Abort these steps.
+
+             1. Run |subscriber|'s {{Subscriber/next()}} given |nextRecord|'s \[[Value]].
+
+    1. If [=IsPromise=](|value|) is true, then:
+
+       1. Return a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
+          algorithm that takes a {{Subscriber}} |subscriber| and does the following:
+
+          1. [=Upon fulfillment=] of |value|, run the following steps, given |resolution|:
+
+             1. Run |subscriber|'s {{Subscriber/next()}} method, given |resolution|.
+
+             1. Run |subscriber|'s {{Subscriber/complete()}} method.
+
+          1. [=Upon rejection=] of |value|, run the following steps, given |rejection|:
+
+             1. Run |subscriber|'s {{Subscriber/error()}} method, given |rejection|.
+
+    1. Throw a {{TypeError}}.
+
+</div>
+
+
 <div algorithm>
   To <dfn for=Observable>subscribe to an {{Observable}}</dfn> given an
   {{ObserverUnion}}-or-[=internal observer=] |observer|, and a {{SubscribeOptions}} |options|, run
@@ -556,15 +693,25 @@ For now, see [https://github.com/wicg/observable#operators](https://github.com/w
 
 <h4 id=observable-from>{{Observable/from()}}</h4>
 
-<p class=XXX>Spec the exact semantics of {{Observable/from()}} conversion.</p>
+<div algorithm>
+  The <dfn for=Observable method><code>from(|value|)</code></dfn> method steps
+  are:
+
+    1. Return the result of <a for=Observable lt="convert to an Observable">
+       converting</a> |value| to an Observable.
+
+</div>
 
 <h4 id=observable-returning-operators>{{Observable}}-returning operators</h4>
 
 <div algorithm>
-  The <dfn for=Observable method><code>takeUntil(|notifier|)</code></dfn> method steps are:
+  The <dfn for=Observable method><code>takeUntil(|value|)</code></dfn> method steps are:
 
     1. Let |sourceObservable| be [=this=].
 
+    1. Let |notifier| be the result of <a for=Observable lt="convert to an Observable">
+       converting</a> |value| to an Observable.
+
     1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
        algorithm that takes a {{Subscriber}} |subscriber| and does the following: