Added EitherObservable

src/EitherObservable.ts

@@ -0,0 +1,40 @@
+import { Observable, Operator, TeardownLogic, Subscriber } from "rxjs"
+
+/**
+ * Represents a disjoint union of two Observables.
+ * 
+ * An instance of `EitherObservable` is either an instance of `LeftObservable` or `RightObservable`.
+ * Conviention is that `LeftObservable` is used for errors and `RightObservable` is used for successes.
+ * 
+ * `EitherObservable` is right-biased, `Right` is the default case to operate on.
+ */
+export type EitherObservable<E, A> = LeftObservable<E, A> | RightObservable<E, A>
+
+/**
+ * Interface to ensure that LeftObservable and RightObservable have the same methods.
+ */
+interface IEitherObservable<E, A> extends Observable<A> {
+    readonly tag: "left" | "right";
+}
+
+export class LeftObservable<E, A> implements IEitherObservable<E, A> {
+    readonly tag: "left" = "left";
+
+    /** From RxJS docs: Internal implementation detail. */
+    public _isScalar = false;
+
+    /** Previous Observable in the chain. */
+    source: Observable<any>;
+    operator: Operator<any, A>;
+
+    private readonly _subscribe?: (this: Observable<A>, subscriber: Subscriber<A>) => TeardownLogic
+
+    constructor() {}
+}
+
+export class RightObservable<E, A> implements IEitherObservable<E, A> {
+    readonly tag: "right" = "right";
+
+    /** From RxJS docs: Internal implementation detail. */
+    public _isScalar = false;
+}