diff --git a/src/operators/finalize.ts b/src/operators/finalize.ts
index 02c9ce6..7a17616 100644
--- a/src/operators/finalize.ts
+++ b/src/operators/finalize.ts
@@ -2,7 +2,31 @@ import { Observable } from '../Observable';
 import { OperatorFunction } from './types';
 
 export function finalize<T>(callback: () => void): OperatorFunction<T, T> {
-  void Observable;
-  void callback;
-  throw new Error('TODO');
+  return source => {
+    return new Observable(observer => {
+      let finalized = false;
+
+      const finalizeFn = () => {
+        if (!finalized) {
+          finalized = true;
+          callback();
+        }
+      };
+
+      const complete = () => {
+        observer.complete();
+        finalizeFn();
+      };
+
+      const sub = source.subscribe({
+        next: val => observer.next(val),
+        error: err => observer.error(err),
+        complete: () => complete(),
+      });
+
+      sub.add(finalizeFn);
+
+      return sub;
+    });
+  };
 }
diff --git a/src/operators/retry.ts b/src/operators/retry.ts
index ed26d09..725f8fc 100644
--- a/src/operators/retry.ts
+++ b/src/operators/retry.ts
@@ -1,8 +1,37 @@
 import { Observable } from '../Observable';
+import { Subscription } from '../Subscription';
 import { OperatorFunction } from './types';
 
 export function retry<T>(limit?: number): OperatorFunction<T, T> {
-  void Observable;
-  void limit;
-  throw new Error('TODO');
+  return source => {
+    return new Observable(observer => {
+      let count: number | undefined = limit;
+      let sub: Subscription = new Subscription();
+
+      const makeSubscription = (onError: (err: unknown) => void) => {
+        sub = source.subscribe({
+          next: value => observer.next(value),
+          error: onError,
+          complete: () => observer.complete(),
+        });
+      };
+
+      const error = (err: unknown) => {
+        if (count === undefined) {
+          makeSubscription(error);
+        } else if (count > 0) {
+          count = count - 1;
+          makeSubscription(error);
+        } else {
+          observer.error(err);
+        }
+      };
+
+      makeSubscription(error);
+
+      return () => {
+        sub.unsubscribe();
+      };
+    });
+  };
 }
diff --git a/src/operators/scan.ts b/src/operators/scan.ts
index a3f65e4..78384e3 100644
--- a/src/operators/scan.ts
+++ b/src/operators/scan.ts
@@ -2,7 +2,24 @@ import { Observable } from '../Observable';
 import { OperatorFunction } from './types';
 
 export function scan<T, R>(accumulator: (acc: R, value: T) => R, seed: R): OperatorFunction<T, R> {
-  void Observable;
-  void accumulator, seed;
-  throw new Error('TODO');
+  return source => {
+    return new Observable(observer => {
+      let acc: R = seed;
+
+      const next = (value: T) => {
+        try {
+          acc = accumulator(acc, value);
+          observer.next(acc);
+        } catch (e) {
+          observer.error(e);
+        }
+      };
+
+      return source.subscribe({
+        next,
+        error: err => observer.error(err),
+        complete: () => observer.complete(),
+      });
+    });
+  };
 }
diff --git a/src/operators/switchMap.ts b/src/operators/switchMap.ts
index 12e4c6e..f5dbde1 100644
--- a/src/operators/switchMap.ts
+++ b/src/operators/switchMap.ts
@@ -1,7 +1,62 @@
 import { Observable } from '../Observable';
+import { Subscription } from '../Subscription';
 import { OperatorFunction } from './types';
 
 export function switchMap<T, R>(project: (value: T) => Observable<R>): OperatorFunction<T, R> {
-  void project;
-  throw new Error('TODO');
+  return source => {
+    return new Observable(obs => {
+      let sourceIsCompleted = false;
+      let innerSubscription = new Subscription();
+
+      const innerNext = (resultValue: R) => {
+        obs.next(resultValue);
+      };
+
+      const innerError = (err: unknown) => {
+        obs.error(err);
+      };
+
+      const innerComplete = () => {
+        if (sourceIsCompleted) {
+          obs.complete();
+        }
+      };
+
+      const next = (value: T) => {
+        innerSubscription.unsubscribe();
+        try {
+          innerSubscription = project(value).subscribe({
+            next: innerNext,
+            error: innerError,
+            complete: innerComplete,
+          });
+        } catch (e) {
+          obs.error(e);
+        }
+      };
+
+      const error = (err: unknown) => {
+        obs.error(err);
+      };
+
+      const complete = () => {
+        sourceIsCompleted = true;
+
+        if (innerSubscription.closed) {
+          obs.complete();
+        }
+      };
+
+      const mainSub = source.subscribe({
+        next,
+        error,
+        complete,
+      });
+
+      return () => {
+        mainSub.unsubscribe();
+        innerSubscription.unsubscribe();
+      };
+    });
+  };
 }