feat: add Liskov substitution principle implementation

Author: angelo.yanve

04-SOLID/04x02_OCP/good.ts

@@ -10,8 +10,8 @@ interface IEmailService {
     sendMail(mail: Mail): Promise<EmailTransmissionResult>;
 }
 
-class Mail {
-    // Define properties and methods for the Mail class
+type Mail = {
+    // Define properties and methods for the Mail interface
     subject: string;
     from: string;
     to: string[];

04-SOLID/04x03_LSP/README.md

@@ -0,0 +1,4 @@
+## Example
+Here we are going to create a simple example to illustrate the Liskov Substitution Principle. 
+
+We will use the same example of the Open-Closed Principle, with `IEmailService` and it's implementations `MailGunEmailService`, `SendGridEmailService`, `MailChimpEmailService`...

04-SOLID/04x03_LSP/good.ts

@@ -0,0 +1,46 @@
+type TransmissionResult = 'Success' | 'Failure' | 'Bounced'
+
+interface IEmailTransmissionResult {
+  result: TransmissionResult;
+  message?: string;
+}
+
+type Mail = {
+    // Define properties and methods for the IMail interface
+    subject: string;
+    from: string;
+    to: string[];
+    body: any;
+}
+
+interface IMailService {
+  sendMail(email: Mail): Promise<IEmailTransmissionResult>
+}
+
+class SendGridEmailService implements IMailService {
+    sendMail(email: Mail): Promise<IEmailTransmissionResult> {
+      // algorithm
+      return Promise.resolve({ result: 'Success' });
+    }
+  }
+  
+  class MailChimpEmailService implements IMailService {
+    sendMail(email: Mail): Promise<IEmailTransmissionResult> {
+      // algorithm
+      return Promise.resolve({ result: 'Success' });
+
+    }
+  }
+  
+  class MailGunEmailService implements IMailService {
+    sendMail(email: Mail): Promise<IEmailTransmissionResult> {
+      // algorithm
+      return Promise.resolve({ result: 'Success' });
+
+    }
+  }
+//   ...
+
+
+
+// Go to use-case.ts to see the why the LSP is respected

04-SOLID/04x03_LSP/use-case.ts

@@ -0,0 +1,8 @@
+
+let emailService : IMailService = new SendGridEmailService();
+
+const mailGunEmailService = new MailGunEmailService();
+
+emailService = mailGunEmailService;
+
+// This is valid because MailGunEmailService is a subtype of IMailService

04-SOLID/README.md

@@ -26,6 +26,9 @@ This principle is about **extending** the behavior of a class without **modifyin
 **Example:** go to file [OpenClosedPrinciple](./04x02_OCP)
 
 ### LisKov-Substitution Principle (LSP)
-***Derived classes must be substitutable for their base classes*** \
+***Derived classes must be substitutable for their base classes*** 
+
+
 This principle is about **inheritance** and **polymorphism**. It states that if a program is using a base class, then the reference to the base class can be replaced with a derived class without affecting the functionality of the program.
 
+**Example:** go to file [LiskovSubstitutionPrinciple](./04x03_LSP)