Dean Wampler, October 2021.
This repo is the code for the Scala 3 Medium post on The Value of Parametric Polymophism.
Try the examples by starting sbt, then use run, then pick the example to run from the list.
The example versions, v1 through v4 evolve the example:
| Version | Description |
|---|---|
v1 |
Don't attempt to use a parametric polymorphic implementation for message handling. Rather, just declare custom handler types for each message type, with lots of duplication. The MessageHandler.apply method could be concrete, as all subtype implementations are identical. I left it as shown because in v3 I deal with the case where they aren't identical. |
v2 |
Use parametric polymorphism, where one type handles all details and given instances are declared for each input message type. |
v3 |
Use parametric polymorphism and the Template Method Pattern to handle the situation where the message handling is actually different for each message. MessageHandler.apply is a concrete template, which implements what's common to all variants. It calls the abstract protected process method that each concrete class must implement for unique handling. Two other protected methods success and failure are helper methods. |
v4 |
Show how easy and robust it is to add a new message type. |
v4java |
A Java implementation of v4 to show you how to apply these techniques in Java code. |
There is also
Finally, see the branch with-enums, which uses enum declarations for the IncomingMessage and OutgoingMessage hierarchies. The code comments discuss a limitation you encounter when using enums, namely an extra given instance has to be declared for MessageHandler[IncomingMessage], which is not needed in the implementation that uses a trait and case classes. The difference is due to typing differences involving enum cases vs. a hierarchy using a base trait with subtypes, as done here.
Also, enums wouldn't work for hierarchies that aren't sealed, i.e., where a client needs the ability to add new message types, as shown in v4, which still plug into the "infrastructure".