Topic 6
Topic 6
Ruby source files are converted directly from the English source tree.
Overview
Why this topic matters 💡
This is where students begin designing small cooperating objects instead of isolated functions. The topic introduces internal state, validation, reusable module behavior, and dependency injection through composition.
Learning outcomes 🎯
By the end of this topic, students should be able to:
Assessment focus ✅
Students should be able to explain why the object owns its state and why the calculator dependency is injected rather than created internally.
Short Note
Ruby's object model supports very small classes that still do meaningful work. Students should learn that a class is not justified by size alone; it is justified by responsibility, state, and collaboration boundaries.
This topic matters because it introduces two powerful habits:
Ruby beauty in this topic:
Ruby caution in this topic:
Reflection prompt:
Worked Examples
Example 1: Wallet or account balance 💡
A balance-holding object is a strong teaching example because it has real state and clear invariants.
Questions students should ask:
Example 2: Checkout with injected calculator 💡
Checkout logic is often composed from pricing rules, discount engines, tax services, or promotional logic. That makes it a natural composition example.
checkout = Checkout . new ( calculator : calculator ) checkout . final_total ( cart ) worked_examples.md ruby Why this is useful: