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mgeisler merged 1 commit into from
Feb 12, 2026
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user-defined-types: add explanatory commentary to solution #3065

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23 changes: 23 additions & 0 deletions src/user-defined-types/solution.md
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Expand Up @@ -3,3 +3,26 @@
```rust,editable
{{#include exercise.rs:solution}}
```

- **Enums with Data:** Rust `enum` variants can carry data. `CarArrived(Floor)`
carries an integer, and `ButtonPressed(Button)` carries a nested `Button`
enum. This allows `Event` to represent a rich set of states in a type-safe
way.
- **Type Aliases:** `type Floor = i32` gives a semantic name to `i32`. This
improves readability, but `Floor` is still just an `i32` to the compiler.
- **`#[derive(Debug)]`:** We use this attribute to automatically generate code
to format the enums for printing with `{:?}`. Without this, we would have to
manually implement the `fmt::Debug` trait.
- **Nested Enums:** The `Button` enum is nested inside `Event::ButtonPressed`.
This hierarchical structure is common in Rust for modeling complex domains.

<details>

- Note that `Event::CarDoorOpened` is a "unit variant" (it carries no data),
while `Event::CarArrived` is a "tuple variant".
- You might discuss why `Button` is a separate enum rather than just having
`LobbyCallButtonPressed` and `CarFloorButtonPressed` variants on `Event`. Both
are valid, but grouping related concepts (like buttons) can make the code
cleaner.

</details>