m14-mental-model

# Mental Models

> **Layer 2: Design Choices**

## Core Question

**What's the right way to think about this Rust concept?**

When learning or explaining Rust:
- What's the correct mental model?
- What misconceptions should be avoided?
- What analogies help understanding?

---

## Key Mental Models

| Concept | Mental Model | Analogy |
|---------|--------------|---------|
| Ownership | Unique key | Only one person has the house key |
| Move | Key handover | Giving away your key |
| `&T` | Lending for reading | Lending a book |
| `&mut T` | Exclusive editing | Only you can edit the doc |
| Lifetime `'a` | Valid scope | "Ticket valid until..." |
| `Box<T>` | Heap pointer | Remote control to TV |
| `Rc<T>` | Shared ownership | Multiple remotes, last turns off |
| `Arc<T>` | Thread-safe Rc | Remotes from any room |

---

## Coming From Other Languages

| From | Key Shift |
|------|-----------|
| Java/C# | Values are owned, not references by default |
| C/C++ | Compiler enforces safety rules |
| Python/Go | No GC, deterministic destruction |
| Functional | Mutability is safe via ownership |
| JavaScript | No null, use Option instead |

---

## Thinking Prompt

When confused about Rust:

1. **What's the ownership model?**
   - Who owns this data?
   - How long does it live?
   - Who can access it?

2. **What guarantee is Rust providing?**
   - No data races
   - No dangling pointers
   - No use-after-free

3. **What's the compiler telling me?**
   - Error = violation of safety rule
   - Solution = work with the rules

---

## Trace Up ↑

To design understanding (Layer 2):

```
"Why can't I do X in Rust?"
    ↑ Ask: What safety guarantee would be violated?
    ↑ Check: m01-m07 for the rule being enforced
    ↑ Ask: What's the intended design pattern?
```

---

## Trace Down ↓

To implementation (Layer 1):

```
"I understand the concept, now how do I implement?"
    ↓ m01-ownership: Ownership patterns
    ↓ m02-resource: Smart pointer choice
    ↓ m07-concurrency: Thread safety
```

---

## Common Misconceptions

| Error | Wrong Model | Correct Model |
|-------|-------------|---------------|
| E0382 use after move | GC cleans up | Ownership = unique key transfer |
| E0502 borrow conflict | Multiple writers OK | Only one writer at a time |
| E0499 multiple mut borrows | Aliased mutation | Exclusive access for mutation |
| E0106 missing lifetime | Ignoring scope | References have validity scope |
| E0507 cannot move from `&T` | Implicit clone | References don't own data |

## Deprecated Thinking

| Deprecated | Better |
|------------|--------|
| "Rust is like C++" | Different ownership model |
| "Lifetimes are GC" | Compile-time validity scope |
| "Clone solves everything" | Restructure ownership |
| "Fight the borrow checker" | Work with the compiler |
| "`unsafe` to avoid rules" | Understand safe patterns first |

---

## Ownership Visualization

```
Stack                          Heap
+----------------+            +----------------+
| main()         |            |                |
|   s1 ─────────────────────> │ "hello"        |
|                |            |                |
| fn takes(s) {  |            |                |
|   s2 (moved) ─────────────> │ "hello"        |
| }              |            | (s1 invalid)   |
+----------------+            +----------------+

After move: s1 is no longer valid
```

## Reference Visualization

```
+----------------+
| data: String   |────────────> "hello"
+----------------+
       ↑
       │ &data (immutable borrow)
       │
+------+------+
| reader1    reader2    (multiple OK)
+------+------+

+----------------+
| data: String   |────────────> "hello"
+----------------+
       ↑
       │ &mut data (mutable borrow)
       │
+------+
| writer (only one)
+------+
```

---

## Learning Path

| Stage | Focus | Skills |
|-------|-------|--------|
| Beginner | Ownership basics | m01-ownership, m14-mental-model |
| Intermediate | Smart pointers, error handling | m02, m06 |
| Advanced | Concurrency, unsafe | m07, unsafe-checker |
| Expert | Design patterns | m09-m15, domain-* |

---

## Related Skills

| When | See |
|------|-----|
| Ownership errors | m01-ownership |
| Smart pointers | m02-resource |
| Concurrency | m07-concurrency |
| Anti-patterns | m15-anti-pattern |