M05 Type Driven

Type-Driven Design

Layer 1: Language Mechanics

Core Question

How can the type system prevent invalid states?

Before reaching for runtime checks:

• Can the compiler catch this error?
• Can invalid states be unrepresentable?
• Can the type encode the invariant?

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Error → Design Question

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Thinking Prompt

Before adding runtime validation:

1. Can the type encode the constraint?

• Numeric range → bounded types or newtypes
• Valid states → type state pattern
• Semantic meaning → newtype

2. When is validation possible?

• At construction → validated newtype
• At state transition → type state
• Only at runtime → Result with clear error

3. Who needs to know the invariant?

• Compiler → type-level encoding
• API users → clear type signatures
• Runtime only → documentation

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Trace Up ↑

When type design is unclear:

"Need to validate email format"
    ↑ Ask: Is this a domain value object?
    ↑ Check: m09-domain (Email as Value Object)
    ↑ Check: domain-* (validation requirements)

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Trace Down ↓

From design to implementation:

"Need type-safe wrapper for primitives"
    ↓ Newtype: struct UserId(u64);

"Need compile-time state validation"
    ↓ Type State: Connection<Connected>

"Need to track phantom type parameters"
    ↓ PhantomData: PhantomData<T>

"Need capability markers"
    ↓ Marker Trait: trait Validated {}

"Need gradual construction"
    ↓ Builder: Builder::new().field(x).build()

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Quick Reference

Pattern Examples

Newtype

struct Email(String);  // Not just any string

impl Email {
    pub fn new(s: &str) -> Result<Self, ValidationError> {
        // Validate once, trust forever
        validate_email(s)?;
        Ok(Self(s.to_string()))
    }
}

Type State

struct Connection<State>(TcpStream, PhantomData<State>);

struct Disconnected;
struct Connected;
struct Authenticated;

impl Connection<Disconnected> {
    fn connect(self) -> Connection<Connected> { ... }
}

impl Connection<Connected> {
    fn authenticate(self) -> Connection<Authenticated> { ... }
}

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Decision Guide

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Anti-Patterns

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Related Skills