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?

Error → Design Question

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

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)

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()

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> { ... }}

Decision Guide

Anti-Patterns

Related Skills