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

Pattern	Don't Just Say	Ask Instead
Primitive obsession	"It's just a string"	What does this value represent?
Boolean flags	"Add an is_valid flag"	Can states be types?
Optional everywhere	"Check for None"	Is absence really possible?
Validation at runtime	"Return Err if invalid"	Can we validate at construction?

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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)

Situation	Trace To	Question
What types to create	m09-domain	What's the domain model?
State machine design	m09-domain	What are valid transitions?
Marker trait usage	m04-zero-cost	Static or dynamic dispatch?

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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()