Type Safety

Wooks provides compile-time type safety with zero runtime overhead. Every primitive — key, cached, slot, defineEventKind, defineWook — carries its type through the entire system. There are no casts, no any escape hatches, and no runtime type checking.

How It Works

The core mechanism is type branding. Each slot stores a phantom type parameter _T that TypeScript tracks but JavaScript never sees:

interface Key<T> {
  readonly _id: number
  readonly _name: string
  readonly _T?: T  // ← phantom type brand, not used at runtime
}

When you create a key, TypeScript locks in the type:

const userId = key<string>('userId')
// userId is Key<string> — TypeScript remembers this everywhere

Typed Get/Set

EventContext.get() and EventContext.set() extract the type from the accessor:

get<T>(accessor: Key<T> | Cached<T>): T
set<T>(key: Key<T> | Cached<T>, value: T): void

Both operations share the same <T>, tied to the accessor's type brand. TypeScript enforces consistency automatically:

const userId = key<string>('userId')

ctx.set(userId, 'abc')    // ✓ string matches Key<string>
ctx.set(userId, 123)      // ✗ Type 'number' is not assignable to 'string'

const id = ctx.get(userId) // id is string — no cast needed

Inferred Factory Types

cached<T> and defineWook<T> infer their type from the factory function's return value — you never need to spell out the generic:

// TypeScript infers T = number from the return value
const requestSize = cached((ctx) => {
  const req = ctx.get(httpKind.keys.req)
  return parseInt(req.headers['content-length'] || '0')
})

ctx.get(requestSize) // number — inferred, not annotated

The same applies to defineWook:

// TypeScript infers the full return type from the factory
export const useJob = defineWook((ctx) => ({
  getJobId: () => ctx.get(jobKind.keys.jobId),
  getStatus: () => ctx.has(statusKey) ? ctx.get(statusKey) : 'pending',
}))

const { getJobId, getStatus } = useJob()
// getJobId: () => string
// getStatus: () => 'pending' | 'running' | 'done'

No manual type annotation. The factory return type flows through defineWook and becomes the wook's return type:

function defineWook<T>(factory: (ctx: EventContext) => T): (ctx?: EventContext) => T

Parameterized Caching

cachedBy<K, V> tracks two independent types — the key and the value:

const headerValue = cachedBy((name: string, ctx) => {
  return ctx.get(httpKind.keys.req).headers[name] || ''
})

headerValue('content-type')  // returns string
headerValue(42)              // ✗ Type 'number' is not assignable to 'string'

Event Kind Schemas

defineEventKind uses slot<T>() markers to declare a typed schema. Each slot is a zero-cost type brand:

function slot<T>(): SlotMarker<T>  // returns {} at runtime — purely a type marker

When you pass a schema to defineEventKind, TypeScript uses conditional types with infer to transform each SlotMarker<T> into a Key<T>:

type EventKind<S> = {
  keys: { [K in keyof S]: S[K] extends SlotMarker<infer V> ? Key<V> : never }
}

In practice:

const jobKind = defineEventKind('job', {
  jobId: slot<string>(),
  payload: slot<unknown>(),
  priority: slot<number>(),
})

// TypeScript infers:
// jobKind.keys.jobId    → Key<string>
// jobKind.keys.payload  → Key<unknown>
// jobKind.keys.priority → Key<number>

ctx.get(jobKind.keys.priority) // number

Seed Validation

ctx.seed() enforces that every slot in the schema is provided with the correct type. The seed type is derived from the schema using a mapped conditional type:

type EventKindSeeds<K> =
  K extends EventKind<infer S>
    ? { [P in keyof S]: S[P] extends SlotMarker<infer V> ? V : never }
    : never

This means:

// ✓ All slots provided with correct types
ctx.seed(jobKind, {
  jobId: 'abc',
  payload: { data: 1 },
  priority: 5,
})

// ✗ Missing 'priority'
ctx.seed(jobKind, {
  jobId: 'abc',
  payload: { data: 1 },
})

// ✗ Wrong type for 'priority'
ctx.seed(jobKind, {
  jobId: 'abc',
  payload: { data: 1 },
  priority: 'high',  // Type 'string' is not assignable to 'number'
})

No runtime validation code. The compiler catches mismatches before you run anything.

Summary

Primitive	Type mechanism	What it enforces
key<T>()	Phantom type brand	get/set must use the same type
cached<T>(fn)	Inferred from factory return	Read type matches computed type
cachedBy<K,V>(fn)	Two independent generics	Key and value types from factory signature
slot<T>()	Type-level marker (zero runtime cost)	Schema slot type for defineEventKind
defineEventKind	Mapped type with infer	Transforms SlotMarker<T> → Key<T> for each slot
defineWook<T>(fn)	Inferred from factory return	Wook return type matches factory return type
ctx.seed()	Conditional mapped type	All required slots present with correct types

All type safety is compile-time only — no runtime checks, no instanceof, no validation libraries. The context is a flat Map<number, unknown> at runtime, with TypeScript enforcing correctness through generics, phantom types, and conditional type inference.