Contexts And Tags

A Context is an immutable collection of concrete services. Services are addressed by tags, not by string lookups at call sites.

import { Context } from '@fuiste/dependencies'

const Clock = Context.Tag<{ now: () => Date }>('app/clock')
const context = Context.of(Clock, { now: () => new Date() })

Context.get(context, Clock).now()

Stable Tag Keys

Tags are stable by key.

const A = Context.Tag<string>('service/name')
const B = Context.Tag<string>('service/name')

A === B
// true

This makes tags convenient to share across modules. The type parameter still carries the service type, so the tag is the boundary between runtime identity and TypeScript inference.

Immutability

Context operations return new contexts.

const Name = Context.Tag<string>('name')
const Age = Context.Tag<number>('age')

const withName = Context.of(Name, 'Ada')
const withBoth = Context.add(withName, Age, 36)

Context.has(withName, Age)
// false

Context.get(withBoth, Age)
// 36

Context.merge rejects duplicate service tags. If two contexts intentionally need the same tag with the right side winning, use Context.override.

Safe And Unsafe Reads

Context.get is typed for tags known to exist in the context. Context.unsafeGet accepts any tag and throws at runtime if the service is missing.

Use get in normal application code. Reach for unsafeGet only at dynamic boundaries, where the type system has already been politely shown the door.