pattern

bun add @stopcock/pattern

Patterns are plain values plus small predicate objects. Matching has no runtime service, code-generation step, or global registry.

import { match, select, string } from '@stopcock/pattern'

type Event =
  | { type: 'created'; id: number }
  | { type: 'renamed'; id: number; name: string }
  | { type: 'deleted'; id: number }

const label = (event: Event) =>
  match(event)
    .with({ type: 'created' }, ({ id }) => `created ${id}`)
    .with(
      { type: 'renamed', name: select('name', string) },
      ({ id }, { name }) => `renamed ${id} to ${name}`,
    )
    .with({ type: 'deleted' }, ({ id }) => `deleted ${id}`)
    .exhaustive()

exhaustive() becomes available only after every member of a discriminated union is covered. otherwise(handler) adds an explicit fallback, while run() returns undefined when no branch matches.

Patterns include literals, nested objects and tuples, primitive predicates, unions and intersections, negation, guards, arrays, records, maps, sets, nullable and optional values, strict object keys, and typed named captures. Focused imports are available from @stopcock/pattern/match and @stopcock/pattern/pattern.