Immutability

update returns a new Model instead of mutating the current one. Foldkit provides modifyFields for these immutable field updates. It wraps Effect's Struct.evolve with stricter key checking, so removing or renaming a Model field produces errors at every stale update site.

import { Schema } from 'effect'
import { modifyFields } from 'foldkit/struct'

const Model = Schema.Struct({
  count: Schema.Number,
  status: Schema.Literals(['Idle', 'Counting']),
})
type Model = typeof Model.Type

const model: Model = { count: 0, status: 'Idle' }

const nextModel = modifyFields(model, {
  count: count => count + 1,
  status: () => 'Counting',
})

Each property in the transform object receives that field's current value and returns its next value. Omitted properties remain unchanged.

Use the current value when the change depends on it, such as incrementing a count. Use () => value when a Message payload or Command result replaces the field. Keep the transformer deterministic. Time, randomness, browser state, and other outside values belong behind a Command.