Sixty seconds: say what is shared

import { Option, Schema } from 'effect'
import { defineMessageUnion } from 'foldkit/message'
import { modifyFields } from 'foldkit/struct'
import type * as Update from 'foldkit/update'
import { MessageSet, Projection, Surface } from 'foldkit-surface'
import { DocumentId, Sync } from 'foldkit-sync'

const Model = Schema.Struct({
  todos: Schema.Array(Schema.Struct({ id: Schema.String, title: Schema.String })),
  selectedTodoId: Schema.Option(Schema.String),
  lastError: Schema.Option(Schema.String),
})
type Model = typeof Model.Type

const Message = defineMessageUnion({
  CreatedTodo: { id: Schema.String, title: Schema.String },
  RenamedTodo: { id: Schema.String, title: Schema.String },
  SelectedTodo: { id: Schema.String },
})
type Message = typeof Message.Type

const initial: Model = { todos: [], selectedTodoId: Option.none(), lastError: Option.none() }

type Return = Update.Return<Model, Message>

const update = (model: Model, message: Message): Return =>
  Message.match<Return>(message, {
    CreatedTodo: ({ id, title }) => ({
      model: modifyFields(model, { todos: () => [...model.todos, { id, title }] }),
    }),
    RenamedTodo: ({ id, title }) => ({
      model: modifyFields(model, {
        todos: () => model.todos.map(todo => (todo.id === id ? { ...todo, title } : todo)),
      }),
    }),
    SelectedTodo: ({ id }) => ({
      model: modifyFields(model, { selectedTodoId: () => Option.some(id) }),
    }),
  })

const App = Surface.application({ Model, Message, initial, update })

const TodoSync = Sync.forApplication(App).make({
  documentId: DocumentId.make('todos'),
  shared: Projection.pick(App.model.todos),
  durable: MessageSet.make(App, [Message.CreatedTodo, Message.RenamedTodo]),
})

todos            shared     replicated, persisted, journaled
selectedTodoId   local      this device only
lastError        local

CreatedTodo      durable    recorded in the outbox, committed by the server
RenamedTodo      durable
SelectedTodo     local      runs through update and goes nowhere