Composing features

One editor saves one note. A list has to load many notes, mount an editor for each, and know when one of them saved.

Features talk to each other the way React components do: props down, outputs up. The child announces Saved and the parent receives it through an onSaved prop. The child's inputs are its props and its outputs are declared, so the same editor mounts under any parent, or under none.

1. Add a list endpoint to the service

// notes-api.ts
import { Context, Effect, Layer, Schema } from "effect";

export const Note = Schema.Struct({ id: Schema.String, text: Schema.String });

export class NotesApi extends Context.Service<
  NotesApi,
  {
    readonly list: Effect.Effect<ReadonlyArray<typeof Note.Type>, Error>;
    readonly save: (note: {
      readonly id: string;
      readonly text: string;
    }) => Effect.Effect<string, Error>;
  }
>()("NotesApi") {}

export const notesApiLayer = Layer.succeed(NotesApi)({
  list: Effect.succeed([{ id: "n1", text: "Buy milk" }]),
  save: (note) => Effect.succeed(`${note.id}@${Date.now()}`),
});
// runtime.ts
import { createRuntime } from "@wych/react";

export const { component } = createRuntime(notesApiLayer);

2. Announce the save

Action.output declares outbound messages, as a record like Action. Command.output emits one. The editor is the chapter 2 file with those two additions and the cancel button left out.

// note-editor.tsx
import { Action, Command, Task, define } from "@wych/react";

const actions = Action({
  TextChanged: { text: Schema.String },
  Reverted: {},
  SaveClicked: {},
});
const outputs = Action.output({ Saved: { id: Schema.String, revision: Schema.String } });

const saveNote = Task("Save", {
  success: Schema.String,
  mode: "first",
  run: (note: { readonly id: string; readonly text: string }) =>
    Effect.gen(function* () {
      const api = yield* NotesApi;
      return yield* api.save(note);
    }),
});

const Editor = define({
  props: Schema.Struct({ noteId: Schema.String, initialText: Schema.String }),
  state: Schema.Struct({ text: Schema.String, dirty: Schema.Boolean }),
  tasks: { save: saveNote },
  actions,
  outputs,
});

const editor = Editor.create({
  initialState: (props) => ({ text: props.initialText, dirty: false }),
  reducer: {
    TextChanged: ({ text }, { draft, props, tasks }) => {
      draft.text = text;
      draft.dirty = text !== props.initialText;
      return tasks.save.cancel();
    },
    Reverted: (_payload, { draft, props, tasks }) => {
      draft.text = props.initialText;
      draft.dirty = false;
      return tasks.save.cancel();
    },
    SaveClicked: (_payload, { state, props, tasks }) =>
      tasks.save.start({ id: props.noteId, text: state.text }),
    SaveResolved: ({ value }, { draft, props }) => {
      draft.dirty = false;
      return [draft, Command.output(outputs.Saved, { id: props.noteId, revision: value })];
    },
  },
  render: ({ state, dispatch }) => (
    <form>
      <textarea
        value={state.text}
        onChange={(event) => dispatch(actions.TextChanged, { text: event.target.value })}
      />
      <button type="button" disabled={!state.dirty} onClick={() => dispatch(actions.Reverted)}>
        Revert
      </button>
      <button type="button" onClick={() => dispatch(actions.SaveClicked)}>
        Save
      </button>
    </form>
  ),
});

export const NoteEditor = component(editor, { name: "NoteEditor" });

Command.output takes the message and its payload, the same shape as dispatch. It rides beside the draft, which already holds the resolved save field: the runtime writes the settle before SaveResolved runs. Saved has no reducer handler, and writing one is a compile error. An output leaves the feature and never comes back. NoteEditor now takes a required onSaved prop, derived from the tag, with the payload minus _tag.

const one = <NoteEditor noteId="n1" initialText="Buy milk" onSaved={({ id }) => console.log(id)} />;

// @ts-expect-error Property 'onSaved' is missing
const missing = <NoteEditor noteId="n1" initialText="Buy milk" />;

3. Declare the parent

The list holds the loaded notes in a task field and the id of the note that saved last. Children lets a caller put its own nodes inside the list.

// note-list.tsx
import { Children } from "@wych/react";

const NoteSaved = Action("NoteSaved", { id: Schema.String, revision: Schema.String });

const loadNotes = Task("Load", {
  success: Schema.Array(Note),
  run: () =>
    Effect.gen(function* () {
      const api = yield* NotesApi;
      return yield* api.list;
    }),
});

const List = define({
  props: Schema.Struct({ title: Schema.String, children: Schema.optionalKey(Children) }),
  state: Schema.Struct({ lastSaved: Schema.String }),
  tasks: { notes: loadNotes },
  actions: NoteSaved,
});

NoteSaved is the list's one action, so Action("Tag", fields) declares it alone. loadNotes takes no input, so run declares none and the handle's start takes none. Children validates any node and is invisible to change detection. A parent that passes a fresh node on every render raises no PropsChanged.

4. Load the notes on Mounted

Mounted fires once per mount, after the initial state exists. It is where startup work goes.

const listReducer = List.reducer({
  Mounted: (_payload, { tasks }) => tasks.notes.start(),
  NoteSaved: ({ id }, { draft }) => {
    draft.lastSaved = id;
    return draft;
  },
});

A loaded list has nothing to do beyond landing in its field, and the runtime writes the field, so the reducer has no settle handler for LoadResolved or LoadRejected. Lifecycle handlers are optional and take the same shape as any other handler. The five tags are Mounted, PropsChanged, HookChanged, Error and Unmounted. Their payloads and firing order are in the lifecycle reference.

5. Split a piece of the view out

A plain React component under the mount reads the same snapshot render has, through useFeature. It takes no props.

const LastSaved = () => {
  const { state } = NoteList.useFeature();
  return state.lastSaved === "" ? null : <footer>Last saved: {state.lastSaved}</footer>;
};

LastSaved is part of the list's view, so it reads the list's state. That is the line between a view fragment and a child feature: a fragment reaches into its own feature through useFeature, a child feature such as NoteEditor receives props and announces outputs.

Called outside a <NoteList>, LastSaved throws TypeError: NoteList.useFeature() called outside <NoteList>.

6. Render the children

Each note gets a NoteEditor. onSaved receives the output payload with _tag stripped, and turns it into an action of the parent's own.

const noteList = List.create({
  initialState: () => ({ lastSaved: "" }),
  reducer: listReducer,
  render: ({ state, props, dispatch }) => (
    <section>
      <h1>{props.title}</h1>
      {Task.match(state.notes, {
        Idle: () => null,
        Pending: () => <p>Loading notes...</p>,
        Rejected: ({ error }) => <p role="alert">{error}</p>,
        Resolved: ({ value }) => (
          <ul>
            {value.map((note) => (
              <li key={note.id}>
                <NoteEditor
                  noteId={note.id}
                  initialText={note.text}
                  onSaved={({ id, revision }) => dispatch(NoteSaved, { id, revision })}
                />
              </li>
            ))}
          </ul>
        ),
      })}
      <LastSaved />
      {props.children}
    </section>
  ),
});

export const NoteList = component(noteList, { name: "NoteList" });

The child is a feature of its own with its own state. The parent reaches it through props and hears from it through onSaved, the same way any React component pair works.

7. Mount it

// main.tsx
import { createRoot } from "react-dom/client";

const root = createRoot(document.getElementById("root")!);
root.render(
  <NoteList title="Notes">
    <p>Every note saves on its own.</p>
  </NoteList>,
);

The paragraph lands where props.children is rendered. Change it on a parent render and the new node paints, while the list's state machine sees no props change at all.

8. Watch the output cross

feature.run collects outputs in their own array. They are never folded back into the reducer.

const saved = await Effect.runPromise(
  editor.run([actions.SaveClicked.make()], {
    props: { noteId: "n1", initialText: "Buy milk" },
    hooks: {},
    layer: notesApiLayer,
  }),
);

console.log(saved.emitted.map((action) => action._tag));
// => ["SaveResolved"]
console.log(saved.outputs.map((output) => output._tag));
// => ["Saved"]
console.log(saved.state.dirty);
// => false

SaveResolved is an action, so it appears in emitted and changed the state. Saved is an output, so it appears in outputs and changed nothing.

The files

src/
  main.tsx         # the mount
  note-editor.tsx  # the child feature, with its Saved output
  note-list.tsx    # the parent feature and its view fragment
  notes-api.ts     # list and save
  runtime.ts       # createRuntime(notesApiLayer)

Split across the files, the imports between them are:

import { notesApiLayer } from "./notes-api"; // runtime.ts
import { Note, NotesApi } from "./notes-api"; // note-editor.tsx and note-list.tsx
import { component } from "./runtime"; // note-editor.tsx and note-list.tsx
import { NoteEditor } from "./note-editor"; // note-list.tsx
import { NoteList } from "./note-list"; // main.tsx
import { actions, editor } from "./note-editor"; // main.tsx, for the run in step 8

Next

You have the whole model: schemas, actions, outputs, a pure reducer, commands, tasks, and composition.

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