Part 2: Read it with a view

The account decides, but it doesn’t answer questions. Add two views: a summary with the balance, and a statement with one line per movement.

Show the balance

import * as Domain from "alchemy/Domain";
import * as Schema from "effect/Schema";
import { Account } from "./Account.aggregate.ts";

export class AccountSummary extends Domain.View("AccountSummary", {
  from: [Account],
  key: Account,
  state: Schema.Struct({ owner: Schema.NullOr(Schema.String), balance: Schema.Int }),
  initial: { owner: null, balance: 0 },
  evolve: {
    AccountOpened: (v, e) => ({ ...v, owner: e.owner }),
    MoneyDeposited: (v, e) => ({ ...v, balance: e.balanceAfter }),
    MoneyWithdrawn: (v, e) => ({ ...v, balance: e.balanceAfter }),
  },
}) {}

A view folds events into state, one copy per key. key: Account gives every account its own summary.

Host the view

import * as Domain from "alchemy/Domain";
import { Account } from "./Account.aggregate.ts";
import { AccountSummary } from "./AccountSummary.view.ts";

export class Bank extends Domain.Model("Bank", {
  aggregates: [Account],
  views: [AccountSummary],
}) {}

Check the balance

import { it } from "@effect/vitest";
import { Story } from "alchemy/Domain/Story";
import { Account } from "../src/Account.aggregate.ts";
import { AccountSummary } from "../src/AccountSummary.view.ts";
import { Bank } from "../src/Bank.model.ts";
import { DepositMoney } from "../src/DepositMoney.command.ts";
import { AccountOpened } from "../src/OpenAccount.command.ts";
import { WithdrawMoney } from "../src/WithdrawMoney.command.ts";

const story = Story.make(Bank);
const account = Account.ref("a-1");

it.effect("the summary follows the balance", () =>
  story(
    Story.given(account, new AccountOpened({ owner: "sam" })),
    Story.when(account, new DepositMoney({ amount: 100 })),
    Story.when(account, new WithdrawMoney({ amount: 30 })),
    Story.view(AccountSummary, account, { owner: "sam", balance: 70 }),
  ),
);

view checks a key’s state after everything before it has been applied. A partial object checks only the fields you give.

Describe a statement line

import * as Domain from "alchemy/Domain";
import * as Schema from "effect/Schema";

export class StatementLine extends Domain.Event("StatementLine", {
  kind: Schema.Literals(["deposit", "withdrawal"]),
  amount: Schema.Int,
  balance: Schema.Int,
}) {}

A view can record events of its own. One StatementLine describes any movement of money.

Emit a line for every movement

import * as Domain from "alchemy/Domain";
import * as Schema from "effect/Schema";
import { Account } from "./Account.aggregate.ts";

export class StatementLine extends Domain.Event("StatementLine", {

  balance: Schema.Int,
}) {}

export class Statement extends Domain.View("Statement", {
  from: [Account],
  key: Account,
  events: [StatementLine],
  emit: {
    MoneyDeposited: (e) =>
      new StatementLine({ kind: "deposit", amount: e.amount, balance: e.balanceAfter }),
    MoneyWithdrawn: (e) =>
      new StatementLine({ kind: "withdrawal", amount: e.amount, balance: e.balanceAfter }),
  },
}) {}

emit takes each account event and returns the line to record. The statement keeps no state of its own; its lines are its history.

Host the statement

import { Account } from "./Account.aggregate.ts";
import { AccountSummary } from "./AccountSummary.view.ts";
import { Statement } from "./Statement.view.ts";

export class Bank extends Domain.Model("Bank", {
  aggregates: [Account],
  views: [AccountSummary],
  views: [AccountSummary, Statement],
}) {}

Check the statement

import { DepositMoney } from "../src/DepositMoney.command.ts";
import { AccountOpened } from "../src/OpenAccount.command.ts";
import { Statement } from "../src/Statement.view.ts";
import { WithdrawMoney } from "../src/WithdrawMoney.command.ts";

  ),
);

it.effect("the statement has one line per movement", () =>
  story(
    Story.given(account, new AccountOpened({ owner: "sam" })),
    Story.when(account, new DepositMoney({ amount: 100 })),
    Story.when(account, new WithdrawMoney({ amount: 30 })),
    Story.emitted(Statement, account, [
      { kind: "deposit", amount: 100, balance: 100 },
      { kind: "withdrawal", amount: 30, balance: 70 },
    ]),
  ),
);

emitted checks every event a view key has emitted, in order.

bun vitest run

The account decides, and two views answer: what the balance is, and how it got there.