Events and Rejections

Events record what happened. Rejections explain why something didn't. Both are tagged factories with the same syntax.

Import

import { event, integer, refine, rejection } from '@marcelsud/accord'

Usage

There are exactly two forms: with no shape, or with a shape.

const Cents = refine(integer, (n) => n > 0, 'positive cents')

const Opened = event('Opened')
const Deposited = event('Deposited', { amount: Cents, balance: integer })

const NotOpen = rejection('NotOpen')
const InsufficientFunds = rejection('InsufficientFunds', {
  balance: integer,
  requested: Cents,
})

Opened() // { _tag: 'Opened' }
Deposited({ amount: 100, balance: 100 }) // { amount: 100, balance: 100, _tag: 'Deposited' }
InsufficientFunds({ balance: 100, requested: 150 })

• No shape means no properties.
• A shape fixes the property types and validates them at runtime. Invalid data throws a TypeError.
• There's no schema-free property inference.
• The tag is the name. You can't override _tag.

Using them

Emit events from an aggregate and return rejections with fail():

deposit(amount: number) {
  if (!this.state.open) return fail(NotOpen())
  this.emit(Deposited({ amount, balance: this.state.balance + amount }))
  // ...
}

Subscribe to events in views, policies, and projections by passing the factory: on(Deposited, handler) or on: [Deposited].

Versioning event tags

The tag is what gets persisted. When a payload changes incompatibly, give the new event a new tag and keep the old factory around for history:

// v1.ts
export const DealChanged = event('DealChanged', schemaV1)

// v2.ts
export const DealChanged = event('DealChangedV2', schemaV2)

See Versioning and migrations.