On this pageFacts, Not Instructions

Messages

Facts, Not Instructions

A Message records something that happened in the application. It does not prescribe the response. The update function decides what the fact means for the current Model.

ClickedIncrement does not mean “add one.” It records that the user clicked the increment button. In this counter, update adds one. A later version may return a Command that obtains the next value elsewhere. The Message remains a stable account of the event.

The counter has three Messages:

import { Schema } from 'effect'
import { defineMessageUnion } from 'foldkit/message'

// MESSAGE

// defineMessageUnion() declares the union and its callable constructors together

const Message = defineMessageUnion({
  ClickedDecrement: {},
  ClickedIncrement: {},
  ClickedReset: {},
})
type Message = typeof Message.Type

Messages use verb-first, past-tense names such as ClickedIncrement, not Increment or ADD_COUNT. Prefixes make their causes easy to scan. Clicked* records clicks, and Updated* records input changes. Command results use Succeeded* or Failed* when the distinction matters, and Completed* otherwise. Got* is reserved for results lifted from a child Submodel.

The defineMessageUnion() helper declares the whole union in one place. Each key becomes a callable constructor on the union, so Message.ClickedIncrement() creates the value and Message.match(message, handlers) handles every variant exhaustively. Do not destructure the constructors. Keeping Message or OutMessage at the call site makes the owning domain explicit.

Name the cause

A Message says what happened, not what update intends to do next. That keeps the same fact useful when the application’s response changes.

Messages describe what happened. The update function defines every resulting state transition.