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A Simple Counter Example

Overview

Here’s a complete counter application. It has all the pieces from the Architecture page — a Model, Messages, update, init, and view — wired together and running.

import { Match as M, Schema as S } from 'effect'
import { Command, Runtime } from 'foldkit'
import { Html, html } from 'foldkit/html'
import { m } from 'foldkit/message'

// MODEL

const Model = S.Struct({
  count: S.Number,
})
type Model = typeof Model.Type

// MESSAGE

const ClickedDecrement = m('ClickedDecrement')
const ClickedIncrement = m('ClickedIncrement')
const ClickedReset = m('ClickedReset')

const Message = S.Union(ClickedDecrement, ClickedIncrement, ClickedReset)
type Message = typeof Message.Type

// UPDATE

const update = (
  model: Model,
  message: Message,
): readonly [Model, ReadonlyArray<Command.Command<Message>>] =>
  M.value(message).pipe(
    M.withReturnType<
      readonly [Model, ReadonlyArray<Command.Command<Message>>]
    >(),
    M.tagsExhaustive({
      ClickedDecrement: () => [{ count: model.count - 1 }, []],
      ClickedIncrement: () => [{ count: model.count + 1 }, []],
      ClickedReset: () => [{ count: 0 }, []],
    }),
  )

// INIT

const init: Runtime.ProgramInit<Model, Message> = () => [{ count: 0 }, []]

// VIEW

const { div, button, Class, OnClick } = html<Message>()

const view = (model: Model): Html =>
  div(
    [
      Class(
        'min-h-screen bg-white flex flex-col items-center justify-center gap-6 p-6',
      ),
    ],
    [
      div(
        [Class('text-6xl font-bold text-gray-800')],
        [model.count.toString()],
      ),
      div(
        [Class('flex flex-wrap justify-center gap-4')],
        [
          button([OnClick(ClickedDecrement()), Class(buttonStyle)], ['-']),
          button([OnClick(ClickedReset()), Class(buttonStyle)], ['Reset']),
          button([OnClick(ClickedIncrement()), Class(buttonStyle)], ['+']),
        ],
      ),
    ],
  )

// STYLE

const buttonStyle = 'bg-black text-white hover:bg-gray-700 px-4 py-2 transition'

// RUN

const program = Runtime.makeProgram({
  Model,
  init,
  update,
  view,
  container: document.getElementById('root')!,
})

Runtime.run(program)

Don’t worry about understanding every line yet. The next four pages break this code apart piece by piece. After that, we’ll add new features to the counter — a delayed reset, auto-counting, loading saved state — and each one will introduce a new concept.

Let’s start with the Model — the single data structure that holds everything your application can be.