class CreateCounter < ActiveRecord::Migration[6.0]
  def change
    create_table :counter do |t|
      t.integer :count
    end
  end
end

class Counter < ApplicationRecord
end

Counter::Application.routes.draw do
  match '/incr' => 'counter#incr', via: :post
  match '/decr' => 'counter#decr', via: :post
  match '/count' => 'counter#count', via :get
end

class CounterController < ApplicationController
  def new
    @Counter = Counter.new
  end
  def get
    { count: @Counter.count }.to_json
  end
  def incr
    @Counter.count += 1
    { count: @Counter.count }.to_json
  end
  def decr
    @Counter.count -= 1
    { count: @Counter.count }.to_json
  end
end

# POST /incr
{count: 1}
# POST /decr
{count: 0}
# GET /count
{count: 0}

import React, { useState } from "react";

function App() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <button onClick={() => setCount(count - 1)}>-</button>
      {count}
      <button onClick={() => setCount(count + 1)}>+</button>
    </div>
  );
}

function App() {
  return (
    <div>
      <Counter />
      <CounterCopy count={count}>
    </div>
  )
}

const initialState = { count: 0 };
function reducer(state = initialState, action) {
  switch (action.type) {
    case "INCREMENT":
      return { count: state.count + 1 };
    case "DECREMENT":
      return { count: state.count - 1 };
    default:
      return state;
  }
}

function Counter({count, dispatch}) {
  increment = () => dispatch({ type: "INCREMENT" });
  decrement = () => dispatch({ type: "DECREMENT" });

  return (
    <div>
      <button onClick={decrement}>-</button>
      {count}
      <button onClick={increment}>+</button>
    </div>
  );
}

const mapStateToProps = (state) => ({
  count: state.count;
});

export default connect(mapStateToProps)(Counter);

function CounterCopy({ count }) {
  return <div>Copy: {count}</div>;
}

import Browser
import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)

-- MAIN

main : Program Value Model Msg
main =
    Browser.application
        {
          init = init
        , view = view
        , update = update
        }

-- MODEL

type alias Model = Int

init : Model
init = 0

-- UPDATE

type Msg = Increment | Decrement

update : Msg -> Model -> Model
update msg model =
    case msg of
        Increment ->
            model + 1
        Decrement ->
            model - 1

-- VIEW

view : Model -> Html Msg
view model =
    div []
        [ button [ onClick Decrement ] [ text "-" ]
        , div [] [ text (String.fromInt model) ]
        , button [ onClick Increment ] [ text "+" ]
        ]

type alias User =
    { name : String
    , age : Int
    }

isOldEnoughToVote : User -> Bool
isOldEnoughToVote user =
  user.age >= 18


isOldEnoughToVote : { name : String, age : Int } -> Bool
isOldEnoughToVote user =
  user.age >= 18

type UserStatus
  = Regular
  | Visitor

type alias User =
  { status : UserStatus
  , name : String
  }

thomas = { status = Regular, name = "Thomas" }
kate95 = { status = Visitor, name = "kate95" }

greetUser : User -> String
greetUser user =
    case user.status of
        Regular ->
            "Hi Regular " ++ user.name
        Visitor ->
            "Hi Visitor " ++ user.name

type User
  = Regular String String
  | Visitor String

thomas = Regular "Thomas" "Kahlua"
kate95 = Visitor "kate95"

greetUser : User -> String
greetUser user =
    case user of
        Regular name drink ->
            "Hi Regular " ++ name ++ "Who's favorite drink is: " ++ drink
        Visitor name ->
            "Hi Visitor " ++ name ++ "What would you like to order?"

type Maybe a
  = Just a
  | Nothing

> String.toInt "3"
Just 3
> String.toInt "3.14"
Nothing
> String.toInt "abc"
Nothing
> String.toInt Nothing
-- Compiler Error

> "3".to_i
=> 3
> "3.14".to_i
=> 3
> "abc".to_i
=> 0
> nil.to_i
=> 0

isReasonableAge : String -> Result String Int
isReasonableAge input =
  case String.toInt input of
    Nothing ->
      Err "That is not a number!"

    Just age ->
      if age < 0 then
        Err "Please try again after you are born."

      else if age > 135 then
        Err "Are you some kind of turtle?"

      else
        Ok age

fetch(url).then(res => res.json()).then(res => /* do something */)
// I forgot to add a .catch(), so this program could error out at any time.

-- Main
main =
  Browser.element
    { init = init
    , update = update
    , view = view
    }

-- Model
type Model
  = Failure
  | Loading
  | Success String


init : () -> (Model, Cmd Msg)
init _ =
  ( Loading
  , Http.get
      { url = "https://elm-lang.org/assets/public-opinion.txt"
      , expect = Http.expectString GotText
      }
  )

-- UPDATE
type Msg
  = GotText (Result Http.Error String)


update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    GotText result ->
      case result of
        Ok fullText ->
          (Success fullText, Cmd.none)

        Err _ ->
          (Failure, Cmd.none)

-- VIEW
view : Model -> Html Msg
view model =
  case model of
    Failure ->
      text "I was unable to load your book."

    Loading ->
      text "Loading..."

    Success fullText ->
      pre [] [ text fullText ]