The Power of Elm in JavaScript

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@franzejr

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Before starting...

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This presentation is based on

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open-source code It's not theory.

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we really enjoy using these things

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elm

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Why should I care?

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...many of the common issues that

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front-end developers have to deal with just don't exist in Elm

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don't exist

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How elm can achieve such thing?

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Errors will happen at compile time

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Compiler as an assistant

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http://elm-lang.org/blog/compilers-as-assistants

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Strong Static Types

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No null or undefined

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Purely functional

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Total immutability state

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Reactive by design

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Imperative a = b + c if we change b, a will NOT be updated.

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Reactive a = b + c if we change b, a will BE updated.

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Conﬁdence in Programming

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Me writing Haskell: 30% test coverage. Scala: 60% test coverage. Ruby: 97% test coverage. Guess the order of reliability of code...

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1. Make changes, any changes. 2. The compiler tells what you missed. 3. Go to 1.

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No runtime exceptions

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But I can't use elm now

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Bring these things to JavaScript

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Static Types

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Flow

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// @flow import { Record, Map } from "immutable"; import FormType from "./FormType"; import QuestionSubmissionType from "./QuestionSubmissionType"; export default class Model extends Record({ form: new FormType(), submissions: new Map(), currentStep: 0, submitted: false, errors: new Map(), apiKey: "" }) { form: FormType; submissions: Map; currentStep: number; apiKey: string; }

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// @flow import { Record, Map } from "immutable"; // --> import FormType from "./FormType"; import QuestionSubmissionType from "./QuestionSubmissionType"; export default class Model extends Record({ form: new FormType(), submissions: new Map(), currentStep: 0, submitted: false, errors: new Map(), apiKey: "" }) { form: FormType; submissions: Map; currentStep: number; apiKey: string; }

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src/output_lib/types/Model.js:9 9: form: new FormType(), ^^^^^^^^ identifier `FormType`. Could not resolve name Found 1 error

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➜ formulae_react git:(master) flow status src/output_lib/decoders.js:201 201: return 0; ^ number. This type is incompatible with the expected return type of 198: function decodeForm(data: ApiForm): FormType { ^^^^^^^^ FormType src/output_lib/decoders.js:202 202: return new FormType({ ^ unreachable code Found 2 errors

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function myFunction() { let a = b; }

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➜ formulae_react git:(master) ✗ flow status src/output_lib/decoders.js:199 199: let a = b; ^ identifier `b`. Could not resolve name Found 1 error

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function myFunction() { let a = "hello" * 5; }

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➜ formulae_react git:(master) ✗ flow status src/output_lib/decoders.js:199 199: let a = "hello" * 5; ^^^^^^^ string. The operand of an arithmetic operation must be a number. Found 1 error

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function timesTwo(x: number) { return x * 2; } function main() { timesTwo("hello"); }

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➜ formulae_react git:(master) ✗ flow status src/output_lib/decoders.js:203 203: timesTwo("hello"); ^^^^^^^ string. This type is incompatible with the expected param type of 198: function timesTwo(x: number) { ^^^^^^ number Found 1 error

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TypeScript

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Decoders

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Encode // Decode

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Convert at the boundaries

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The boundary between Two Worlds

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back-end <> front-end

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const Person = { get: (name: string): Promise => { return api.get(`${name}`).then(resp => decodePersonType(resp.data)); } };

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import { Record } from 'immutable' export default class PersonType extends Record({ name: '', age: 0, github: '' }) { name: string age: number github: string }

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type ApiPerson = { name: string, age: number, github: string } decodePersonType(person: ApiPerson) : PersonType{ return new PersonType({ name: person.name, age: person.age, github: person.github }) }

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All our code now uses our own types

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You make the conversion between your server data and application data very explicitly.

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Your app will now recognize weird JSON structures.

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function encodeForm(form: FormType): ApiForm { return { id: form.persisted ? form.id : undefined, application_id: form.applicationId, completion_content: form.completionContent, sections: form.sections.toArray().map(encodeSection), uuid: form.persisted ? undefined : form.id }; }

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// @flow import { Record, List } from "immutable"; import SectionType from "./SectionType"; export default class FormType extends Record({ id: "", applicationId: "", sections: List(), completionContent: "", persisted: false }) { id: string; applicationId: string; sections: List; completionContent: string; persisted: boolean; }

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type ApiForm = { id: number, application: ApiApplication, completion_content: string, sections: Array, questions: Array };

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It adds one more validation phase in getting your data from your API

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It turns server JSON into typed immutable records

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❤

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Redux elm based architecture People are using Elm and never thought about that

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import Html exposing (..) -- MODEL type alias Model = { ... } -- UPDATE type Msg = Reset | ... update : Msg -> Model -> Model update msg model = case msg of Reset -> ... ... -- VIEW view : Model -> Html Msg view model = ...

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http://redux.js.org/docs/introduction/PriorArt.html

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Redux First Router

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Route State + Date State = ❤

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... const routesMap = { HOME: "/", // action <-> url path PRODUCT: "/product/:id" // :id is a dynamic segment }; ...

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const productsReducer = (state = init(), action = {}) => { switch (action.type) { case "HOME": ... case "PRODUCT": ... ...

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... const mapDispatchToProps = dispatch => ({ goToProducts: id => dispatch({ type: "PRODUCT", payload: { id: id } }) }); ...

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Stay tuned!

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Questions? :)

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References → https://www.dailydrip.com/ → https://www.dailydrip.com/topics/elm → https://www.dailydrip.com/blog/understanding- the-elm-architecture

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References → https://www.dailydrip.com/blog/general-aspects- between-elm-and-js → http://slides.com/danielbachler/fearless- refactoring-with-elm#/ → http://elm-lang.org/blog/compilers-as-assistants → https://www.dailydrip.com/blog/using-decoders- in-your-api-the-boundary-between-two- worlds.html