elm: frontend code without runtime exceptions

Frontend code without runtimeexceptions

@PietroGrandi3D

elm

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Pietro Grandi

1. Frontend developer

2. Write on pietrograndi.com

3. Born as 3D Artist

Hi!

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Runtime exception

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When does it happen?

calling unexisting methods

var aString = 1;var first = aString.slice(0,1);

Runtime exception

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When does it happen?

calling unexisting methods

var aString = 1;var first = aString.slice(0,1);

your program expects a type to be a di�erent one

Runtime exception

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You discover it by the timeyou run that specific code

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What causes it?

1. inconsistent data from external sources

Runtime exception

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What causes it?

1. inconsistent data from external sources

2. error loading dependencies

Runtime exception

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What causes it?

1. inconsistent data from external sources

2. error loading dependencies

3. bugs

Runtime exception

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What causes it?

1. inconsistent data from external sources

2. error loading dependencies

3. bugs

we can control bugs

Runtime exception

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Can't I prevent bugs?

you should be able to

a simple JS bug

function contains(c, s) { var index = s.indexOf(c); return index > -1; }

console.log(contains("hi", "there")); // false console.log(contains("hi", "hi!")); // true console.log(contains("hi", 1)); // TypeError: s.indexOf is // not a function

Bugs

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Bring back type checking

apply defensive techniques

check against type

function contains(c, s) { var index = typeof s === 'string' ? s.indexOf(c) : -1; return index > -1; }

console.log(contains("hi", "there")); // false console.log(contains("hi", "hi!")); // true console.log(contains("hi", 1)); // false

Bugs

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Figthing bugs in JS

use a linter

put runtime checks/assertions

handle corner cases

write more tests

try to rule your growing codebase

Bugs

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Isn't this a job for acompiler?

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Catch'em at compile time

static type checking

non-nullable types

no implicit casts

referential transparency

Bugs

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How does Elm behave? contains c s = (List.length (String.indexes c s)) > 0

v = contains "hi" 1

Bugs

runnable example here 16 / 41

Doesn't compile contains c s = (List.length (String.indexes c s)) > 0

v = contains "hi" 1

Bugs

runnable example here 17 / 41

Cool.What is Elm?

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What is Elm

functional language

declarative UI design

ML-like syntax

immutable data structures

statically and strongly typed

Elm

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Strong typing if 0 then doStuff else differentStuff

Elm

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Immutable data structures a = 2 a = a + 3

Elm

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Functional

first-class functions

all functions can be partially applied

contains c s = (List.length (String.indexes c s)) > 0

containsH = contains "h" -- a new function

all functions always return

Elm

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Features

compiles to Javascript

follows Elm Architecture

inference engine

enforces semantic versioning

Javascript interoperability

Elm

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Elm Architecture?

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The pattern

model: the state

update: modify your state

view: show your state

Elm architecture

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The pattern

model: the state

update: modify your state

view: show your state

update changes model using messages

Elm architecture

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The counter!main = beginnerProgram { model = 0, view = view, update = update }

type Msg = Increment | Decrement

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

view model = div [] [ button [ onClick Decrement ] [ text "-" ] , div [] [ text (toString model) ] , button [ onClick Increment ] [ text "+" ] ]

A trivial example

try it live! 27 / 41

Looks like Redux

Elm architecture

actually, Redux has been inspired by the Elm architecture... 28 / 41

Looks like Redux

it is part of the language

no plugins, libraries, hacks

just one way to go

Elm architecture

actually, Redux has been inspired by the Elm architecture... 29 / 41

The virtual DOM

evaluate what changes

render what is needed

lazy evaluation

Performances

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JS output

use arrays instead of dictionaries

references instead of looking up

fully initialize objects

hooks to requestAnimationFrame to save work

Performances

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How does it compare tocompetitors?

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source 33 / 41

Sorry, I liedYou can still get runtime exceptions

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Sorry, I liedYou can still get runtime exceptions

If you use Javascript.

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Manual bootstrap var mountNode = document.getElementById('main'); var myApp = Elm.Main.embed(mountNode);

can act on a subset of the page

allows for slow integration

can build a small module

Javascript interoperability

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Ports

get data from JS code

send data to JS code

typed ports

runtime assertions

Javascript interoperability

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Inbound ports

define the port

type alias Move = (Float, Float)

port scroll : (Move -> msg) -> Sub msg

send data from JS code

myApp.ports.scroll.send([scroll, newScroll]);

Javascript interoperability

code from MaltaJS Elm app 38 / 41

Outbound ports

define the port

port todoListChanges : (List TodoItem) -> Cmd msg

listen for data

myApp.ports.todoListChanges.subscribe((updatedTodoList) => { /*...*/ })

Javascript interoperability

code from TodoMVC done in Elm 39 / 41

Elm Package Manager

forces documentation with a precise format

must document every public item

automatic semantic versioning

Package system

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References:

Elm website

Elm slack community

Elm architecture

Move fast and don't break things

Thank you!

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