What's Elm?

Why should I be interested?

10/02/2016 - Elm London Meetup

@doppioslash

+ClaudiaDoppioslash

www.lambdacat.com

About me

Functional Programmer & Game Developer

Peer Stritzinger GmbH

Functional and Failure Tolerant Programming for Embbedded, Industrial Control and Automotive

Table of Contents

 

  • What is cool about Elm?
  • Time Travelling Debugger
  • Meet Elm
  • Signals
  • Structuring programs in Elm
  • Why use statically typed pure Functional languages?

What is cool about Elm?

Elm is a Functional Reactive Language for interactive applications.

 

  • Functional Reactive Programming
  • Time Travelling Debugger
  • Avoid NULL pointers
  • Escape callback hell
  • Compiles to fast Javascript/Html/CSS

What does the best programming workflow look like?

Inventing

on

Principle

As seen on LambdaCat:

Inventing on Principle

Hot reload: reload changes to the code without stopping the game

Inventing on Principle

Time Travel:

being able to scrub to any point in the session

Inventing on Principle

Omniscience:

see all the state in the session

Inventing on Principle

Could we possibly have that?

Elm is getting near to that ideal

(debug.elm-lang.org 

Mario demo)

What sort of language is Elm?

What sort of language is Elm?

Purely Functional

Functional Reactive

(FRP)

What sort of language is Elm?

Eager

What sort of language is Elm?

Static Type System

What sort of language is Elm?

Compiles to Javascript, HTML, CSS

What sort of language is Elm?

Interoperates with Javascript while still being type safe

What sort of language is Elm?

Small

What sort of language is Elm?

Invented by Evan Czaplicki for his thesis

What sort of language is Elm?

Meant to be approachable and practical

What sort of language is Elm?

Meant to be approachable and practical

No 'scary' terms like Monad

In Production at

Elm Reactor

(the Time Travelling Debugger)

Elm Reactor

Inspired by Inventing on Principle

Elm Reactor

Fundamentals made in a few days by Laszlo

Elm Reactor

Elm's language design is accidentally 'compatible'

Elm Reactor

You can "step through" while moving the mouse

Elm Fundamentals

Elm Fundamentals

  • Signals

  • Foldp

  • Mailboxes

  • Tasks

  • Effects

What are Signals?

Stream of values

What are Signals?

What are Signals?

What are Signals?

a different way of thinking about variables

What are Signals?

an explicit model of variable mutation in time

Demo

What are Signals?

non-awkward way of structuring callbacks

What are Signals?

they are wired in signal graphs 

(directed acyclic graphs)

What are Signals?

What kind of functions can we apply on signals?

What kind of functions can we apply on signals?

map : (a -> result) -> Signal a -> Signal result

applies a function on a signal

returns another, transformed, signal

What kind of functions can we apply on signals?

makes a signal that depends on the past values of a signal

foldp : (a -> state -> state) -> state -> Signal a -> 

Signal state

What kind of functions can we apply on signals?

merge : Signal a -> 

Signal a -> Signal a

What kind of functions can we apply on signals?

 

filter : (a -> Bool) -> a -> 

Signal a -> Signal a

What kind of functions can we apply on signals?

map2 : (a -> b -> result) -> 

Signal a -> Signal b -> 

Signal result

applies a function on two signals

How do you structure programs in Elm?

The Functional Triforce

Maybe

Union Types

Pattern Matching

The Functional Triforce

Union Types

define the Model:

type Tile
    = Door Size
    | Column
    | BackGround BackGroundTile
    | Shadow ShadowTile

The Functional Triforce

Maybe

gets rid of NULL errors

type Maybe a 
    = Just a 
    | Nothing

The Functional Triforce

Pattern Matching

String.toInt : String -> Maybe Int

toMonth : String -> Maybe Int toMonth rawString = 
    case String.toInt rawString of
      Nothing -> 
         Nothing 
      Just n -> 
         if n > 0 && n <= 12 then Just n else Nothing

Elm Architecture

Elm is opinionated

Elm Architecture

Use the

"Elm Architecture"

Elm Architecture

Model

View

Update

Overview

Elm Architecture

Model =

the Data Structure we pass around

Model

type alias UserInput = {}

userInput : Signal UserInput

userInput Signal.constant {}

type alias Input =

    { timeDelta : Float

​    , userInput : UserInput

    }

Model

type alias UserInput = {}

userInput : Signal UserInput

userInput Signal.constant {}

type alias Input =

    { timeDelta : Float

​    , userInput : UserInput

    }

Type signature

Model

type alias UserInput = {}

userInput : Signal UserInput

userInput Signal.constant {}

type alias Input =

    { timeDelta : Float

​    , userInput : UserInput

    }

A record

Type alias

Signal

Model

Record =

set of key value pairs

Model

Type alias =

give name to a set of fields in a record

Model

Signal =

updates every time variable changes

Model

type alias GameState = {}

defaultGame : GameState

defaultGame {}

Elm Architecture

View =

the code that renders from the Model

Display

display : (Int,Int) -> GameState -> Element

display (w,h) gameState show gameState

Display

display : (Int,Int) -> GameState -> Element

display (w,h) gameState show gameState

Type signature

Elm Architecture

Update =

the function that does the change of state 

Update

stepGame : Input -> GameState -> GameState

stepGame {timeDelta,userInput} gameState 

                    gameState

Update

stepGame uses the current Input

(which is UserInput and a timeDelta) 

to make a new GameState

Elm Architecture

Signals

Signals

delta : Signal Float

delta Time.fps 30

input : Signal Input

input Signal.sampleOn delta    

     (Signal.map2 Input delta userInput)

sampleOn

Signals

delta : Signal Float

delta Time.fps 30

input : Signal Input

input Signal.sampleOn delta    

     (Signal.map2 Input delta userInput)

map2

Signals

gameState : Signal GameState

gameState Signal.foldp stepGame

           defaultGame input

Signals

gameState : Signal GameState

gameState Signal.foldp stepGame

           defaultGame input

foldp

Signals

Signal.map2 = 

applies a function that takes 2 arguments 

to a signal

Signals

Signal.sampleOn = samples from 2nd input anytime an event occurs in the 1st

Signals

Signal.foldp =

a signal dependent on the past 

Main

(where you wire everything up)

Elm Architecture

Main

main : Signal Element

main Signal.map2 display

       Window.dimensions gameState

Main

main : Signal Element

main Signal.map2 display

       Window.dimensions gameState

Signal of Element values

Overview (again)

What is a foldp?

Fold from the past

Fold from the past

Fold from the past

What is a foldp?

foldp takes (a -> state -> state) 

a function

What is a foldp?

... -> state -> ...

a default state

What is a foldp?

... -> Signal a -> ...

an input signal

What is this foldp thing?

... -> Signal state

returns a signal

(= next state of the program, after applying update)

StartApp

app structure built from best practices

StartApp

mainly for web UIs

(games are better modelled with signals)

StartApp

will save you time, at the price of hiding some complexity

StartApp

How Elm Reactor works

Record Inputs

How Elm Reactor works

Reapplies functions to inputs

How Elm Reactor works

previous state

+

previous inputs =

next state

How Elm Reactor works

snapshotting for performance

How Elm Reactor works

How Elm Reactor works

changes to types will break hot swapping

also if a change doesn't compile

How Elm Reactor works

Why is Elm good for Time Travelling?

Why Elm Reactor works

Applying the same inputs will return the same output

(referential transparency)

Why Elm Reactor works

No side effects =

can replay code

Why Elm Reactor works

All mutable state is stored in the foldp

Static signal graph

Why Elm Reactor works

Why use statically typed pure Functional languages?

Why it's worth to use new language research

No runtime exceptions

Why it's worth to use new language research

No race conditions

Why it's worth to use new language research

Better tools

Why it's worth to use new language research

Less code

Why it's worth to use new language research

Ease of parallelisation

elm-lang.org/docs

@elmlang

#elmlang <- twitter hashtag

elmlang.herokuapp.com <- get a Slack invite here

Where to learn Elm?

lambdacat.com

@lambda_cat

Read LambdaCat!

What's Elm? Why should I be interested? at London Elm Meetup

By Claudia Doppioslash

What's Elm? Why should I be interested? at London Elm Meetup

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