haskell @ han arnhem 2013-2014

HaskellTjeerd Hans TerpstraMicrosoft Consultant

Atos

Agenda

• Introduction• History of Haskell• Real world Haskell• I/O• Step back• Project Euler

Introduction – who?

• Microsoft Consultant @ Atos• Microsoft Competence Leader for Atos East NL• Developer, Team lead, Scrum Master

@tjeerdhans

Introduction – who too?

• A Taste of Haskell - http://research.microsoft.com/en-us/um/people/simonpj/papers/haskell-tutorial/index.htm

Simon Peyton JonesResearcher at Microsoft Research (Cambridge, England)

http://research.microsoft.com/en-us/people/simonpj/

Introduction – what?

• Haskell is a programming language that is• purely functional• lazy• higher order• strongly typed• general purpose

Introduction – why?

• Functional programming will make you think differently about programming• Mainstream languages are all about state• Functional programming is all about values

• Whether or not you drink the Haskell whisky, you'll be a better programmer in whatever language you regularly use.

History of Haskell

History of Haskell

History of Haskell 3

History of Haskell 4

Real world Haskell© xkcd http://xkcd.com/1312/

Real world Haskell

• Text-based tools• Imperative input/output programming• Parsing, transforming data

• GUI programming• Event driven programming• Hook up events to callback functions in Haskell

• Database programming• Network programming

• Examples: http://book.realworldhaskell.org/read

Real world Haskell – xmonad

• xmonad is an X11 tiling window manager written entirely in Haskell

…

xmo..-wut?! DEMO!

Real world Haskell – xmonad

• xmonad is an X11 tiling window manager written entirely in Haskell

Why xmonad?

• Because it's • A real program• of manageable size• that illustrates many Haskell programming techniques• is open-source software• is being actively developed by an active community

Manageable size

Code Comments Language

metacity >50000 C

ion3 20000 700 C

larswm 6000 130 C

wmii 6000 100 C

dwm 4.2 1500 200 C

xmonad 0.2 500 700 Haskell

Inside xmonad

Things to notice

• Purity makes the interface explicit

• Takes a stack, and returns a stack; that is all

• Takes a stack; may modify it; may modify other persistent state; may do I/O

I/O

Where’s the I/O in xmonad?• All this pure stuff is nice, but sooner or later we

have to:• Talk to X11, whose interface is not at all pure• Do input/output (other programs)

Doing I/O

• Idea:

• But: nowmight do arbitrary stateful things • And what does this do?

• What order are the things printed?• Are they printed at all?

Order of evaluation!

Laziness!

The main idea

A value of type (IO t) is an “action” that, when performed, may do some input/output

before delivering a result of type t

A helpful picture

A value of type (IO t) is an “action” that, when performed, may do some input/output

before delivering a result of type t

Simple I/O

getLine :: IO StringputStr :: String -> IO ()

main :: IO ()Main = putStr “Hello World”

Main program is an action of type IO ()

Connecting actions up

Goal:read a line and then write it back out

Connecting actions up

We have connected two actions tomake a new, bigger action.

Getting two lines

We want to just return (s1,s2)

The return combinator

Taking a step back

The central challenge

The challenge of effects

Project Euler

http://projecteuler.net

Thanks