This document provides an introduction and overview of the Haskell programming language. It discusses: - The speaker's background and an introduction to Simon Peyton Jones, a researcher at Microsoft. - An overview of what Haskell is - a purely functional, lazy, higher-order, strongly typed general purpose language. - Why learning Haskell is useful even if you don't use it primarily - it encourages different thinking that can improve programming in any language. - A brief history of the development of Haskell and how it has evolved over time through various versions. - Examples of real-world applications of Haskell like text tools, GUI programming, databases, networking, and the tiling window manager xmonad

1. Haskell
Tjeerd Hans Terpstra
Microsoft Consultant
Atos

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

3. Introduction – who?
• Microsoft Consultant @ Atos
• Microsoft Competence Leader for Atos East NL
• Developer, Team lead, Scrum Master
@tjeerdhans

4. Introduction – who too?
Simon Peyton Jones
Researcher at Microsoft Research (Cambridge, England)
http://research.microsoft.com/en-us/people/simonpj/
• A Taste of Haskell - http://research.microsoft.com/enus/um/people/simonpj/papers/haskell-tutorial/index.htm

5. Introduction – what?
• Haskell is a programming language that is
•
•
•
•
•
purely functional
lazy
higher order
strongly typed
general purpose

6. 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.

7. History of Haskell

8. History of Haskell

9. History of Haskell 3

10. History of Haskell 4

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

12. 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

13. Real world Haskell – xmonad
• xmonad is an X11 tiling window manager written
entirely in Haskell
…
xmo..-wut?! DEMO!

14. Real world Haskell – xmonad
• xmonad is an X11 tiling window manager written
entirely in Haskell

15. 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

16. Manageable size
Code
metacity
ion3
Comments
Language
>50000
C
20000
700
C
larswm
6000
130
C
wmii
6000
100
C
dwm 4.2
1500
200
C
500
700
Haskell
xmonad 0.2

17. Inside xmonad

26. 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

27. I/O

28. 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)

29. Doing I/O
• Idea:
• But: now
might do arbitrary stateful things 
• And what does this do?
• What order are the things printed?
• Are they printed at all?
Order of
evaluation!
Laziness!

30. 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

31. 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

32. Simple I/O
getLine :: IO String
putStr :: String -> IO () Main program is an
action of type IO ()
main :: IO ()
Main = putStr “Hello World”

33. Connecting actions up
Goal:
read a line and then write it back out

34. Connecting actions up
We have connected two actions to
make a new, bigger action.

35. Getting two lines
We want to just return (s1,s2)

36. The return combinator

37. Taking a step back
The central challenge

38. The challenge of effects

39. Project Euler
http://projecteuler.net

40. Thanks