A short talk on what makes Functional Programming - and especially Haskell - different. We'll take a quick overview of Haskell's features and coding style, and then work through a short but complete example of using it for a Real World problem. http://lanyrd.com/2011/geekup-liverpool-may/sdykh/

1. Haskell in the Real World
Functional Programming Night
Geekup Liverpool, 31st May, 2011
hakim.cassimally@gmail.com
http://www.fickr.com/photos/jef_saf/3493852795/

2. What makes FP different?
● MJD quoting Norvig (on Lisp):
● “big, important features, features like frst-class
functions, dynamic access to the symbol table,
and automatic storage management.”
● gluing functions together
● declarative

3. Popular FP languages
● Excel
● SQL?
● Linq (based on Haskell's monads)
● Lisp → Scheme → Clojure
● Strongly Typed FP (Hindley/Milner)
● ML → Ocaml → F#
● Haskell

4. What makes Haskell different?
● Purity
● Laziness
● High Level
● Strong Typing
● Memory Managed
● Modular
● Mathematical rigour
● category theory

5. Sounds a bit ivory tower?
●
http://prog21.dadgum.com/31.html
● Q “When will Haskell fnally go
mainstream?”
● A “most of it already has.”

6. Imperative programming
●
records =
[ "one", "two", "three", "four", "five" ]
filtered = []; j = 0;
for (i = 0; i < length records; i++) {
if (records[i] matches “o”) {
filtered[j++] = records[i];
}
}

7. Imperative programming
●
records =
[ "one", "two", "three", "four", "five" ]
filtered = []; j = 0;
for (i = 0; i < length records; i++) {
if (records[i] matches “o”) {
filtered[j++] = records[i];
}
}

8. Functional version
●
filtered =
filter (=~ “o”)
records

9. Why is this better?
● less code. less bugs
● no synthetic variables
●
i, j, length records
● no risk of off-by-one error
● intent clear from skimming
● intent clear to compiler
● parallelize (MapReduce, Hadoop)

10. Why is this better?
● fewer codes. fewer bugs
● no synthetic variables
●
i, j, length records
● no risk of off-by-one error
● intent clear from skimming
● intent clear to compiler
● parallelize (MapReduce, Hadoop)

11. Your language has this construct
● Perl: my @filtered = grep /o/, @records;
● .Net: var filtered = from r in records
where r.match('o') select r
● Ruby: @filtered = @records.grep /o/
● Python: filtered = [x for x in records
if re.match('o', x)]
● etc.

12. Another example
●
countCaps = length
. filter (isUpper . head)
. words
> countCaps “Hello there, Fred”
2

13. Real World Haskell
● JSON library
● barcode reading
● database

14. Quizzes
● important subsystem of Makini's attention
management system
● real world (has customers, pays wages)
● currently written in Perl
● could it be ported to Haskell?

15. Haskell Quiz – proof of concept
● full code at:
●
https://github.com/
osfameron/geekup-talk-haskell/
● overview, to give a favour of programming
in Haskell

16. Modelling the quiz

17. Modelling the quiz

18. Modelling the quiz

19. Modelling the quiz
2x

20. Modelling the quiz

21. Modelling the quiz
1x

22. Quiz tree data types
● Quizzes
● Sections
● (randomized sections)
● Questions

23. Quiz data type
●
data QuizNode =
Quiz Name [QuizNode]
| Section Name Score [QuizNode]
| RandomSection Name Score Choose [QuizNode]
| Question Name Score Answer

24. Quiz data type
●
data QuizNode =
Quiz Name [QuizNode]
| Section Name Score [QuizNode]
| RandomSection Name Score Choose [QuizNode]
| Question Name Score Answer
●
type Name = String
type Score = Int
type Choose = Int

25. Quiz data type
●
data QuizNode =
Quiz Name [QuizNode]
| Section Name Score [QuizNode]
| RandomSection Name Score Choose [QuizNode]
| Question Name Score Answer
●
data Answer = MultiChoice [BoolAnswer]
| StringChoice [String]

26. Quiz data type
●
data QuizNode =
Quiz Name [QuizNode]
| Section Name Score [QuizNode]
| RandomSection Name Score Choose [QuizNode]
| Question Name Score Answer
●
data Answer = MultiChoice [BoolAnswer]
| StringChoice [String]
●
data BoolAnswer = BoolAnswer Bool String

27. Quiz data type
quiz,geo,pop :: QuizNode
quiz = Quiz “General Knowledge Quiz” [ pop, geo ]
geo = RandomSection “Geography” 40 2 [
Question “What is the capital of England?”
2 $ StringChoice [“London”],
Question “What is the capital of France?”
2 $ StringChoice [“Paris”],
Question “What is the capital of Finland?”
2 $ StringChoice [“Helsinki”],
Question “What is the capital of Italy?”
2 $ StringChoice [“Rome”, “Roma”],
]

28. Quiz data type
quiz,geo,pop :: QuizNode
quiz = Quiz “General Knowledge Quiz” [ pop, geo ]
geo = RandomSection “Geography” 40 2 [
Question “What is the capital of England?”
2 $ StringChoice [“London”],
Question “What is the capital of France?”
2 $ StringChoice [“Paris”],
Question “What is the capital of Finland?”
2 $ StringChoice [“Helsinki”],
Question “What is the capital of Italy?”
2 $ StringChoice [“Rome”, “Roma”],
]

29. Quiz data type
quiz,geo,pop :: QuizNode
quiz = Quiz “General Knowledge Quiz” [ pop, geo ]
geo = RandomSection “Geography” 40 2 [
Question “What is the capital of England?”
2 $ StringChoice [“London”],
Question “What is the capital of France?”
2 $ StringChoice [“Paris”],
Question “What is the capital of Finland?”
2 $ StringChoice [“Helsinki”],
Question “What is the capital of Italy?”
2 $ StringChoice [“Rome”, “Roma”],
]

30. Quiz data type
pop = Section “Pop music” 60 [
Question “Which of these are Beatles?” 5
$ MultiChoice [
y “John”,
y “Paul”,
y “George”,
y “Ringo”,
n “Bob”,
n “Jason” ],
...

31. Quiz data type
pop = Section “Pop music” 60 [
Question “Which of these are Beatles?” 5
$ MultiChoice [
BoolAnswer True “John”,
BoolAnswer True “Paul”,
BoolAnswer True “George”,
BoolAnswer True “Ringo”,
BoolAnswer False “Bob”,
BoolAnswer False “Jason” ],
...

32. Quiz data type
pop = Section “Pop music” 60 [
Question “Which of these are Beatles?” 5
$ MultiChoice [
y “John”,
y “Paul”,
y “George”,
y “Ringo”,
n “Bob”,
n “Jason” ],
...
y,n :: String -> BoolAnswer
y = BoolAnswer True
n = BoolAnswer False

33. Stamping the quiz
1x 2x

34. Stamping the quiz
1x 2x

35. Stamping the quiz
stamp ::
QuizNode → ...
1x 2x

36. Stamping the quiz
stamp ::
QuizNode → QuizNode?
1x 2x

37. Functions
●
increment :: Num → Num
●
increment 4 => 5
●
increment 10 => 11

38. Functions
●
increment :: Num → Num
●
increment 4 => 5
●
increment 10 => 11

39. Functions
●
increment :: Num → Num
●
increment 4 => 5
●
increment 10 => 11

40. Functions
●
increment :: Num → Num
●
increment x = x+1

41. Functions
let x = 42
addx :: Num → Num
add y = x + y

42. Functions
(cannot
let x = 42 change!)
addx :: Num → Num
add y = x + y

43. Functions
let x = 42
addx :: Num → Num
add y = x + y

44. Stamping the quiz
stamp ::
QuizNode → QuizNode?
1x 2x

45. Stamping the quiz
stamp ::
QuizNode → QuizNode?
1x 2x

46. Stamping the quiz
stamp ::
QuizNode → IO QuizNode
1x 2x

47. Monads
● Useful data-structure
● Lets us model various thing...
● including IO in a pure language
● Concept is a little confusing
● Using them is (mostly) not too bad.

48. Stamping the quiz
stamp ::
QuizNode → IO QuizNode

49. Stamping the quiz
stamp ::
QuizNode → IO QuizNode
1x 2x

50. Stamping the quiz
stamp ::
QuizNode → IO QuizNode
1x 2x

51. Stamping the quiz
stamp ::
QuizNode → IO QuizNode

52. Stamping the quiz
stamp ::
QuizNode → IO QuizNode

53. The stamp function
stamp :: QuizNode -> IO QuizNode
stamp q@(Question _ _ _) = return q
stamp (Quiz s ns) = Quiz s <$> mapM stamp ns
stamp (Section s i ns) = Section s i <$> mapM stamp ns
stamp (RandomSection s i r ns)
= do selected <- pickN r ns
Section s i <$> mapM stamp selected

54. The stamp function
stamp :: QuizNode -> IO QuizNode
stamp q@(Question _ _ _) = return q
stamp (Quiz s ns) = Quiz s <$> mapM stamp ns
stamp (Section s i ns) = Section s i <$> mapM stamp ns
stamp (RandomSection s i r ns)
= do selected <- pickN r ns
Section s i <$> mapM stamp selected

55. The stamp function
map stamp ns
stamp :: QuizNode -> IO QuizNode – “stamp all the
child nodes in
stamp q@(Question _ _ _) = return q turn”
stamp (Quiz s ns) = Quiz s <$> mapM stamp ns
stamp (Section s i ns) = Section s i <$> mapM stamp ns
stamp (RandomSection s i r ns)
= do selected <- pickN r ns
Section s i <$> mapM stamp selected

56. The stamp function
stamp :: QuizNode -> IO QuizNode
stamp q@(Question _ _ _) = return q
stamp (Quiz s ns) = Quiz s <$> mapM stamp ns
stamp (Section s i ns) = Section s i <$> mapM stamp ns
stamp (RandomSection s i r ns)
= do selected <- pickN r ns
Section s i <$> mapM stamp selected

57. The stamp function
stamp :: QuizNode -> IO QuizNode
stamp q@(Question _ _ _) = return q
stamp (Quiz s ns) = Quiz s <$> mapM stamp ns
stamp (Section s i ns) = Section s i <$> mapM stamp ns
stamp (RandomSection s i r ns)
= do selected <- pickN r ns
Section s i <$> mapM stamp selected
1x

58. Taking the quiz!
●
takeNode ::
QuizNode -> IO CompletedNode
●
printQuestion ::
QuizNode -> IO ()
●
showBoolTextAnswers ::
[BoolAnswer] -> String
●
checkAnswer ::
String -> Answer -> Bool

59. Taking the quiz!
●
takeNode ::
QuizNode -> IO CompletedNode
●
printQuestion ::
QuizNode -> IO ()
●
showBoolTextAnswers ::
[BoolAnswer] -> String
●
checkAnswer ::
String -> Answer -> Bool

60. Taking the quiz!
●
takeNode ::
QuizNode -> IO CompletedNode
●
printQuestion ::
QuizNode -> IO ()
●
showBoolTextAnswers ::
[BoolAnswer] -> String
●
checkAnswer ::
String -> Answer -> Bool

61. Taking the quiz!
●
takeNode ::
QuizNode -> IO CompletedNode
●
printQuestion ::
QuizNode -> IO ()
●
showBoolTextAnswers ::
[BoolAnswer] -> String
●
checkAnswer ::
String -> Answer -> Bool

62. takeNode
takeNode node@(Question s i a) = do
printQuestion node
ans <- getLine
let correct = checkAnswer ans a
let score = if correct
then (i,i) else (0,i)
putStrLn $ if correct
then “Correct!” else “Wrong!”
return $
CompletedNode ans score [] node

63. main
main :: IO ()
main = stamp quiz >>= takeQuiz
Function, not entrypoint

64. main
main :: IO ()
main = stamp quiz >>= takeQuiz
Live Demo!

65. Should you go Haskell?
● Power
● Speed?
● can be faster than C (supercompilation)
● can be tricky to optimize
● Jobs?
● In NorthWestUK?
● Libraries & Tools
● Haskell Platform. Hackage. Cabal

66. Should you learn Haskell?
● Powerful
● Interesting techniques
● … and ways of thinking about problems
● Ready for future shift to FP
● … possibly in your own language

67. Thank you! Questions?
● full code at:
●
https://github.com/
osfameron/geekup-talk-haskell/
● hakim.cassimally@gmail.com
http://www.fickr.com/photos/jef_saf/3493852795/