The document discusses a type-driven approach to functional design. It presents a line break algorithm that takes a string and breaks it into words, lines, and joined lines using a series of functions. Each function builds on the previous one by accepting and returning the same types, keeping the algorithm in the same "shape". This approach focuses on the data and favors a combinator style where types guide the design.

1. A Type Driven Approach to
Functional Design
Michael Feathers
Groupon

2. A Type Driven Approach to
Functional Design
(just because)
Michael Feathers
Groupon

4. a

5. [a]

6. map :: (a -> b) -> [a] -> [b]

7. map :: (a -> b) -> [a] -> [b]
map (+2) [1,2,3]

8. region 7 9 “expertsexchange”

9. region 7 9 “expertsexchange”
region :: Int -> Int -> String -> String

10. regionFrom7 9 “expertsexchange”
regionFrom7 :: Int -> Int -> String -> String

11. Hoping

12. Line Break algorithm

13. String -> String

14. String -> [String] -> [[String]] -> [String] -> String

15. String -> [String] -> [[String]] -> [String] -> String
breakTextIntoWords

16. String -> [String] -> [[String]] -> [String] -> String
breakTextIntoWords
breakWordsIntoLines

17. String -> [String] -> [[String]] -> [String] -> String
breakTextIntoWords
breakWordsIntoLines
joinWordsInBrokenLines

18. String -> [String] -> [[String]] -> [String] -> String
breakTextIntoWords
breakWordsIntoLines
joinWordsInBrokenLines
joinBrokenLines

19. String -> [String] -> [[String]] -> [String] -> String

20. String -> [String] -> [[String]] -> [String] -> String
String -> [String] -> (Int -> [[String]]) -> [[String]] ->
[String] -> String

21. String -> [String] -> [[String]] -> [String] -> String
String -> [String] -> (Int -> [[String]]) -> [[String]] ->
[String] -> String

22. String -> [String] -> [[String]] -> [String] -> String
String -> [String] -> (Int -> [[String]]) -> [[String]] ->
[String] -> String
Abuse!

23. A Design Device

24. (via Scott Ambler)

25. (via Scott Ambler)

33. String -> [String] -> [[String]] -> [String] -> String
words

34. String -> [String] -> [[String]] -> [String] -> String
words

35. String -> [String] -> [[String]] -> [String] -> String
words
brokenLines

36. String -> [String] -> [[String]] -> [String] -> String
words
brokenLines
wordJoinedLines

37. String -> [String] -> [[String]] -> [String] -> String
words
brokenLines
wordJoinedLines
joinedLines

40. Favors combinator style

41. Favors combinator style
[String] -> Int

42. Favors combinator style
[String] -> Int
[String] -> [Int] -> [Int] -> [Int] -> Int
(repeated types in endomorphic chain)

43. Affordances

44. Separate bins for your head and
your notation
String -> [String] -> (Int -> [[String]]) -> [[String]] ->
[String] -> String

45. Concentrates on Data

46. Concentrates on Data
..in naming

47. Favors combinator style
Staying in the same shape is the easiest way to get from here to there

48. Thank you