1. Haskell for Scala-ists
Chris Eidhof

2. Three things
• Datatypes
• Purity
• Laziness

3. List
Type-parameter
Constructor
• data [a] = [] | a : [a]
Case distinction
Type
Constructor

4. Functions
Give me a list of a And I’ll return a
list of a
• reverse :: [a] -> [a]
• reverse [] = []
• reverse (x:xs) = reverse xs ++ [x]
Pattern Matching

5. Purity
• State = Evil

6. Purity
• Same Input
• ntial
Refere cy
Transparen
=
• Same output

7. Purity
• No side effects:
• * Variables / Mutable state
• * I/O
• * launchMissiles()

8. Example
• sort :: [Int] -> [Int]
How do we know sort doesn’t launch missiles?

9. Doing I/O
• putStrLn :: String -> IO ()
The IO type shows us it’s not pure

10. Doing I/O
(>>=) :: IO a -> (a -> IO b) -> IO b
main = putStrLn "hello! what's your name?"
>>= () -> getLine
>>= name -> putStrLn ("hello, " ++ name)

11. Doing I/O
main = do
putStrLn "hello! what's your name?"
name <- getLine
putStrLn ("hello, " ++ name)

12. Fusion
• myFunction = map square . map toInt
• = map (square . toInt)

13. Quickcheck
• Automatic testing of pure code.

14. Parallel code
• map :: (a -> b) -> [a] -> [b]
• parMap :: (a -> b) -> [a] -> [b]

15. Composability
• Pure code is easy to compose, it is
• * Stateless
• * Self-contained

16. Effects
Useful
Most Nirvana
languages
Haskell
Useless
Dangerous Safe
Simon Peyton-Jones, Caging The Effects Monster

17. Laziness
• if (x < 10 && x > 5)

18. Laziness: Quicksort
qsort [] = []
qsort (x:xs) = qsort (filter (< x) xs)
++ [x]
++ qsort (filter (>= x) xs)
minimum ls = head (quickSort ls)

19. Read more
• Real World Haskell - http://book.realworldhaskell.org/
• Haskell.org - http://haskell.org
• Haskell Café - http://haskell.org/haskellwiki/Mailing_lists
• Planet Haskell - http://planet.haskell.org/
• Haskell reddit - http://haskell.reddit.com
•

20. Getting Started
• 1. Install the Haskell Platform
• http://hackage.haskell.org/platform/
• 2. Haskell in 10 minutes
• http://haskell.org/haskellwiki/
Learn_Haskell_in_10_minutes

21. Keep in touch
• http://github.com/chriseidhof
• @chriseidhof