Haskell for Scala-ists

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Haskell for Scala-ists

  1. 1. Haskell for Scala-ists Chris Eidhof
  2. 2. Three things • Datatypes • Purity • Laziness
  3. 3. List Type-parameter Constructor • data [a] = [] | a : [a] Case distinction Type Constructor
  4. 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. 5. Purity • State = Evil
  6. 6. Purity • Same Input • ntial Refere cy Transparen = • Same output
  7. 7. Purity • No side effects: • * Variables / Mutable state • * I/O • * launchMissiles()
  8. 8. Example • sort :: [Int] -> [Int] How do we know sort doesn’t launch missiles?
  9. 9. Doing I/O • putStrLn :: String -> IO () The IO type shows us it’s not pure
  10. 10. Doing I/O (>>=) :: IO a -> (a -> IO b) -> IO b main = putStrLn "hello! what's your name?" >>= () -> getLine >>= name -> putStrLn ("hello, " ++ name)
  11. 11. Doing I/O main = do putStrLn "hello! what's your name?" name <- getLine putStrLn ("hello, " ++ name)
  12. 12. Fusion • myFunction = map square . map toInt • = map (square . toInt)
  13. 13. Quickcheck • Automatic testing of pure code.
  14. 14. Parallel code • map  :: (a -> b) -> [a] -> [b] • parMap :: (a -> b) -> [a] -> [b]
  15. 15. Composability • Pure code is easy to compose, it is • * Stateless • * Self-contained
  16. 16. Effects Useful Most Nirvana languages Haskell Useless Dangerous Safe Simon Peyton-Jones, Caging The Effects Monster
  17. 17. Laziness • if (x < 10 && x > 5)
  18. 18. Laziness: Quicksort qsort [] = [] qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++ qsort (filter (>= x) xs) minimum ls = head (quickSort ls)
  19. 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. 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. 21. Keep in touch • http://github.com/chriseidhof • @chriseidhof

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