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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)