Haskell for Scala-ists

Haskell for Scala-ists Chris Eidhof

Three things • Datatypes • Purity • Laziness

List • data [a] = [] | a : [a]

List • data [a] = [] | a : [a] Type

List Type-parameter • data [a] = [] | a : [a] Type

List Type-parameter • data [a] = [] | a : [a] Type Constructor

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

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

Functions • reverse :: [a] -> [a] • reverse [] = [] • reverse (x:xs) = reverse xs ++ [x]

Functions Give me a list of a • reverse :: [a] -> [a] • reverse [] = [] • reverse (x:xs) = reverse xs ++ [x]

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]

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

Purity • State = Evil

Purity • Same Input • = • Same output

Purity • Same Input • ntial Refere cy Transp aren = • Same output

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

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

Doing I/O • putStrLn :: String -> IO ()

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

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

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

Fusion • myFunction = map square . map toInt

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

Quickcheck • Automatic testing of pure code.

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

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

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

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

Laziness • if (x < 10 && x > 5)

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

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 •

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

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

Haskell for Scala-ists

by chriseidhof on Oct 12, 2010

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