A new paradigm

Express your ideas clearly and learn a new way of thinking about programming. Based on lambda calculus Haskell is a purely functional programming language that features referential transparency, immutability and lazy evaluation. Concepts that will blow your mind — relearn programming while having an absolute blast.

Composition and predictability

Reason about large pieces of code and compose them easily. There is no global state or mutable variables obscuring the meaning of your program. The strong type system makes sure there are no surprises — never again will you have to guess what your program does at execution time.

Declarative

Write your programs declaratively by utilizing the power of pure functions and algebraic data types. In Haskell we don't write how a program should be executed, we just describe its logic — never again be forced to think about evaluation order or execution details.

Performance

Squeeze out the last ticks of your multi-core processors, thanks to best-in-class support for async, concurrent and parallel programming... made possible via garbage collection and green threads. Use advanced streaming libraries for ultra efficient data processing.

Abstraction

Build powerful abstractions that are not possible in other languages. Only your imagination is the limit, due to polymorphism, type classes and more advanced typesystem features. Haskell has its roots in programming language research and will always be at the forefront of expressivity.

Excellent tooling

A tooling story that's truly amazing: spawn your toolchain with GHCup, build your project with cabal, get editor integration with haskell-language-server — everything at your fingertips. GHC is the next generation compiler that supports all of your favorite platforms.

Statically typed

Every expression in Haskell has a type which is determined at compile time. All the types composed together by function application have to match up. If they don't, the program will be rejected by the compiler. Types become not only a form of guarantee, but a language for expressing the construction of programs.

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char = 'a'    :: Char
int = 123     :: Int
fun = isDigit :: Char -> Bool

isDigit 1

bytes = Crypto.Hash.SHA1.hash "hello" :: ByteString
text = decodeUtf8 bytes               :: Text

doubleDecode = decodeUtf8 (decodeUtf8 bytes)

Purely functional

Every function in Haskell is a function in the mathematical sense (i.e., "pure"). Even side-effecting IO operations are but a description of what to do, produced by pure code. There are no statements or instructions, only expressions which cannot mutate variables (local or global) nor access state like time or random numbers.

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square :: Int -> Int
square x = x * x

"Hello: " ++ "World!" 

"Name: " ++ getLine

Type inference

You don't have to explicitly write out every type in a Haskell program. Types will be inferred by unifying every type bidirectionally. However, you can write out types if you choose, or ask the compiler to write them for you for handy documentation.

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main :: IO ()
main = do line :: String <- getLine
          print (parseDigit line)
  where parseDigit :: String -> Maybe Int
        parseDigit ((c :: Char) : _) =
          if isDigit c
             then Just (ord c - ord '0')
             else Nothing

main = do line <- getLine
          print (parseDigit line)
  where parseDigit (c : _) =
          if isDigit c
             then Just (ord c - ord '0')
             else Nothing

do ss <- decode "[\"Hello!\",\"World!\"]"
   is <- decode "[1,2,3]"
   return (zipWith (\s i -> s ++ " " ++ show (i + 5)) ss is)
 => Just ["Hello! 6","World! 7"]

do ss <- decode "[1,2,3]"
   is <- decode "[null,null,null]"
   return (zipWith (\s i -> s ++ " " ++ show (i + 5)) ss is)
 => Nothing

Concurrent

Haskell lends itself well to concurrent programming due to its explicit handling of effects. Its flagship compiler, GHC, comes with a high-performance parallel garbage collector and light-weight concurrency library containing a number of useful concurrency primitives and abstractions.

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main = do
  done <- newEmptyMVar
  forkIO (do putStrLn "I'm one thread!"
             putMVar done "Done!")
  second <- forkIO (do threadDelay 100000
                       putStrLn "I'm another thread!")
  killThread second
  msg <- takeMVar done
  putStrLn msg

do a1 <- async (getURL url1)
  a2 <- async (getURL url2)
  page1 <- wait a1
  page2 <- wait a2
  ...

transfer :: Account -> Account -> Int -> IO ()
transfer from to amount =
  atomically (do deposit to amount
                 withdraw from amount)

main = atomically (putStrLn "Hello!")

Lazy

Functions don't evaluate their arguments. This means that programs can compose together very well, with the ability to write control constructs (such as if/else) just by writing normal functions. The purity of Haskell code makes it easy to fuse chains of functions together, allowing for performance benefits.

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when p m = if p then m else return ()
main = do args <- getArgs
          when (null args)
               (putStrLn "No args specified!") 

if c then t else False

and c t = if c then t else False

any :: (a -> Bool) -> [a] -> Bool
any p = or . map p

Packages

Open source contribution to Haskell is very active with a wide range of packages available on the public package servers.

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