Haskell Language

Time 2020-07-14 15:16:25

Web Name: Haskell Language

WebSite: http://www.haskell.org

ID:25299

Keywords:

Haskell,Language,

Description:

Escape from the ivory tower: The Haskell journey, by Simon Peyton-Jones Haskell taketh away: limiting side effects for parallel programming, by Ryan Newton Haskell is Not For Production and Other Tales, by Katie Miller Your First Web Application with Spock, by Oskar Wickström 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. Click to expand All Haskell values have a type: char = 'a' :: Charint = 123 :: Intfun = isDigit :: Char - Bool You have to pass the right type of values to functions, or the compiler will reject the program: Type error isDigit 1 You can decode bytes into text: bytes = Crypto.Hash.SHA1.hash "hello" :: ByteStringtext = decodeUtf8 bytes :: Text But you cannot decode Text, which is already a vector of Unicode points: Type error 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. Click to expand The following function takes an integer and returns an integer. By the type it cannot do any side-effects whatsoever, it cannot mutate any of its arguments. square :: Int - Intsquare x = x * x The following string concatenation is okay: "Hello: " ++ "World!" The following string concatenation is a type error: Type error "Name: " ++ getLine Because getLine has type IO String and not String, like Name: is. So by the type system you cannot mix and match purity with impurity. 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. Click to expand This example has a type signature for every binding: 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 But you can just write: main = do line - getLine print (parseDigit line) where parseDigit (c : _) = if isDigit c then Just (ord c - ord '0') else Nothing You can also use inference to avoid wasting time explaining what you want: 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"] Types give a parser specification for free, the following input is not accepted: 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. Click to expand Easily launch threads and communicate with the standard library: 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 Use an asynchronous API for threads: do a1 - async (getURL url1) a2 - async (getURL url2) page1 - wait a1 page2 - wait a2 ... Atomic threading with software transactional memory: transfer :: Account - Account - Int - IO ()transfer from to amount = atomically (do deposit to amount withdraw from amount) Atomic transactions must be repeatable, so arbitrary IO is disabled in the type system: Type error 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. Click to expand Define control structures easily: when p m = if p then m else return ()main = do args - getArgs when (null args) (putStrLn "No args specified!") If you notice a repeated expression pattern, like if c then t else False you can give this a name, like and c t = if c then t else False and then use it with the same effect as the orginal expression. Get code re-use by composing lazy functions. It's quite natural to express the any function by reusing the map and or functions: any :: (a - Bool) - [a] - Boolany p = or . map p Reuse the recursion patterns in map, filter, foldr, etc. Packages Open source contribution to Haskell is very active with a wide range of packages available on the public package servers. Click to expand There are 6,954 packages freely available. Here is a sample of the most common ones: DataDog provides powerful, customizable 24/7 metrics and monitoring integration for all of Haskell.org, and complains loudly for us when things go wrong. Fastly's Next Generation CDN provides low latency access for all of Haskell.org's downloads and highest traffic services, including the primary Hackage server, Haskell Platform downloads, and more. Packet provides compute, storage, and networking resources, powering almost all of Haskell.org in several regions around the world. Galois provides infrastructure, funds, administrative resources and has historically hosted critical Haskell.org infrastructure, as well as helping the Haskell community at large with their work. DreamHost has teamed up to provide Haskell.org with redundant, scalable object-storage through their Dream Objects service. Webmon provides monitoring and escalation for core haskell.org infrastructure. Psst! Looking for the wiki? This is the new Haskell home page! The wiki has moved to wiki.haskell.org. Got changes to contribute to the site? Fork or comment on Github Proudly hosted by

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