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Haskell (pronounced [ˈhæskəl][2][3]) is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. [edit] HistoryFollowing the release of Miranda by Research Software Ltd, in 1985, interest in lazy functional languages grew.[citation needed] By 1987, more than a dozen non-strict, purely functional programming languages existed. Of these, Miranda was the most widely used, but was not in the public domain. At the conference on Functional Programming Languages and Computer Architecture (FPCA '87) in Portland, Oregon, a meeting was held during which participants formed a strong consensus that a committee should be formed to define an open standard for such languages. The committee's purpose was to consolidate the existing functional languages into a common one that would serve as a basis for future research in functional-language design.[4] The first version of Haskell ("Haskell 1.0") was defined in 1990.[5] The committee's efforts resulted in a series of language definitions. In late 1997, the series culminated in Haskell 98, intended to specify a stable, minimal, portable version of the language and an accompanying standard library for teaching, and as a base for future extensions. The committee expressly welcomed the creation of extensions and variants of Haskell 98 via adding and incorporating experimental features.[4] In February 1999, the Haskell 98 language standard was originally published as "The Haskell 98 Report".[4] In January 2003, a revised version was published as "Haskell 98 Language and Libraries: The Revised Report".[6] The language continues to evolve rapidly, with the GHC implementation representing the current de facto standard. In early 2006, the process of defining a successor to the Haskell 98 standard, informally named Haskell′ ("Haskell Prime"), was begun.[7] This is an ongoing incremental process to revise the language definition, producing a new revision once per year. The first revision, named Haskell 2010, was announced in November 2009.[8] [edit] Overview and distinguishing featuresHaskell is a purely functional language, which means that in general, functions in Haskell do not have side effects. There is a distinct type for representing side effects, orthogonal to the type of functions. A pure function may return a side effect which is subsequently executed, modeling the impure functions of other languages. Haskell has a non-strict semantics. Most implementations of Haskell use lazy evaluation. Haskell has a strong, static, type system based on Hindley-Milner type inference. Haskell's principal innovation in this area is to add type classes, which were originally conceived as a principled way to add overloading to the language,[9] but have since found many more uses[10]. The type which represents side effects is an example of a monad. Monads are a general framework which can model different kinds of computation, including error handling, nondeterminism, parsing, and software transactional memory. Monads are defined as ordinary datatypes, but Haskell provides some syntactic sugar for their use. The language has an open, published, specification,[6] and multiple implementations exist. There is an active community around the language, and more than 1700 third-party open-source libraries and tools are available in the online package repository Hackage.[11] The main implementation of Haskell, GHC, is both an interpreter and native-code compiler that runs on most platforms. GHC is noted for its high-performance implementation of concurrency and parallelism,[12] and for having a rich type system incorporating recent innovations such as generalized algebraic data types and Type Families. [edit] Features
[edit] Syntax
[edit] LayoutHaskell allows indentation to be used to indicate the beginning of a new declaration. For example, in a where clause: product xs = prod xs 1 where prod [] a = a prod (x:xs) a = prod xs (a*x) The two equations for the nested function prod are aligned vertically, which allows the semi-colon separator to be omitted. In Haskell, indentation can be used in several syntactic constructs, including do, let, case, class, and instance. The use of indentation to indicate program structure originates in Landin's ISWIM language, where it was called the off-side rule. This was later adopted by Miranda, and Haskell adopted a similar (but rather more complicated) version of Miranda's off-side rule, which it called "layout". Other languages to adopt whitespace-sensitive syntax include Python and F#. The use of layout in Haskell is optional. For example, the function product above can also be written: product xs = prod xs 1 where { prod [] a = a; prod (x:xs) a = prod xs (a*x) } The explicit open brace after the where keyword indicates that the programmer has opted to use explicit semi-colons to separate declarations, and that the declaration-list will be terminated by an explicit closing brace. One reason for wanting support for explicit delimiters is that it makes automatic generation of Haskell source code easier. Haskell's layout rule has been criticised for its complexity. In particular, the definition states that if the parser encounters a parse error during processing of a layout section, then it should try inserting a close brace (the "parse error" rule). Implementing this rule in a traditional parsing/lexical-analysis combination requires two-way cooperation between the parser and lexical analyser, whereas in most languages these two phases can be considered independently. [edit] The rest of the syntax
[edit] Algebraic data types
Algebraic data types are used extensively in Haskell. Some examples of these are the built in list, -- A list of a's ([a]) is either an a consed (:) onto another list of a's, or an empty list ([]) data [a] = a : [a] | [] -- Something of type Maybe a is either Just something, or Nothing data Maybe a = Just a | Nothing -- Something of type Either atype btype is either a Left atype, or a Right btype data Either a b = Left a | Right b Users of the language can also define their own abstract data types. An example of an ADT used to represent a person's name, sex and age might look like: data Sex = Male | Female data Person = Person String Sex Int -- Notice that Person is both a constructor and a type -- An example of creating something of type Person tom :: Person tom = Person "Tom" Male 27 [edit] Pattern matchingPattern matching is used to match on the different constructors of algebraic data types. Here are some functions, each using pattern matching on each of the types above: -- This type signature says that empty takes a list containing any type, and returns a Bool empty :: [a] -> Bool empty (x:xs) = False empty [] = True -- Will return a value from a Maybe a, given a default value in case a Nothing is encountered fromMaybe :: a -> Maybe a -> a fromMaybe x (Just y) = y fromMaybe x Nothing = x isRight :: Either a b -> Bool isRight (Right _) = True isRight (Left _) = False getName :: Person -> String getName (Person name _ _) = name getSex :: Person -> Sex getSex (Person _ sex _) = sex getAge :: Person -> Int getAge (Person _ _ age) = age Using the above functions, along with the map empty [[1,2,3],[],[2],[1..]] -- returns [False,True,False,False] map (fromMaybe 0) [Just 2,Nothing,Just 109238, Nothing] -- returns [2,0,109238,0] map isRight [Left "hello", Right 6, Right 23, Left "world"] -- returns [False, True, True, False] map getName [Person "Sarah" Female 20, Person "Alex" Male 20, tom] -- returns ["Sarah", "Alex", "Tom"], using the definition for tom above
[edit] TuplesTuples in haskell can be used to hold a fixed number of elements. They are used to group pieces of data of differing types: account :: (String, Integer, Double) -- The type of a three-tuple, representing a name, balance, interest rate account = ("John Smith",102894,5.25) Tuples are commonly used in the zip* functions to place adjacent elements in separate lists together in tuples (zip4 to zip7 are provided in the Data.List module): -- The definition of the zip function. Other zip* functions are defined similarly zip :: [a] -> [b] -> [(a,b)] zip (a:as) (b:bs) = (a,b) : zip as bs zip _ _ = [] zip [1..5] "hello" -- returns [(1,'h'),(2,'e'),(3,'l'),(4,'l'),(5,'o')] -- and has type [(Integer, Char)] zip3 [1..5] "hello" [False, True, False, False, True] -- returns [(1,'h',False),(2,'e',True),(3,'l',False),(4,'l',False),(5,'o',True)] -- and has type [(Integer,Char,Bool)] In the GHC compiler, tuples are defined with sizes from 2 elements up to 62 elements.
[edit] Type system
[edit] Monads and input/output
[edit] ST monadThe ST monad allows programmers to write imperative algorithms in Haskell, using mutable variables (STRef's) and mutable arrays (STArrays and STUArrays). The advantage of the ST monad is that it allows programmers to write code that has internal side effects, such as destructively updating mutable variables and arrays, while containing these effects inside the monad. The result of this is that functions written using the ST monad appear completely pure to the rest of the program. This allows programmers to produce imperative code where it may be impractical to write functional code, while still keeping all the safety that pure code provides. Here is an example program (taken from the Haskell wiki page on the ST monad) that takes a list of numbers, and sums them, using a mutable variable: import Control.Monad.ST import Data.STRef import Control.Monad sumST :: Num a => [a] -> a sumST xs = runST $ do -- runST takes stateful ST code and makes it pure. summed <- newSTRef 0 -- Create an STRef (a mutable variable) forM_ xs $ \x -> do -- For each element of the argument list xs .. modifySTRef summed (+x) -- add it to what we have in n. readSTRef summed -- read the value of n, which will be returned by the runST above. [edit] STM monadMain article: Concurrent Haskell The STM monad is an implementation of Software Transactional Memory in Haskell. It is implemented in the GHC compiler, and allows for mutable variables to be modified in transactions. [edit] Arrows
As Haskell is a pure functional language, functions cannot have side effects. Being non-strict, it also does not have a well-defined evaluation order. This is a challenge for real programs, which among other things need to interact with an environment. Haskell solves this with monadic types that leverage the type system to ensure the proper sequencing of imperative constructs. The typical example is I/O, but monads are useful for many other purposes, including mutable state, concurrency and transactional memory, exception handling, and error propagation. Haskell provides a special syntax for monadic expressions, so that side-effecting programs can be written in a style similar to current imperative programming languages; no knowledge of the mathematics behind monadic I/O is required for this. The following program reads a name from the command line and outputs a greeting message: main = do putStrLn "What's your name?" name <- getLine putStr ("Hello, " ++ name ++ "!\n") The do-notation eases working with monads. This do-expression is equivalent to, but (arguably) easier to write and understand than, the de-sugared version employing the monadic operators directly: main = putStrLn "What's your name?" >> getLine >>= \ name -> putStr ("Hello, " ++ name ++ "!\n")
[edit] Concurrency and parallelismThe Haskell language definition itself does not include either concurrency or parallelism, although GHC supports both. [edit] ConcurrencyMain article: Concurrent Haskell Concurrent Haskell is an extension to Haskell that provides support for threads and synchronization.[13] GHC's implementation of Concurrent Haskell is based on multiplexing lightweight Haskell threads onto a few heavyweight OS threads,[14] so that Concurrent Haskell programs run in parallel on a multiprocesor. The runtime can support millions of simultaneous threads.[15] The GHC implementation employs a dynamic pool of OS threads, allowing a Haskell thread to make a blocking system call without blocking other running Haskell threads.[16] Hence the lightweight Haskell threads have the characteristics of heavyweight OS threads, and the programmer is unaware of the implementation details. Recently, Concurrent Haskell has been extended with support for Software Transactional Memory (STM), which is a concurrency abstraction in which compound operations on shared data are performed atomically, as transactions.[17] GHC's STM implementation is the only STM implementation to date to provide a static compile-time guarantee preventing non-transactional operations from being performed within a transaction. The Haskell STM library also provides two operations not found in other STMs: retry and orElse, which together allow blocking operations to be defined in a modular and composable fashion. [edit] Parallelism
[edit] Programming in the large
[edit] Semantics
[edit] Extensions to HaskellA number of extensions to Haskell have been proposed. These extensions provide features not described in the language specification, or they redefine existing constructs. As such, each extension may not be supported by all Haskell implementations. There is an ongoing effort[18] to describe extensions and select those which will be included in future versions of the language specification. The extensions[19] supported by the Glasgow Haskell Compiler include:
[edit] Type system extensionsAn expressive static type system is one of the major defining features of Haskell. Accordingly, much of the work in extending the language has been directed towards types and type classes. The Glasgow Haskell Compiler supports an extended type system based on the theoretical System Fc[21]. Major extensions to the type system include:
Extensions relating to type classes include:
[edit] Examples[edit] FactorialA simple example that is often used to demonstrate the syntax of functional languages is the factorial function for non-negative integers, shown in Haskell: factorial :: Integer -> Integer factorial 0 = 1 factorial n = n * factorial (n-1) Or in one line: factorial n = if n > 0 then n * factorial (n-1) else 1 This describes the factorial as a recursive function, with one terminating base case. It is similar to the descriptions of factorials found in mathematics textbooks. Much of Haskell code is similar to standard mathematical notation in facility and syntax. The first line of the factorial function describes the type of this function; while it is optional, it is considered to be good style[22] to include it. It can be read as the function factorial (factorial) has type (::) from integer to integer (Integer -> Integer). That is, it takes an integer as an argument, and returns another integer. The type of a definition is inferred automatically if the programmer didn't supply a type annotation. The second line relies on pattern matching, an important feature of Haskell. Note that parameters of a function are not in parentheses but separated by spaces. When the function's argument is 0 (zero) it will return the integer 1 (one). For all other cases the third line is tried. This is the recursion, and executes the function again until the base case is reached. A guard protects the third line from negative numbers for which a factorial is undefined. Without the guard this function would, if called with a negative number, recurse through all negative numbers without ever reaching the base case of 0. As it is, the pattern matching is not complete: if a negative integer is passed to the factorial function as an argument, the program will fail with a runtime error. A final case could check for this error condition and print an appropriate error message instead. Using the product function from the Prelude, a number of small functions analogous to C's standard library, and using the Haskell syntax for arithmetic sequences, the factorial function can be expressed in Haskell as follows: factorial n = product [1..n] Here [1..n] denotes the arithmetic sequence 1, 2, …, n in list form. Using the Prelude function enumFromTo, the expression [1..n] can be written as enumFromTo 1 n, allowing the factorial function to be expressed as factorial n = product (enumFromTo 1 n) which, using the function composition operator (expressed as a dot in Haskell) to compose the product function with the curried enumeration function can be rewritten in point-free style:[23] factorial = product . enumFromTo 1 In the Hugs interpreter, one often needs to define the function and use it on the same line separated by a where or let..in. For example, to test the above examples and see the output 120: let { factorial n | n > 0 = n * factorial (n-1); factorial _ = 1 } in factorial 5 or factorial 5 where factorial = product . enumFromTo 1 The GHCi interpreter doesn't have this restriction and function definitions can be entered on one line and referenced later. [edit] More complex examplesA simple Reverse Polish notation calculator expressed with the higher-order function calc :: String -> [Float] calc = foldl f [] . words where f (x:y:zs) "+" = (y + x):zs f (x:y:zs) "-" = (y - x):zs f (x:y:zs) "*" = (y * x):zs f (x:y:zs) "/" = (y / x):zs f xs y = read y : xs The empty list is the initial state, and f interprets one word at a time, either matching two numbers from the head of the list and pushing the result back in, or parsing the word as a floating-point number and prepending it to the list. The following definition produces the list of Fibonacci numbers in linear time: fibs = 0 : 1 : zipWith (+) fibs (tail fibs) The infinite list is produced by corecursion — the latter values of the list are computed on demand starting from the initial two items 0 and 1. This kind of a definition relies on lazy evaluation, an important feature of Haskell programming. For an example of how the evaluation evolves, the following illustrates the values of fibs and tail fibs after the computation of six items and shows how zipWith (+) has produced four items and proceeds to produce the next item: fibs = 0 : 1 : 1 : 2 : 3 : 5 : ... + + + + + + tail fibs = 1 : 1 : 2 : 3 : 5 : ... = = = = = = zipWith ... = 1 : 2 : 3 : 5 : 8 : ... fibs = 0 : 1 : 1 : 2 : 3 : 5 : 8 : ... The same function, written using GHC's parallel list comprehension syntax (GHC extensions must be enabled using a special command-line flag '-fglasgow-exts'; see GHC's manual for more): fibs = 0 : 1 : [ a+b | a <- fibs | b <- tail fibs ] The factorial we saw previously can be written as a sequence of functions: factorial n = (foldl (.) id [\x -> x*k | k <- [1..n]]) 1 A remarkably concise function that returns the list of Hamming numbers in order: hamming = 1 : map (2*) hamming `merge` map (3*) hamming `merge` map (5*) hamming where merge (x:xs) (y:ys) | x < y = x : xs `merge` (y:ys) | x > y = y : (x:xs) `merge` ys | otherwise = x : xs `merge` ys Like the various In this case the producer [edit] ImplementationsThe following all comply fully, or very nearly, with the Haskell 98 standard, and are distributed under open source licenses. There are currently no proprietary Haskell implementations.
[edit] Tools
[edit] Distribution
[edit] HackageSince January 2007, libraries and applications written in Haskell have been collected on Hackage, an online database of open source Haskell software using Cabal packaging tool. As of December 2009 there are about 1700 packages available.[11] Hackage provides a central point for the distribution of Haskell software, via Cabal, and has become a hub for new Haskell development activity. Installing new Haskell software via Hackage is possible via the $ cabal install xmonad which recursively installs required dependencies if they are available on Hackage. This makes installation of Haskell code easier than had been possible previously. [edit] Cabal
[edit] The Haskell PlatformTo cope with the growing number of libraries, the Haskell Platform was launched in September 2008 to provide a standard, quality-assured suite of Haskell libraries, available on every machine. The library standardisation project is modelled on GNOME's release process. The first release of the Haskell Platform was in May 2009. [edit] Libraries
[edit] Applications
Haskell is increasingly being used in commercial situations[24]. Audrey Tang's Pugs is an implementation for the long-forthcoming Perl 6 language with an interpreter and compilers that proved useful after just a few months of its writing; similarly, GHC is often a testbed for advanced functional programming features and optimizations. Darcs is a revision control system written in Haskell, with several innovative features. Linspire GNU/Linux chose Haskell for system tools development.[25] Xmonad is a window manager for the X Window System, written entirely in Haskell. Bluespec SystemVerilog is a language for semiconductor design that is an extension of Haskell. Additionally, Bluespec, Inc.'s tools are implemented in Haskell. Cryptol, a language and toolchain for developing and verifying cryptographic algorithms, is implemented in Haskell. Notably, the first formally verified microkernel, seL4 was developed in Haskell. [edit] Open source applicationsThere are many open source Haskell applications, most distributed from Hackage. A few of note: [edit] DevelopmentComplete listing in Hackage:Development
[edit] DatabasesComplete listing in Hackage:Databases
[edit] GamesComplete listing in Hackage:Games.
[edit] GraphicsComplete listing in Hackage:Graphics.
[edit] Languages and compilersComplete listing in Hackage:Language
[edit] System toolsComplete listing at Hackage:System
[edit] Text processingComplete listing in Hackage:Text
[edit] NetworkingComplete listing in Hackage:Network
[edit] Office tools
[edit] Commercial applications
Further accounts of the use of Haskell in industry are collected via the Commercial Users of Functional Programming workshop series, and collected on Haskell.org's industry page. [edit] Research projects
[edit] Community
[edit] Related languagesConcurrent Clean is a close relative of Haskell, whose biggest deviation from Haskell is in the use of uniqueness types instead of monads for I/O and side-effects. A series of languages inspired by Haskell, but with different type systems, have been developed, including:
Other related languages include:
[edit] Haskell variantsHaskell has served as a testbed for many new ideas in language design. There have been a wide number of Haskell variants produced, exploring new language ideas, including:
[edit] CriticismJan-Willem Maessen, in 2002, and Simon Peyton Jones, in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motivation for it[26][27], in addition to purely practical considerations such as improved performance.[28] They note that, in addition to adding some performance overhead, laziness makes it more difficult for programmers to reason about the performance of their code (particularly its space usage). Bastiaan Heeren, Daan Leijen, and Arjan van IJzendoorn in 2003 also observed some stumbling blocks for Haskell learners: "The subtle syntax and sophisticated type system of Haskell are a double edged sword — highly appreciated by experienced programmers but also a source of frustration among beginners, since the generality of Haskell often leads to cryptic error messages."[29] To address these, they developed an advanced interpreter called Helium which improved the user-friendliness of error messages by limiting the generality of some Haskell features, and in particular removing support for type classes. [edit] Haskell conferences and workshopsThe Haskell community meets regularly for research and development activities. The primary events are:
Since 2007 there has been a series of organized "hackathons" - the Hac series - aimed at improving the programming language tools and libraries:
Since 2005, a growing number of Haskell User Groups have formed, in the USA, Canada, Australia, South America, Europe and Asia. [edit] References
[edit] External links
[edit] Tutorials
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