Haskell (programming language)
Haskell // is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. The latest standard of Haskell is Haskell 2010. As of May 2016[update], a group is working on the next version, Haskell 2020.
Haskell features a type system with type inference and lazy evaluation. Type classes first appeared in the Haskell programming language. Its main implementation is the Glasgow Haskell Compiler.
Haskell is based on the semantics, but not the syntax, of the language Miranda, which served to focus the efforts of the initial Haskell working group. Haskell is used widely in academia and industry.
Following the release of Miranda by Research Software Ltd, in 1985, interest in lazy functional languages grew. By 1987, more than a dozen non-strict, purely functional programming languages existed. Of these, Miranda was used most widely, but it was proprietary software. 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.
Haskell 1.0 to 1.4
The first version of Haskell ("Haskell 1.0") was defined in 1990. The committee's efforts resulted in a series of language definitions (1.0, 1.1, 1.2, 1.3, 1.4).
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 creating extensions and variants of Haskell 98 via adding and incorporating experimental features.
In February 1999, the Haskell 98 language standard was originally published as The Haskell 98 Report. In January 2003, a revised version was published as Haskell 98 Language and Libraries: The Revised Report. The language continues to evolve rapidly, with the Glasgow Haskell Compiler (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 Prime, began. This was intended to be an ongoing incremental process to revise the language definition, producing a new revision up to once per year. The first revision, named Haskell 2010, was announced in November 2009 and published in July 2010.
Haskell 2010 adds the foreign function interface (FFI) to Haskell, allowing for bindings to other programming languages, fixes some syntax issues (changes in the formal grammar), and bans so-called n-plus-k-patterns, that is, definitions of the form
fact (n+1) = (n+1) * fact n are no longer allowed. It introduces the Language-Pragma-Syntax-Extension which allows for code designating a Haskell source as Haskell 2010 or requiring certain extensions to the Haskell language. The names of the extensions introduced in Haskell 2010 are DoAndIfThenElse, HierarchicalModules, EmptyDataDeclarations, FixityResolution, ForeignFunctionInterface, LineCommentSyntax, PatternGuards, RelaxedDependencyAnalysis, LanguagePragma and NoNPlusKPatterns.
Haskell features lazy evaluation, pattern matching, list comprehension, type classes, and type polymorphism. It is a purely functional language, which means that in general, functions in Haskell have no side effects. A distinct construct exists to represent 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 strong, static type system based on Hindley–Milner type inference. Haskell's principal innovation in this area is to add type classes, originally conceived as a principled way to add overloading to the language, but since finding many more uses.
The construct 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.
Haskell has an open, published specification, and multiple implementations exist. Its main implementation, the Glasgow Haskell Compiler (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, and for having a rich type system incorporating recent innovations such as generalized algebraic data types and type families.
A growing active community exists around the language, and more than 5,400 third-party open-source libraries and tools are available in the online package repository Hackage.
module Main where main :: IO () main = putStrLn "Hello, World!"
Here is the factorial function in Haskell, defined in a few different ways:
-- Type annotation (optional) factorial :: (Integral a) => a -> a -- Using recursion factorial n | n < 2 = 1 factorial n = n * factorial (n - 1) -- Using recursion, with guards factorial n | n < 2 = 1 | otherwise = n * factorial (n - 1) -- Using recursion but written without pattern matching factorial n = if n > 0 then n * factorial (n-1) else 1 -- Using a list factorial n = product [1..n] -- Using fold (implements product) factorial n = foldl (*) 1 [1..n] -- Point-free style factorial = foldr (*) 1 . enumFromTo 1
-- Type annotation (optional) fib :: Int -> Integer -- With self-referencing data fib n = fibs !! n where fibs = 0 : scanl (+) 1 fibs -- 0,1,1,2,3,5,... -- Same, coded directly fib n = fibs !! n where fibs = 0 : 1 : next fibs next (a : t@(b:_)) = (a+b) : next t -- Similar idea, using zipWith fib n = fibs !! n where fibs = 0 : 1 : zipWith (+) fibs (tail fibs) -- Using a generator function fib n = fibs (0,1) !! n where fibs (a,b) = a : fibs (b,a+b)
The Int type refers to a machine-sized integer (used as a list subscript with the !! operator), while Integer is an arbitrary-precision integer. For example, using Integer, the factorial code above easily computes
factorial 100000 as a huge number, of 456,574 digits, with no loss of precision.
This is an implementation of an algorithm similar to quick sort over lists, in which the first element is taken as the pivot:
quickSort :: Ord a => [a] -> [a] quickSort  =  -- The empty list is already sorted quickSort (x:xs) = quickSort [a | a <- xs, a < x] -- Sort the left part of the list ++ [x] ++ -- Insert pivot between two sorted parts quickSort [a | a <- xs, a >= x] -- Sort the right part of the list
Implementations which comply fully, or very nearly, with the Haskell 98 standard, include:
- The Glasgow Haskell Compiler (GHC) compiles to native code on many different processor architectures, and to ANSI C, via one of two intermediate languages: C--, or in more recent versions, LLVM (formerly Low Level Virtual Machine) bitcode. GHC has become the de facto standard Haskell dialect. There are libraries (e.g., bindings to OpenGL) that will work only with GHC. GHC is also distributed along with the Haskell platform.
- The Utrecht Haskell Compiler (UHC) is a Haskell implementation from Utrecht University. It supports almost all Haskell 98 features plus many experimental extensions. It is implemented using attribute grammars and is currently mainly used for research into generated type systems and language extensions.
- Jhc is a Haskell compiler written by John Meacham emphasising speed and efficiency of generated programs, and exploring new program transformations.
- Ajhc is a fork of Jhc.
- LHC is a whole-program optimizing backend for GHC. It is based on Urban Boquist’s compiler intermediate language, GRIN. Older versions of LHC were based on Jhc rather than GHC.
Implementations no longer being actively maintained include:
- The Haskell User's Gofer System (Hugs) is a bytecode interpreter. It used to be one of the implementations used most widely, alongside the GHC compiler, but has now been mostly replaced by GHCi. It also comes with a graphics library.
- nhc98 is a bytecode compiler focusing on minimizing memory use.
- HBC is an early implementation supporting Haskell 1.4. It was implemented by Lennart Augustsson in, and based on, Lazy ML. It has not been actively developed for some time.
Implementations not fully Haskell 98 compliant, and using a variant Haskell language, include:
- Gofer was an educational dialect of Haskell, with a feature called constructor classes, developed by Mark Jones. It was supplanted by Hugs (see above).
- Helium is a newer dialect of Haskell. The focus is on make learning easier via clearer error messages. It currently lacks full support for type classes, rendering it incompatible with many Haskell programs.
Darcs is a revision control system written in Haskell, with several innovative features, such as more precise control of the patches to be applied. Cabal is a tool for building and packaging Haskell libraries and programs. Linspire GNU/Linux chose Haskell for system tools development. Xmonad is a window manager for the X Window System, written fully in Haskell. GHC is also often a testbed for advanced functional programming features and optimizations in other programming languages. Pandoc is a tool to convert one markup format into another.
- Facebook implements its anti-spam programs in Haskell, as open-source software.
- Bluespec SystemVerilog (BSV) is a language for semiconductor design that is an extension of Haskell. Also, Bluespec, Inc.'s tools are implemented in Haskell.
- Cryptol, a language and toolchain for developing and verifying cryptography algorithms, is implemented in Haskell.
- The first formally verified microkernel, seL4, used Haskell as a prototyping language for the OS developer.:p.2 At the same time the Haskell code defined an executable specification with which to reason, for automatic translation by the theorem-proving tool.:p.3 The Haskell code thus served as an intermediate prototype before final C refinement.:p.3
Jan-Willem Maessen, in 2002, and Simon Peyton Jones, in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motives for it, in addition to purely practical considerations such as improved performance. They note that, in addition to adding some performance overhead, lazy evaluation makes it more difficult for programmers to reason about the performance of their code (particularly its space use).
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." To address these, researchers from Utrecht University 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.
Ben Lippmeier designed Disciple as a strict-by-default (lazy by explicit annotation) dialect of Haskell with a type-and-effect system, to address Haskell's difficulties in reasoning about lazy evaluation and in using traditional data structures such as mutable arrays. He argues (p. 20) that "destructive update furnishes the programmer with two important and powerful tools... a set of efficient array-like data structures for managing collections of objects, and ... the ability to broadcast a new value to all parts of a program with minimal burden on the programmer."
Robert Harper, one of the authors of Standard ML, has given his reasons for not using Haskell to teach introductory programming. Among these are the difficulty of reasoning about resource use with non-strict evaluation, that lazy evaluation complicates the definition of data types and inductive reasoning, and the "inferiority" of Haskell's (old) class system compared to ML's module system.
A series of languages inspired by Haskell, but with different type systems, have been developed, including:
- Agda, a functional language with dependent types
- Idris, a general purpose functional language with dependent types, developed at the University of St Andrews
- Epigram, a functional language with dependent types suitable for proving properties of programs
- Cayenne, with dependent types
- Ωmega, strict and more
- Elm, a functional language to create web front-end apps, no support for higher-kinded types
Java virtual machine (JVM) based:
- Frege, a Haskell-like language with Java's scalar types and good Java integration.
- Jaskell, a functional scripting language that runs in Java VM.
Other related languages include:
- Curry, a functional/logic programming language based on Haskell
Haskell has served as a testbed for many new ideas in language design. There have been many Haskell variants produced, exploring new language ideas, including:
- Parallel Haskell:
- Distributed Haskell (formerly Goffin) and Eden.
- Eager Haskell, based on speculative evaluation.
- Several object-oriented versions: Haskell++, and Mondrian.
- Generic Haskell, a version of Haskell with type system support for generic programming.
- O'Haskell, an extension of Haskell adding object-orientation and concurrent programming support which "has ... been superseded by Timber."
- Disciple, a strict-by-default (laziness available by annotation) dialect of Haskell which supports destructive update, computational effects, type directed field projections and allied functional aspects.
- Scotch, a kind of hybrid of Haskell and Python.
- Hume, a strict functional language for embedded systems based on processes as stateless automata over a sort of tuples of one element mailbox channels where the state is kept by feedback into the mailboxes, and a mapping description from outputs to channels as box wiring, with a Haskell-like expression language and syntax.
Conferences and workshops
The Haskell community meets regularly for research and development activities. The main events are:
- International Conference on Functional Programming (ICFP)
- Haskell Symposium (formerly the Haskell Workshop)
- Haskell Implementors Workshop
- Commercial Users of Functional Programming (CUFP)
Since 2006, a series of organized hackathons has occurred, the Hac series, aimed at improving the programming language tools and libraries.
Since 2005, Haskell users' groups are growing in number.
- 'Hello world' is meant as the introductory prototype of a read-eval-print_loop. The IO tool putStrLn prints a string, which is the only essential line of this example. The second line of this example is a type definition, which is unnecessary for Haskell, because the compiler infers the type; instead, the second line serves to communicate the programmer's intention to the reader. The first line of the example isn't needed, either, because the start symbol
mainin this simple example makes the module
Maina nicety, which instead would have been a necessity in a multi-module example. Rather, the first two lines are provided for consistency with larger examples.
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- Syme, Don; Granicz, Adam; Cisternino, Antonio (2007). Expert F#. Apress. p. 2.
F# also draws from Haskell particularly with regard to two advanced language features called sequence expressions and workflows.
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- "Facebook Introduces 'Hack,' the Programming Language of the Future". WIRED. 20 March 2014.
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- "LiveScript Inspiration". Retrieved 2014-02-04.
- "Glossary of Terms and Jargon". Perl Foundation Perl 6 Wiki. The Perl Foundation. Retrieved 9 February 2012.
- Kuchling, A. M. "Functional Programming HOWTO". Python v2.7.2 documentation. Python Software Foundation. Retrieved 9 February 2012.
- "The Rust Reference: Appendix: Influences". Retrieved 2016-02-03.
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The Swift language is the product of tireless effort from a team of language experts, documentation gurus, compiler optimization ninjas, and an incredibly important internal dogfooding group who provided feedback to help refine and battle-test ideas. Of course, it also greatly benefited from the experiences hard-won by many other languages in the field, drawing ideas from Objective-C, Rust, Haskell, Ruby, Python, C#, CLU, and far too many others to list.
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- This allows finer control over the expression evaluation strategy
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- Edward Kmett, Edward Kmett - Type Classes vs. the World
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- Lazy evaluation can lead to excellent performance, such as in The Computer Language Benchmarks Game 
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- Yet Another Haskell Tutorial, by Hal Daumé III; assumes far less prior knowledge than official tutorial
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- Haskell Wiki
- Planet Haskell an aggregator of Haskell related blogs and other Haskell-related news sites
- Hackage – central package archive
- Hoogle – API search engine