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    (Top)
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    1 History

    Toggle History subsection
    -
        1.1 Haskell 1.0 to 1.4
    -
        1.2 Haskell 98
    -
        1.3 Haskell 2010
    -
        1.4 Future standards
-
    2 Features
-
    3 Code examples
-
    4 Implementations
-
    5 Notable applications

    Toggle Notable applications subsection
    -
        5.1 Industry
    -
        5.2 Web
-
    6 Criticism
-
    7 Related languages
-
    8 Conferences and workshops
-
    9 See also
-
    10 References
-
    11 Bibliography
-
    12 External links

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Haskell

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From Wikipedia, the free encyclopedia

Functional programming language

For other uses, see Haskell (disambiguation).

Haskell (/ˈhæskəl/^([25])) is a general-purpose, statically typed,
purely functional programming language with type inference and lazy
evaluation.^([26][27]) Haskell pioneered several programming language
features including type classes for type-safe operator overloading and
monadic input/output (IO). It is named after logician Haskell
Curry.^([1]) Haskell's main implementation is the Glasgow Haskell
Compiler (GHC).

Haskell's semantics are historically based on those of the Miranda
programming language, which served to focus the efforts of the initial
Haskell working group.^([28]) The last formal specification of the
language was made in July 2010, while the development of GHC continues
to expand Haskell via language extensions.

Haskell is used in academia and industry.^([29][30][31]) As of
May 2021^([update]), Haskell was the 28th most popular programming
language by Google searches for tutorials,^([32]) and made up less than
1% of active users on the GitHub source code repository.^([33])

History

[edit]

After 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. Miranda was
the most widely used, but it was proprietary software. At the conference
on Functional Programming Languages and Computer Architecture (FPCA '87)
in Portland, Oregon, there was a strong consensus that a committee be
formed to define an open standard for such languages. The committee's
purpose was to consolidate existing functional languages into a common
one to serve as a basis for future research in functional-language
design.^([34])

Haskell 1.0 to 1.4

[edit]

Haskell was developed by a committee, attempting to bring together
off-the-shelf solutions where possible.

Type classes were first proposed by Philip Wadler and Stephen Blott to
address the ad hoc handling of equality types and arithmetic overloading
in languages at the time.^([35])

In early versions of Haskell up until and including version 1.2, user
interaction and input/output (IO) were handled by both streams-based and
continuation-based mechanisms, which were widely considered
unsatisfactory.^([36]) In version 1.3, monadic IO was introduced, along
with the generalisation of type classes to higher kinds (type
constructors). Along with "do notation", which provides syntactic sugar
for the Monad type class, this gave Haskell an effect system that
maintained referential transparency and was convenient.

[]

Hierarchy of type classes in the Haskell Prelude as of GHC 7.10. The
inclusion of Foldable and Traversable (with corresponding changes to the
type signatures of some functions), and of Applicative as intermediate
between Functor and Monad, are deviations from the Haskell 2010
standard.

Haskell 98

[edit]

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.^([34])

Haskell 2010

[edit]

In early 2006, the process of defining a successor to the Haskell 98
standard, informally named Haskell Prime, began.^([38]) 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^([2]) and
published in July 2010.

Haskell 2010 added several well-used and uncontroversial features
previously enabled via compiler-specific flags.^([citation needed])

-   Hierarchical module names. Module names are allowed to consist of
    dot-separated sequences of capitalized identifiers, rather than only
    one such identifier. This lets modules be named in a hierarchical
    manner (e.g., Data.List instead of List), although technically
    modules are still in a single monolithic namespace. This extension
    was specified in an addendum to Haskell 98 and was in practice
    universally used.
-   The foreign function interface (FFI) allows bindings to other
    programming languages. Only bindings to C are specified in the
    Report, but the design allows for other language bindings. To
    support this, data type declarations were permitted to contain no
    constructors, enabling robust nonce types for foreign data that
    could not be constructed in Haskell. This extension was also
    previously specified in an Addendum to the Haskell 98 Report and
    widely used.
-   So-called n+k patterns (definitions of the form
    fact (n+1) = (n+1) * fact n) were no longer allowed. This syntactic
    sugar had misleading semantics, in which the code looked like it
    used the (+) operator, but in fact desugared to code using (-) and
    (>=).

Future standards

[edit]

The next formal specification had been planned for 2020.^([3]) On 29
October 2021, with GHC version 9.2.1, the GHC2021 extension was
released. While this is not a formal language spec, it combines several
stable, widely used GHC extensions to Haskell 2010.^([39][40])

Features

[edit]

Main article: Haskell features

See also: Glasgow Haskell Compiler § Extensions to Haskell

Haskell features lazy evaluation, lambda expressions, pattern matching,
list comprehension, type classes and type polymorphism. It is a purely
functional programming language, which means that functions generally
have no side effects. A distinct construct exists to represent side
effects, orthogonal to the type of functions. A pure function can return
a side effect that is subsequently executed, modeling the impure
functions of other languages.

Haskell has a strong, static type system based on Hindley–Milner type
inference. Its principal innovation in this area is type classes,
originally conceived as a principled way to add overloading to the
language,^([41]) but since finding many more uses.^([42])

Code examples

[edit]

See also: Haskell features § Examples

A "Hello, World!" program in Haskell (only the last line is strictly
necessary):

    module Main (main) where          -- not needed in interpreter, is the default in a module file

    main :: IO ()                     -- the compiler can infer this type definition
    main = putStrLn "Hello, World!"

The factorial function in Haskell, defined in a few different ways (the
first line is the type annotation, which is optional and is the same for
each implementation):

    factorial :: (Integral a) => a -> a

    -- Using recursion (with the "ifthenelse" expression)
    factorial n = if n < 2
                  then 1
                  else n * factorial (n - 1)

    -- Using recursion (with pattern matching)
    factorial 0 = 1
    factorial n = n * factorial (n - 1)

    -- Using recursion (with guards)
    factorial n
       | n < 2     = 1
       | otherwise = n * factorial (n - 1)

    -- Using a list and the "product" function
    factorial n = product [1..n]

    -- Using fold (implements "product")
    factorial n = foldl (*) 1 [1..n]

    -- Point-free style
    factorial = foldr (*) 1 . enumFromTo 1

Using Haskell's Fixed-point combinator allows this function to be
written without any explicit recursion.

    import Data.Function (fix)

    factorial = fix fac
      where fac f x
              | x < 2     = 1
              | otherwise = x * f (x - 1)

As the Integer type has arbitrary-precision, this code will compute
values such as factorial 100000 (a 456,574-digit number), with no loss
of precision.

An implementation of an algorithm similar to quick sort over lists,
where the first element is taken as the pivot:

    -- Type annotation (optional, same for each implementation)
    quickSort :: Ord a => [a] -> [a]

    -- Using list comprehensions
    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

    -- Using filter
    quickSort []     = []
    quickSort (x:xs) = quickSort (filter (<x) xs)
                       ++ [x] ++
                       quickSort (filter (>=x) xs)

Implementations

[edit]

All listed implementations are distributed under open source
licenses.^([45])

Implementations that fully or nearly comply 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.^([46][47]) GHC has
    become the de facto standard Haskell dialect.^([48]) There are
    libraries (e.g., bindings to OpenGL) that work only with GHC. GHC
    was also distributed with the Haskell platform. GHC features an
    asynchronous runtime that also schedules threads across multiple CPU
    cores similar to the Go runtime.
-   Jhc, a Haskell compiler written by John Meacham, emphasizes speed
    and efficiency of generated programs and exploring new program
    transformations.
    -   Ajhc is a fork of Jhc.
-   The Utrecht Haskell Compiler (UHC) is a Haskell implementation from
    Utrecht University.^([49]) It supports almost all Haskell 98
    features plus many experimental extensions. It is implemented using
    attribute grammars and is primarily used for research on generated
    type systems and language extensions.

Implementations no longer actively maintained include:

-   The Haskell User's Gofer System (Hugs) is a bytecode interpreter. It
    was once one of the implementations used most widely, alongside the
    GHC compiler,^([50]) but has now been mostly replaced by GHCi. It
    also comes with a graphics library.
-   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.
-   nhc98 is a bytecode compiler focusing on minimizing memory use.
    -   The York Haskell Compiler (Yhc) was a fork of nhc98, with the
        goals of being simpler, more portable and efficient, and
        integrating support for Hat, the Haskell tracer. It also had a
        JavaScript backend, allowing users to run Haskell programs in
        web browsers.

Implementations not fully Haskell 98 compliant, and using a variant
Haskell language, include:

-   Eta and Frege are dialects of Haskell targeting the Java virtual
    machine.
-   Gofer is an educational dialect of Haskell, with a feature called
    constructor classes, developed by Mark Jones. It is supplanted by
    Haskell User's Gofer System (Hugs).
-   Helium, a newer dialect of Haskell. The focus is on making learning
    easier via clearer error messages by disabling type classes as a
    default.

Notable applications

[edit]

-   Agda is a proof assistant written in Haskell.^([51])
-   Cabal is a tool for building and packaging Haskell libraries and
    programs.^([52])
-   Darcs is a revision control system written in Haskell, with several
    innovative features, such as more precise control of patches to
    apply.
-   Glasgow Haskell Compiler (GHC) is also often a testbed for advanced
    functional programming features and optimizations in other
    programming languages.
-   Git-annex is a tool to manage (big) data files under Git version
    control. It also provides a distributed file synchronization system
    (git-annex assistant).
-   Linspire Linux chose Haskell for system tools development.^([53])
-   Pandoc is a tool to convert one markup format into another.
-   Pugs is a compiler and interpreter for the programming language then
    named Perl 6, but since renamed Raku.
-   TidalCycles is a domain special language for live coding musical
    patterns, embedded in Haskell.^([54])
-   Xmonad is a window manager for the X Window System, written fully in
    Haskell.^([55])
-   GarganText^([56]) is a collaborative tool to map through semantic
    analysis texts on any web browser, written fully in Haskell and
    PureScript, which is used for instance in the research community to
    draw up state-of-the-art reports and roadmaps.^([57])

Industry

[edit]

-   Bluespec SystemVerilog (BSV) is a language extension of Haskell, for
    designing electronics. It is an example of a domain-specific
    language embedded into Haskell. Further, Bluespec, Inc.'s tools are
    implemented in Haskell.
-   Cryptol, a language and toolchain for developing and verifying
    cryptography algorithms, is implemented in Haskell.
-   Facebook implements its anti-spam programs^([58]) in Haskell,
    maintaining the underlying data access library as open-source
    software.^([59])
-   The Cardano blockchain platform is implemented in Haskell.^([60])
-   GitHub implemented Semantic, an open-source library to analyze,
    diff, and interpret untrusted source code for many languages, in
    Haskell.^([61])
-   Standard Chartered's financial modelling language Mu is syntactic
    Haskell running on a strict runtime.^([62])
-   seL4, the first formally verified microkernel,^([63]) used Haskell
    as a prototyping language for the OS developer.^([63]: 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.^([63]: p.3 ) The Haskell code thus served as an intermediate
    prototype before final C refinement.^([63]: p.3 )
-   Target stores' supply chain optimization software is written in
    Haskell.^([64])
-   Co–Star^([65])
-   Mercury Technologies' back end is written in Haskell.^([66])

Web

[edit]

Notable web frameworks written for Haskell include:^([67])

-   IHP
-   Servant
-   Snap
-   Yesod

Criticism

[edit]

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.^([68][69]) In addition to purely practical
considerations such as improved performance,^([70]) they note that 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."^([71]) To address the error messages 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. In particular it disables type
classes by default.^([72])

Ben Lippmeier designed Disciple^([73]) 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.^([74]) 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 datatypes
and inductive reasoning,^([75]) and the "inferiority" of Haskell's (old)
class system compared to ML's module system.^([76])

Haskell's build tool, Cabal, has historically been criticized for poorly
handling multiple versions of the same library, a problem known as
"Cabal hell". The Stackage server and Stack build tool were made in
response to these criticisms.^([77]) Cabal has since addressed this
problem by borrowing ideas from the package manager Nix,^([78]) with the
new approach becoming the default in 2019.

Related languages

[edit]

Clean is a close, slightly older relative of Haskell. Its biggest
deviation from Haskell is in the use of uniqueness types instead of
monads for input/output (I/O) and side effects.

A series of languages inspired by Haskell, but with different type
systems, have been developed, including:

-   Agda, a functional language with dependent types.
-   Cayenne, with dependent types.
-   Elm, a functional language to create web front-end apps, no support
    for user-defined or higher-kinded type classes or instances.
-   Epigram, a functional language with dependent types suitable for
    proving properties of programs.
-   Idris, a general purpose functional language with dependent types,
    developed at the University of St Andrews.
-   PureScript transpiles to JavaScript.
-   Ωmega, a strict language that allows introduction of new kinds, and
    programming at the type level.

Other related languages include:

-   Curry, a functional/logic programming language based on Haskell.

Notable Haskell variants include:

-   Generic Haskell, a version of Haskell with type system support for
    generic programming.
-   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

[edit]

Haskell events have been regularly held concurrent with the
International Conference on Functional Programming (ICFP) including
workshops such as the Haskell Symposium (formerly the Haskell Workshop)
and Commercial Users of Functional Programming.^([79]) Other events
include the Haskell Implementors Workshop,^([80]) ZuriHac in
Zurich,^([81]) and Hac: The Haskell Hackathon.^([82])

See also

[edit]

-   Rosetta-lang

References

[edit]

1.  ^ ^(a) ^(b) ^(c) Hudak et al. 2007.
2.  ^ ^(a) ^(b)
    Marlow, Simon (24 November 2009). "Announcing Haskell 2010". Haskell
    (Mailing list). Retrieved 12 March 2011.
3.  ^ ^(a) ^(b) Riedel, Herbert (28 April 2016). "ANN: Haskell Prime
    2020 committee has formed". Haskell-prime (Mailing list). Retrieved
    6 May 2017.
4.  ^ ^(a) ^(b) ^(c) ^(d) ^(e) ^(f) ^(g) ^(h) ^(i) ^(j) ^(k) ^(l) ^(m)
    Peyton Jones 2003, p. xi
5.  ^ Norell, Ulf (2008). "Dependently Typed Programming in Agda" (PDF).
    Gothenburg: Chalmers University. Retrieved 9 February 2012.
6.  ^ Hudak et al. 2007, pp. 12–38, 43.
7.  ^ Stroustrup, Bjarne; Sutton, Andrew (2011). "Design of Concept
    Libraries for C++" (PDF). Software Language Engineering. Archived
    from the original (PDF) on 10 February 2012.
8.  ^ ^(a) ^(b) ^(c) ^(d) ^(e) ^(f) ^(g) ^(h) ^(i) ^(j) Hudak et al.
    2007, pp. 12-45–46.
9.  ^ ^(a) ^(b) Meijer, Erik (2006). "Confessions of a Used Programming
    Language Salesman: Getting the Masses Hooked on Haskell". Oopsla
    2007. CiteSeerX 10.1.1.72.868.
10. ^ Meijer, Erik (1 October 2009). "C9 Lectures: Dr. Erik Meijer –
    Functional Programming Fundamentals, Chapter 1 of 13". Channel 9.
    Microsoft. Archived from the original on 16 June 2012. Retrieved 9
    February 2012.
11. ^ Drobi, Sadek (4 March 2009). "Erik Meijer on LINQ". InfoQ. QCon SF
    2008: C4Media Inc. Retrieved 9 February 2012.{{cite news}}: CS1
    maint: location (link)

Bibliography

[edit]

Reports

-   Peyton Jones, Simon, ed. (2003). Haskell 98 Language and Libraries:
    The Revised Report. Cambridge University Press. ISBN 978-0521826143.
-   Marlow, Simon, ed. (2010). Haskell 2010 Language Report (PDF).
    Haskell.org.

Textbooks

-   Davie, Antony (1992). An Introduction to Functional Programming
    Systems Using Haskell. Cambridge University Press.
    ISBN 978-0-521-25830-2.
-   Bird, Richard (1998). Introduction to Functional Programming using
    Haskell (2nd ed.). Prentice Hall Press. ISBN 978-0-13-484346-9.

Tutorials

-   Hudak, Paul; Peterson, John; Fasel, Joseph (June 2000). "A Gentle
    Introduction To Haskell, Version 98". Haskell.org.
-   Learn You a Haskell for Great Good! - A community version
    (learnyouahaskell.github.io). An up-to-date community maintained
    version of the renowned "Learn You a Haskell" (LYAH) guide.

History

-   Hudak, Paul; Hughes, John; Peyton Jones, Simon; Wadler, Philip
    (2007). "A history of Haskell" (PDF). Proceedings of the third ACM
    SIGPLAN conference on History of programming languages. pp. 12–1–55.
    doi:10.1145/1238844.1238856. ISBN 978-1-59593-766-7. S2CID 52847907.
-   Hamilton, Naomi (19 September 2008). "The A-Z of Programming
    Languages: Haskell". Computerworld.

External links

[edit]

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