Haskell Programming Language

 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^..

History

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 the most widely used, 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).

Haskell 98

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.

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.

Features

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 do not have side effects. There is a distinct construct 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 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, but have since found 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, non determinism, 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, and multiple implementations exist. 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, and for having a rich type system incorporating recent innovations such as generalized algebraic data types and type families.

There is an active community around the language, and more than 5400 third-party open-source libraries and tools are available in the online package repository Hackage.

Code examples

The following is a Hello world program written in Haskell (note that all but the last line can be omitted):

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

An efficient implementation of the Fibonacci numbers, as an infinite list, is this:

-- 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 an incredibly large 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

All listed implementations are distributed under open source licenses^.

The following implementations comply fully, or very nearly, with the Haskell 98 standard.

·         The Glasgow Haskell Compiler (GHC) compiles to native code on a number of different architectures—as well as to ANSI C—using C-- as an intermediate language. 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. UHC 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 as well as exploration of 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.

The following implementations are no longer being actively maintained:

·         Hugs, the Haskell User's Gofer System, is a bytecode interpreter. It used to be one of the most widely used implementations alongside the GHC compiler, but has now been mostly replaced by GHCi. It also comes with a graphics library.

·         nhc98 is another bytecode compiler. Nhc98 focuses on minimizing memory usage.

·         Yhc, the York Haskell Compiler was a fork of nhc98, with the goals of being simpler, more portable and more efficient, and integrating support for Hat, the Haskell tracer. It also featured a JavaScript backend, allowing users to run Haskell programs in a Web browser.

·         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.

The following implementations are not fully Haskell 98 compliant, and use a language that is a variant of Haskell:

·         Gofer was an educational dialect of Haskell, with a feature called "constructor classes", developed by Mark Jones. It was supplanted by Hugs.

·         Helium is a newer dialect of Haskell. The focus is on making it easy to learn by providing clearer error messages. It currently lacks full support for type classes, rendering it incompatible with many Haskell programs.

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