Haskell

Haskell is a general-purpose, statically typed, purely functional programming language with non-strict (lazy) evaluation. It emphasizes referential transparency, equational reasoning, and a high level of abstraction. Core features include algebraic data types, pattern matching, higher-order functions, and a robust type system based on type classes and type inference.

A key design goal is to enable concise, safe, and composable code. Programs express computations with pure functions, while side effects are controlled through monads, most famously the IO monad. This separation of pure and impure code helps maintain referential transparency while still performing real-world tasks.

Haskell arose in the late 1980s through a series of meetings among researchers to standardize non-strict functional languages. The language was released in 1990, with major standardization efforts culminating in Haskell 98 and later updates such as Haskell 2010. The Glasgow Haskell Compiler (GHC) is by far the most widely used implementation, with others including JHC and UHC. The language ships with a large ecosystem around Hackage, Cabal, and Stack.

The type system is Hindley–Milner-like with extensions via type classes, enabling polymorphism, type inference, and overloading. Common abstractions include Functor, Applicative, and Monad, which underpin effects, parsers, and asynchronous operations. Haskell code often emphasizes concise, high-level expressions and strong guarantees about correctness.

Applications include teaching, research, and industry domains such as data analysis and finance. Notable criticisms are the steep learning curve and potential performance pitfalls from lazy evaluation, though well-designed code and strictness annotations can mitigate these issues. Concurrency and parallelism are supported through lightweight threads, Software Transactional Memory, and modern libraries.