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Haskell is the language that computer science professors love and most working programmers avoid. It's purely functional — no mutable state, no side effects (they're sequenced through monads), no imperative loops. Every function is a mathematical function: same input, same output, always. This purity enables equational reasoning, aggressive compiler optimisations, and code that's provably correct.
The type system is Haskell's crown jewel. Type classes, algebraic data types, higher-kinded types, and type-level programming make it possible to encode complex invariants in types — if it compiles, it probably works. GHC (the Glasgow Haskell Compiler) is one of the most sophisticated compilers ever built, producing surprisingly fast executables.
Haskell's influence far exceeds its usage. Rust's traits came from Haskell's type classes. TypeScript's type system borrows Haskell ideas. Swift, Kotlin, and Scala all carry Haskell's DNA. For production use, Haskell powers parts of Facebook's spam filter, Standard Chartered's financial systems, and numerous blockchain projects. But most developers encounter Haskell as an educational language that permanently changes how they think about programming.