Haskell
Haskell is a functional programming language which is pure and statically-typed. It's known for lazy evaluation, where evaluation is deferred until necessary, and its purity, where monads are used for working with side-effects.
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About Haskell
Haskell is a general-purpose programming language known for being purely functional, non-strict with strong static typing and for having type inference.
Purely functional means that you don't update variables or modify state. Pure functions will always return the same value given the same input and will do nothing else. Functions that are referentially transparent are more predictable and more composable. Non-strict (somewhat like lazy) means that you can express infinite data structures. Strong static typing means that a lot of program errors are caught during compilation. Type inference means that the compiler can often figure out the type of a value by itself. The compiler can also tell you if a value has conflicting types in different parts of the code.
There are more than 10,000 free third-party packages available at Hackage, the Haskell community's central package archive, and you can download them using the Stack tool that Exercism also uses.
You can also read the free book Learn You a Haskell for Great Good or follow the interactive tutorial at tryhaskell.org.
Key Features of Haskell
Purely functional
Pure functions always produce the same output for the same input, so are more predictable.
Non-strict
Lazy evaluation only provides results as needed, allowing for infinite data structures.
Strong, static typing
Strong, static types mean more errors are caught before the program is run.
Type inference
Automatic inference of types makes the code lighter and the developer more productive.
Type classes
Categorising types into classes provides type-safe overloading.
Algebraic data types
Powerful data types provide a high degree of convenience and safety.
Purely functional
Pure functions always produce the same output for the same input, so are more predictable.
Non-strict
Lazy evaluation only provides results as needed, allowing for infinite data structures.
Strong, static typing
Strong, static types mean more errors are caught before the program is run.
Type inference
Automatic inference of types makes the code lighter and the developer more productive.
Type classes
Categorising types into classes provides type-safe overloading.
Algebraic data types
Powerful data types provide a high degree of convenience and safety.
