Haskell itself is not written in any specific language, as it is a programming language in its own right. Haskell is an advanced, purely functional programming language that was designed to be expressive, concise, and powerful.

The initial development of Haskell started in the late 1980s, and since then, it has evolved through multiple versions and revisions. The Haskell language is defined by a formal specification known as the Haskell Report, which outlines its syntax, semantics, and standard library.

However, Haskell compilers, which are responsible for translating Haskell code into executable machine code or an intermediate representation, are implemented using various programming languages. Some popular Haskell compilers include: 1. GHC (Glasgow Haskell Compiler) : GHC is the most widely used Haskell compiler and is written primarily in Haskell itself. It is implemented in a mixture of Haskell and C, with some parts written in other low-level languages for performance reasons.

2. Hugs : Hugs is an older Haskell interpreter that is also written in Haskell, with some components implemented in C.

3. nhc98 : nhc98 is another Haskell compiler that is written in C, with some portions implemented in Haskell.

These compilers implement the Haskell language specification and provide the necessary tools and infrastructure to compile Haskell code into executable binaries or run it interactively in an interpreter.