Hugs
Hugs 98 is a Haskell interpreter, based on the Haskell 98 standard. It is a development of Gofer, a functional programming system in a similar language to Haskell. In 1995 a branch of Gofer, Hugs (Haskell User's Gofer System), was released giving greater Haskell compliance to the original program. Subsequent versions have kept the name Hugs with the version of Haskell they support added.

Hugs is distributed under it's own license, which allows for redistribution with or without modification. It is also currently used by the University of Kent, as the default Interpreter for Haskell files.

The following is an edited description of the Hugs feature list as noted from the Hugs website:
  • Lazy evaluation, higher order functions, and pattern matching
  • A wide range of built-in types
  • An advanced polymorphic type system with type and constructor class overloading
  • All of the features of the Haskell 98 expression and pattern syntax
  • An implementation of the Haskell 98 primitives for monadic I/O
  • An almost complete implementation of the Haskell module system