Hugs
Hugs 98 is a Haskell interpreter, based on the Haskell 98 standard.

Hugs is distributed under it's own license, which allows for redistribution with or without modification. HEAT requires the presence of a Haskell interpreter and currently only works with Hugs.

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