From HaskellWiki

The Discus language is an experimental dialect of Haskell which investigates static typing and program transformation in the presence of computational effects. Discus includes region and effect typing, and this extra information provides a handle on the operational behaviour of code that isn't available in other languages.

Programs can be written in either a pure/functional or effecful/imperative style, and one of our goals is to provide both styles coherently in the same language. The two styles can be mixed safely, for example: when using laziness the type system guarantees that computations with visible side effects are not suspended. Many Haskell programs are also Discus programs, or will run with minor changes.

More Information