SugarHaskell is a syntactically extensible variant of Haskell. SugarHaskell organizes language extensions in regular Haskell modules, whose import activates the language extensions at hand. The article "Layout-sensitive Language Extensibility With SugarHaskell" presented at the Haskell Symposium 2012 describes details of SugarHaskell and is available online at http://sugarj.org/sugarhaskell.pdf.
cabal install sugarhaskell-0.1