Hello You might want to look at Wolfram Kahl's update of the modular interpreter code to Haskell 98: http://www.cas.mcmaster.ca/~kahl/FP/2003/ http://www.cas.mcmaster.ca/~kahl/FP/2003/Interpreter.pdf http://www.cas.mcmaster.ca/~kahl/FP/2003/Interpreter.lhs Best wishes Stephen