Kindof vaguely made a start on this, but cant quite see how to handle variables. I guess variables can be stored as a (Map.Map String Double), at least for a first draft? Then, I'm building up two hierarchies in parallel: - a set of parsec functions to parse the incoming string into a Program hierarchy - a set of data types to represent a program Then, there's a class called "Eval" containing a function "eval" which is instanced for each bit of the program hierarchy, so we simply call "eval" on the top level, and the program is executed. That works just fine as long as the only thing eval has to cope with is print statements (so eval has type IO ()), but I'm guessing the clean solution is to thread a Map.Map through that somehow? Solution so far: -- parsing hierarchy (pretty basic, but this bit doesnt seem particularly scary) string :: Parsec.Parser String string = Parsec.many1 Parsec.letter minimprint = do Parsec.string "print" Parsec.many1 (Parsec.char ' ') Parsec.char '"' stringvalue <- string Parsec.char '"' return (Print stringvalue) -- program data type hierarchy data Program = ProgramLeaf Statement | ProgramTree Program Statement deriving(Show) data Statement = PrintStatement Print | AssignmentStatement Assignment deriving(Show) data Print = Print String deriving(Show) data Assignment = VarAssignment Variable Value | Increment Variable | Decrement Variable deriving(Show) data Variable = Variable String deriving(Show) data Value = ValueFromConstant Constant | ValueFromVariable Variable deriving(Show) newtype Constant = Constant Int deriving(Show) -- eval instances class Eval a where eval :: a -> IO() instance Eval Program where eval (ProgramLeaf statement) = eval statement eval (ProgramTree program statement) = do eval program eval statement instance Eval Statement where eval ( PrintStatement print) = eval print eval ( AssignmentStatement assignment) = return () instance Eval Print where eval (Print value) = putStrLn value -- some code to test this minimparse minimsentence = case (Parsec.runParser minimprint () "" minimsentence) of (Right statement) -> eval statement Left error -> putStrLn("error: " ++ show(error)) test = minimparse "print \"hello\"" Running "test" correctly gives an output of "hello", which is a good start.