I'm just trying to pick up the basics....and I've managed to write this code...which remarkably works...... module Main where data SquareType = SquareConstructor Int class ShapeInterface shape where area :: shape->Int data ShapeType = forall a. ShapeInterface a => ShapeType a instance ShapeInterface SquareType where area (SquareConstructor sideLength) = sideLength * sideLength main = do putStrLn (show (area (SquareConstructor 4))) name <- getLine putStrLn "" But my next iteration was to try to parametise SquareType.... So something like data SquareType a = Num a => SquareConstructor a but of course doing this breaks everything.......sepecifically the instance declaration `SquareType' is not applied to enough type arguments Expected kind `*', but `SquareType' has kind `* -> *' In the instance declaration for `ShapeInterface SquareType' And I can't seem to get it to work.....