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…..