[Haskell-cafe] nice simple problem for someone struggling....

[Nicholls, Mark]

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.....

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