Question

The Haskell code below works fine.

data Point = Point Float Float deriving (Show)  
data Shape = Circle Point Float  
surface :: Shape -> Float  
surface (Circle _ r) = pi * r ^ 2  

Result:

*Main> surface $ Circle (Point 0 0) 10  
314.15927  

The Haskell code below does not work. Why? How to write surface function for Shape – Circle correctly?

data Point = Point Float Float deriving (Show)  
data Radius = Radius Float deriving (Show)

data Shape = Circle Point Radius   

surface :: Shape -> Float  
surface (Circle _ (Radius r)) = pi * (Radius r) ^ 2

Answer 1

Your last line is constructing a Radius object and raising that to a power. Since you haven’t defined the power operator for Radius, that can’t work. Remove the constructor call:

surface (Circle _ (Radius r)) = pi * r ^ 2