new tasks

Task/Classes/Haskell/classes-2.hs

@@ -0,0 +1,23 @@
+data Shape = Rectangle Double Double | Circle Double
+{- This Shape is a type rather than a type class. Rectangle and
+Circle are its constructors. -}
+
+perimeter :: Shape -> Double
+{- An ordinary function, not a method. -}
+perimeter (Rectangle width height) = 2 * width + 2 * height
+perimeter (Circle radius)          = 2 * pi * radius
+
+area :: Shape -> Double
+area (Rectangle width height) = width * height
+area (Circle radius)          = pi * radius^2
+
+apRatio :: Shape -> Double
+{- Technically, this version of apRatio is monomorphic. -}
+apRatio shape = area shape / perimeter shape
+
+main = do
+    print $ apRatio $ Circle 5
+    print $ apRatio $ Rectangle 5 5
+{- The value returned by apRatio is determined by the return values
+of area and perimeter, which just happen to be defined differently
+for Rectangles and Circles. -}