new files

Task/Classes/Haskell/classes-1.hs

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+class Shape a where
+    perimeter :: a -> Double
+    area      :: a -> Double
+{- A type class Shape. Types belonging to Shape must support two
+methods, perimeter and area. -}
+
+data Rectangle = Rectangle Double Double
+{- A new type with a single constructor. In the case of data types
+which have only one constructor, we conventionally give the
+constructor the same name as the type, though this isn't mandatory. -}
+
+data Circle = Circle Double
+
+instance Shape Rectangle where
+    perimeter (Rectangle width height) = 2 * width + 2 * height
+    area      (Rectangle width height) = width * height
+{- We made Rectangle an instance of the Shape class by
+implementing perimeter, area :: Rectangle -> Int. -}
+
+instance Shape Circle where
+    perimeter (Circle radius) = 2 * pi * radius
+    area      (Circle radius) = pi * radius^2
+
+apRatio :: Shape a => a -> Double
+{- A simple polymorphic function. -}
+apRatio shape = area shape / perimeter shape
+
+main = do
+    print $ apRatio $ Circle 5
+    print $ apRatio $ Rectangle 5 5
+{- The correct version of apRatio (and hence the correct
+implementations of perimeter and area) is chosen based on the type
+of the argument. -}