Data commit

Task/Classes/Haskell/classes-1.hs

@@ -0,0 +1,33 @@
+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. -}

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. -}

Task/Classes/Haskell/classes-3.hs

@@ -0,0 +1,21 @@
+class Example (x) # 'x' is a field in class
+
+  # method definition
+  method double ()
+    return 2 * x
+  end
+
+  # 'initially' block is called on instance construction
+  initially (x)
+    if /x # if x is null (not given), then set field to 0
+      then self.x := 0
+      else self.x := x
+end
+
+procedure main ()
+  x1 := Example ()  # new instance with default value of x
+  x2 := Example (2) # new instance with given value of x
+  write (x1.x)
+  write (x2.x)
+  write (x2.double ()) # call a method
+end