Initial data commit

Task/Abstract-type/Crystal/abstract-type.crystal

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+abstract class Animal # only abstract class can have abstract methods
+  abstract def move
+  abstract def think
+
+  # abstract class can have normal fields and methods
+  def initialize(@name : String)
+  end
+
+  def process
+    think
+    move
+  end
+end
+
+# WalkingAnimal still have to be declared abstract because `think` was not implemented
+abstract class WalkingAnimal < Animal
+  def move
+    puts "#{@name} walks"
+  end
+end
+
+class Human < WalkingAnimal
+  property in_car = false
+
+  def move
+    if in_car
+      puts "#{@name} drives a car"
+    else
+      super
+    end
+  end
+
+  def think
+    puts "#{@name} thinks"
+  end
+end
+
+# Animal.new # => can't instantiate abstract class
+he = Human.new("Andrew") # ok
+he.process