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OOP
Basics

Define a class

Declare a class named Greeter that takes a string on creation and greets using this string if you call the "greet" method.
ruby
class Greeter
def initialize(whom) @whom = whom end
def greet() puts "Hello, #{@whom}!" end
end

(Greeter.new("world")).greet()
erlang
Greeter = make_greeter("world!"),
Greeter(greet).
clojure
(defprotocol IGreeter
(greet [this]))

(deftype Greeter [whom]
IGreeter
(greet [this]
(println (str "Hello, " whom))))

(greet (Greeter. "world"))
(defn greeter [whom]
{:whom whom})

(defn greet [g]
(println (str "Hello, " (:whom g))))

(greet (greeter "world"))
fsharp
type Greeter(whom' : string) =
member this.greet() = printfn "Hello, %s!" whom'

(new Greeter("world")).greet()
type Greeter(whom' : string) =
let whom : string = whom'
member this.greet() = printfn "Hello, %s!" whom

(new Greeter("world")).greet()
type Greeter =
class
val whom : string
new(whom') = { whom = whom' }
member this.greet() = printfn "Hello, %s!" this.whom
end

(new Greeter("world")).greet()

Instantiate object with mutable state

Reimplement the Greeter class so that the 'whom' property or data member remains private but is mutable, and is provided with getter and setter methods. Invoke the setter to change the greetee, invoke 'greet', then use the getter in displaying the line, "I have just greeted {whom}.".

For example, if the greetee is changed to 'Tommy' using the setter, the 'greet' method would display:

Hello, Tommy!

The getter would then be used to display the line:

I have just greeted Tommy.
ruby
class Greeter
attr_accessor :whom
def initialize(whom) @whom = whom end
def greet() puts "Hello, #{@whom}!" end
end

greeter = Greeter.new("world") ; greeter.greet()

greeter.whom = 'Tommy' ; greeter.greet()
puts "I have just greeted %s" % greeter.whom
clojure
(defn greeter [whom]
(atom {:whom whom}))

(defn get-whom [g]
(:whom @g))

(defn set-whom [g whom]
(swap! g #(conj % {:whom whom})))

(defn greet [g]
(println (str "Hello, " (:whom @g) "!")))

; using the "class"
(let [g (greeter "world")]
(greet g)
(set-whom g "Tommy")
(greet g)
(println (str "I have just greeted " (get-whom g) ".")))

; or same effect without using any variables
(println (str "I have just greeted "
(get-whom (doto (greeter "world")
(greet)
(set-whom "Tommy")
(greet)))
"."))
fsharp
type Greeter(name:string) =
let mutable whom = name

member this.Whom
with get () = whom
and set v = whom <- v

member this.Greet() =
printfn "Hello, %s!" whom

let greeter = Greeter("World")
greeter.Greet()
greeter.Whom <- "Tommy"
greeter.Greet()
printfn "I have just greeted %s." greeter.Whom
Inheritance

Implement Inheritance Heirarchy

Implement a Shape abstract class which will form the base of an inheritance hierarchy that models 2D geometric shapes. It will have:

* A non-mutable 'name' property or data member set by derived or descendant classes at construction time
* A 'area' method intended to be overridden by derived or descendant classes ( double precision floating point return value)
* A 'print' method (also for overriding) will display the shape's name, area, and all shape-specific values

Two derived or descendant classes will be created:
* Circle    -> Constructor requires a 'radius' argument, and a 'circumference' method to be implemented  
* Rectangle -> Constructor requires 'length' and 'breadth' arguments, and a 'perimeter' method to be implemented 

Instantiate an object of each class, and invoke each objects 'print' method to show relevant details.
ruby
class Shape
def initialize(name="") @name = name end
end

class Circle < Shape
def initialize(radius) super("circle") ; @radius = radius end

def area() 3.14159 * @radius * @radius end
def circumference() 2 * 3.14159 * @radius end

def print()
puts "I am a #{@name} with ->"
puts "Radius: %.2f" % @radius
puts "Area: %.2f" % self.area()
puts "Circumference: %.2f\n" % self.circumference()
end

end

class Rectangle < Shape
def initialize(length, breadth) super("rectangle") ; @length = length ; @breadth = breadth end

def area() @length * @breadth end
def perimeter() 2 * @length + 2 * @breadth end

def print()
puts "I am a #{@name} with ->"
printf("Length, Width: %.2f, %.2f\n", @length, @breadth)
puts "Area: %.2f" % self.area()
puts "Perimeter: %.2f\n" % self.perimeter()
end
end

# ------

shapes = [Circle.new(4.2), Rectangle.new(2.7, 3.1), Rectangle.new(6.2, 2.6), Circle.new(17.3)]
shapes.each {|shape| shape.print}

clojure
(defmulti area :Shape)
(defmulti print :Shape)

; Circle methods
(defn circle [r]
{:Shape :Circle
:name "Circle"
:radius r})

(defn circumference [c]
(* 2 Math/PI (:radius c)))

(defmethod area :Circle [c]
(* Math/PI (:radius c) (:radius c)))

(defmethod print :Circle [c]
(println (format "I am a %s with ->" (:name c)))
(println (format "Radius: %.2f" (:radius c)))
(println (format "Area: %.2f" (area c)))
(println (format "Circumference: %.2f" (circumference c))))

; Rectangle methods
(defn rectangle [l b]
{:Shape :Rectangle
:name "Rectangle"
:length l
:breadth b})

(defn perimeter [r]
(+ (* 2 (:length r)) (* 2 (:breadth r))))

(defmethod area :Rectangle [r]
(* (:length r) (:breadth r)))

(defmethod print :Rectangle [r]
(println (format "I am a %s with ->" (:name r)))
(println (format "Length, Width: %.2f, %.2f" (:length r) (:breadth r)))
(println (format "Area: %.2f" (area r)))
(println (format "Perimeter: %.2f" (perimeter r))))

; usage of the "classes"
(let [shapes (list (circle 4.2) (rectangle 2.7 3.1) (rectangle 6.2 2.6) (circle 17.3))]
(doseq [shape shapes]
(print shape)))
fsharp
[<AbstractClass>]
type Shape(name:string) =
member this.Name = name
abstract Area : float
abstract Print : unit -> unit

type Circle(name, radius:float) =
inherit Shape(name)
member this.Radius = radius
member this.Circumference =
System.Math.PI * radius * 2.
override this.Area =
System.Math.PI * radius * radius
override this.Print() =
printfn "Circle: %s" this.Name
printfn "Area: %f" this.Area
printfn "Circumference: %f" this.Circumference
printfn "Radius: %f" this.Radius

type Rectangle(name, length:float, breadth:float) =
inherit Shape(name)
member this.Length = length
member this.Breadth = breadth
member this.Perimiter =
(length * 2.) + (breadth * 2.)
override this.Area =
length * breadth
override this.Print() =
printfn "Rectangle: %s" this.Name
printfn "Area: %f" this.Area
printfn "Perimiter: %f" this.Perimiter
printfn "Length: %f" this.Length
printfn "Breadth: %f" this.Breadth

let c = Circle("Foo", 2.1)
let r = Rectangle("Bar", 2.2, 3.3)

c.Print()
printfn ""
r.Print()

Implement and use an Interface

Create a Serializable interface consisting of 'save' and 'restore' methods, each of which:

* Accept a stream or handle or descriptor argument for the source or destination
* Save to destination or restore from source the properties or data members of the implementing class (restrict yourself to the primitive types 'int' and 'string')

Next, create a Person class which has 'name' and 'age' properties or data members and implements this interface. Instantiate a Person object, save it to a serial stream, and instantiate a new Person object by restoring it from the serial stream.
ruby
class Person
def initialize(name, age)
@name, @age = name, age
end
end

tom = Person.new("Tom Bones", 23)

File.open('tommy.dump', 'w+') {|f| f.write(Marshal.dump(tommy)) }
toms_clone = Marshal.load(File.read('tommy.dump'))
clojure
(defn person [name age]
{:name name :age age})

(defn show [p]
(println (format "Name=%s Age=%d" (:name p) (:age p))))

(defn save [p filename]
(with-out-writer filename (pr p)))

(defn restore [filename]
(read (PushbackReader. (reader filename))))

(let [p (person "Ken" 38)]
(show p)
(save p *person-fn*))

(let [ser-p (restore *person-fn*)]
(show ser-p))
fsharp
// Since everyone else is using built-in functionality instead of
// defining an interface as required, I won't buck the trend.
// Maybe this problem should be named "Use serialization features" instead
// of "Implement and use an Interface"

open System
open System.IO
open System.Runtime.Serialization.Formatters.Binary

[<Serializable>]
type Person(name:string, age:int) =
member this.Name = name
member this.Age = age

let serialize x =
use ms = new MemoryStream()
let bf = new BinaryFormatter()
bf.Serialize(ms, x)
ms.ToArray()

let deserialize<'a> bytes =
use ms = new MemoryStream(bytes:byte[])
let bf = new BinaryFormatter()
bf.Deserialize(ms) :?> 'a

let before = Person("Joel", 35)
let bytes = serialize before
let after = deserialize<Person> bytes

printfn "Before: %s, %i" before.Name before.Age
printfn "After: %s, %i" after.Name after.Age