Modular OOP with Units and Mixins

1
(No Transcript)
2
Modular OOP with Units and Mixins

• Paper by Finder/Flatt
• Presentation by Dan Licata

3
Scheme in Fixnum Minutes

• gt 4
• 4
• gt(define x 4)
• gt x
• 4

4
Scheme in Fixnum Minutes

• gt( 4 4)
• 8
• gt ( ( 4 ( 4 4)) ( 4 4))
• 20

5
Scheme in Fixnum Minutes

• gt 5
• 5

6
Scheme in Fixnum Minutes

• gt (define add2 (lambda (x y) ( x y)))
• gt (add2 4 5)
• 9
• gt add2
• ltprocedure...gt

7
Scheme in Fixnum Minutes

• (define add2(lambda (x y) ( x y))
• gt (add2 4 5)
• 9

(define mk-add(lambda (x) (lambda(y) ( x
y))))
8
Scheme in Fixnum Minutes

• gt (mk-add 4)
• ltproceduregt
• gt (define add4 (mk-add 4))
• gt (add4 5)
• 1

(define mk-add(lambda (x) (lambda(y) ( x
y))))
9
Reading a Paper

1. What problem are they solving?
2. How do they solve it?
3. Why should I care?

10
Reading a Paper

1. What problem are they solving?
2. How do they solve it?
3. Why should I care?

11
Extensibility!

• Same story as last time
• Functional programming gt easy to add a new
  operation
• OOP gt easy to add a new datatype variant
• What would the ideal be?

12
Extensibility
Shape
Clients
13
Add a New Variant
Shape
Existing clients should not need to be modified!
Clients
14
Add a New Variant
Shape
Clients
New Clients
15
Add a New Operation
Shape
Clients
New Clients
16
Presumptions in the Picture

• Add a new operation by subclassing
• If the change gets used behind the original
  interface, unmodified clients should see it
• In traditional OOP, do they?

17
Define Original Datatype

• (define Shape (interface () draw)
• (define Rectangle
• (class null (Shape) (width height)
• (public
• draw (lambda (window x y) ))))

18
Define Original Datatype

• (define Circle
• (class null (Shape) (radius)
• (public
• draw (lambda (window x y) ...))))

19
Define Original Datatype

• (define display-shape
• (lambda (shape)
• (let (window ...)
• (send shape draw window 0 0))))

20
Create A Client

• (display-shape (make-object Rectangle
• 50
• 100))

21
Add a New Variant

• (define Union
• (class null (Shape) (left right)
• (public
• draw (lambda (window x y) ...))))

22
Add a Client that Uses It

• (display-shape
• (make-object Union
• (make-object Rectangle 10 30)
• (make-object Circle 20)))

23
Add a New Operation

• (define BB-Shape
• (interface (shape) bounding-box)
• (define BB-Rectangle
• (class Rectangle (BB-Shape) (w h)
• (public
• bounding-box (lambda ()
• (make-object BB
• 0 0 w h)))
• ...))

24
Add a New Operation

• (define display-shape
• (lambda (shape)
• (let (bb (send shape bounding-box)
• (let (window ... x ... y ...)
• (send shape draw window x y)))))

25
Do the Old Clients Use It?

• (display-shape (make-object Rectangle
• 50
• 100))
• (display-shape
• (make-object Union
• (make-object Rectangle 10 30)
• (make-object Circle 20)))

26
Do the Old Clients Use It?

• (display-shape (make-object Rectangle
• 50
• 100))
• (display-shape
• (make-object Union
• (make-object Rectangle 10 30)
• (make-object Circle 20)))

27
Potential Solution

• Always program using Abstract Factory
• Downsides
• Requires forethought
• Contorts the code
• How can we add language support?

28
Reading a Paper

1. What problem are they solving?
2. How do they solve it?
3. Why should I care?

29
Fortune-Cookie Wisdom

• Every problem in CS can be solved by adding a
  layer of indirection.
• Layers
• Mixins
• Units

30
Mixin

• Class that is parameterized by its superclass

(define BB-Rect (class Rectangle
...))
(define BB-Rect (lambda (Rect) (class Rect
...)))
31
Unit

• Module that is
• Separately compilable
• Black-box reusable (has an interface)
• Multiply instantiable
• Dynamically linkable

32
Define Original Datatype

• (define Basic-Shapes
• (unit (import)
• (export Shape Rectangle Circle)
• (define Shape (interface ...))
• (define Rectangle (class ...))
• (define Circle (class ...))))

33
Define Original Datatype

• (define GUI
• (unit (import Shape)
• (export display-shape)
• (define display-shape ...)))

34
Create a Client

• (define Picture
• (unit (import Shape Rectangle Circle
• display-shape)
• (export)
• (display-shape
• (make-object Rectangle 50 100))))

35
Link Them All Together

• (define Basic-Program
• (compound-unit
• (import)
• (link S (Basic-Shapes)
• G (GUI (S Shape))
• P (Picture (S Rectangle)
• (S Circle)
• (G display-shape)))
• (export)))

36
And Run It

• (invoke-unit Basic-Program)

37
Add a New Variant

• (define Union-Shape
• (unit (import Shape)
• (export Union)
• (define Union (class ...))))

38
Package It Up with the Others

• (define BasicUnion-Shapes
• (compound-unit (import)
• (link S (Basic-Shapes)
• US (Union-Shape (S Shape)))
• (export (S Shape) (S Rectangle)
• (S Circle) (US Union))))

39
Add a Client that Uses It

• (define Union-Picture
• (unit (import Rectangle Circle Union
• display-shape)
• (export)
• (display-shape
• (make-object Union
• (make-object Rectangle 10 30)
• (make-object Circle 20))))

40
Link Them All Together

• (define Union-Program
• (compound-unit
• (import)
• (link S (BasicUnion-Shapes)
• G (GUI (S Shape))
• P (Picture (S Rectangle)
• (S Circle)
• (G display-shape))
• UP (Union-Picture (S Rectangle)
• (S Circle) (S Union)
• (G display-shape)))
• (export)))

Picture is unchanged!
41
Add a New Operation

• (define BB-Shapes
• (unit (import Shape Rectangle
  Circle Union)
• (export BB-Shape BB-Rectangle
• BB-Circle BB-Union)
• (define BB-Shape (interface ...))
• (define BB-Rectangle (class Rectangle ...))
• (define BB-Circle (class Circle ...)
• (define BB-Union (class Union ...)
• (define BB ...)))

Implicit mixin!
42
Package It Up with the Others

• (define BasicUnionBB-Shapes
• (compound-unit (import)
• (link S (BasicUnion-Shapes)
• BS (BB-Shapes (S Shape)
• (S Rectangle)
  (S Circle) (S
  Union)))
• (export (S Shape) (BS BB-Shape) (BS BB)
• (BS (BB-Rectangle Rectangle))
• ...)))

Rename to preserve substitutability!
43
Use the New Functionality

• (define BB-GUI
• (unit (import BB-Shape BB)
• (export display-shape)
• (define display-shape ...)))

44
Link Them All Together
Picture and UnionPicture are unchanged!

• (define BB-Program
• (compound-unit
• (import)
• (link S (BasicUnionBB-Shapes)
• BG (BB-GUI (S BB-Shape) (S BB))
• P (Picture (S Rectangle)
• (S Circle)
• (BG display-shape))
• UP (Union-Picture (S Rectangle)
• (S Circle) (S Union)
• (BG display-shape)))
• (export)))

45
Reading a Paper

1. What problem are they solving?
2. How do they solve it?
3. Why should I care?

46
Solves Extensibility Problem

• You can add both new variants and new operations
  without modifying existing clients
• Unmodified clients may use the new operation
• Or, they may not

47
Synergy Between Modules and Classes

• Modules are good for
• Separate development
• Linking
• Classes are good for
• Extensible datatypes
• Selective reuse
• Language should have both!

48
Separate Definitions and Linking

• Module externally link dependencies
• Classs externally specify superclass
• In both, allow substitution of subtypes!

49
In A Statically-Typed Setting

• Would this work for Java?
• Would this work for ML-with-objects?

50
Bigger Picture

• Do you buy it?
• How could it be improved?