Typesafe Extensible Functional Objects J o n a t h a n I m - PowerPoint PPT Presentation

Typesafe Extensible Functional Objects J o n a t h a n I m m a n u e l B r a c h t h ä u s e r U n i v e r s i t y o f Ma r b u r g , G e r m a n y j o n a t h a n @ b - s t u d i o s . d e HessPL, September 10 th 2014

Why Encode Objects in an OO-Language? Ob j e c t Al g e b r a s [ 1 ] : A l l o w i n g m o d u l a r d e fi n e d f o l d s b y c h o o s i n g a fi r s t c l a s s r e p r e s e n t a t i o n o f a l g e b r a s . My o b j . e x t e n d L i b r a r y : A l l o w i n g m o d u l a r d e fi n e d u n f o l d s b y c h o o s i n g a fi r s t c l a s s e x t e n s i b l e r e p r e s e n t a t i o n o f c o a l g e b r a s . 1 O l i v e i r a , B r u n o C . D . S . , a n d Wi l l i a m R . C o o k . " E x t e n s i b i l i t y f o r t h e Ma s s e s . " 2 E C O O P 2 0 1 2 – O b j e c t - O r i e n t e d P r o g r a m m i n g . S p r i n g e r B e r l i n H e i d e l b e r g , 2 0 1 2 .

Why do I need this, again? E v e r y t i m e y o u fi n d y o u r s e l f w i s h i n g t h a t d e c o r a t o r s w o u l d s u p p o r t l a t e b i n d i n g a n d c o u l d b e a r b i t r a r i l y c o mp o s e d : o b j . e x t e n d i s w h a t y o u a r e l o o k i n g f o r . 3

An Example of Objects in Scala. T w o p l a i n S c a l a t r a i t s : trait Counter { private var i : Int def get : Int = i def inc : Unit = { i += 1 } } trait SkipCounter { self : Counter ⇒ def skip : Unit = { this . inc ; this . inc } } 4

Static Mixin Composition in Scala ... … a l l o w s : val c = new Counter with SkipCounter { var i = 0 } … b u t i t d o e s n o t a l l o w : val c = new Counter { var i = 0 }; c extend SkipCounter Second Class Traits! … w h a t c a n b e a c h i e v e d w i t h o b j . e x t e n d : val c = unfold(Counter, 0); c. extend (SkipCounter, ()) First Class Values! 5

Use Cases for Dynamic Specialization [2]. I n c r e me n t a l c o n s t r u c t i o n o f o b j e c t s p e r f o r m e d b y m o d u l a r i z e d b u i l d e r s A d d i n g me t h o d s f o r p r i n t i n g a n d t r a c i n g i n o r d e r t o f a c i l i t a t e d e b u g g i n g An n o t a t i n g o b j e c t s w i t h a d d i t i o n a l i n f o r m a t i o n , a c q u i r e d a f t e r o b j e c t c r e a t i o n 2 E. Ernst. gbeta – A Language with Virtual Attributes, Block Structure, and 6 Propagating, Dynamic Inheritance. PhD thesis, Department of Computer Science, University of Aarhus, Arhus, Denmark, 1999.

obj .extend Enables Dynamic Specialization by ... … b u i l d i n g o n a c o a l g e b r a i c e n c o d i n g o f o b j e c t s [ 4 ] : – I F . n t e r f a c e s a r e e n c o d e d a s a n i n t e r f a c e e n d o f u n c t o r ⇒ – I S F[S] mp l e me n t a t i o n s a r e e n c o d e d a s c o a l g e b r a s . – I n s t a n t i a t i o n i s e n c o d e d b y u n f o l d i n g a c o a l g e b r a w i t h Fix[F] . a n i n i t i a l s t a t e t o t h e g r e a t e s t fi x e d p o i n t – Ob j e c t s a r e e n c o d e d a s t e r m i n a l c o a l g e b r a s o v e r t h e s e f u n c t o r s . val Counter = (i: Int) ⇒ new CounterF[Int] {...} val c = unfold(Counter, 0) trait CounterF[S] { def get : Int def inc : S 4 B. Jacobs. Objects and Classes, Coalgebraically , pages 83–103. 7 } Springer-Verlag, 1995.

An Example of a Standard Terminal Coalgebra. val c 0 = unfold(Counter, 0) val c 1 = c 0 .inc 8

Same Example with obj .extend. val c 0 = unfold(Counter, 0) val c 1 = c 0 .inc 9

Same Example with obj .extend. val c 0 = unfold(Counter, 0) val c 1 = c 0 .inc val c 2 = c 1 extend (SkipCounter, ()) 10

Same Example with obj .extend. val c 0 = unfold(Counter, 0) val c 1 = c 0 .inc val c 2 = c 1 extend (SkipCounter, ()) T T h h e e t t y y p p e e c c val c 3 = c 2 .skip h h a a n n g g e e s s ! ! 11

obj .extend Enables Dynamic Specialization by ... compose(co 1 , co 2 ) … d e fi n i n g t h e f u n c t i o n o n c o a l g e b r a s a s ≅ ⇒ compose(co 1 , co 2 ) s mix(co 1 (s), co 2 (s)) extend compose , … i m p l e m e n t i n g i n t e r m s o f t h u s unfold(co 1 , s 1 ) extend (co 2 , s 2 ) i s i m p l e m e n t e d a s unfold(compose(co 1 , co 2 ), (s 1 , s 2 )) 12

And there is more to obj .extend... – A l l o w s m u t u a l d e p e n d e n c i e s b e t w e e n c o a l g e b r a s b y e n c o d i n g s e l f - r e f e r e n c e s . – A l l o w s a c c e s s i n g p r i v a t e s l i c e s o f t h e s t a t e u s i n g l e n s e s . – A l l o w s r e f e r e n c e s t o t h e e x t e n d e d b a s e c o a l g e b r a , i m i t a t i n g s u p e r - c a l l s . – A l l o w s s e l e c t i v e o p e n - r e c u r s i o n [ 3 ] b y p a s s i n g t h e c u r r e n t a s w e l l a s t h e l a t e b o u n d s e l f - r e f e r e n c e . ⇒ S o m e o f t h e e x t e n s i o n s h a v e b e e n u s e d t o t r a n s l a t e a s u b s e t o f o p e n j d k w r i t e r s t o t h e e n c o d i n g . 3 J. Aldrich and K. Donnelly. Selective open recursion: Modular reasoning about 13 components and inheritance. SAVCBS 2004 Specification and Verification of Component-Based Systems , page 26, 2004.

Conclusions. We h a v e s h o w n : – We c a n e n c o d e d y n a m i c s p e c i a l i z a t i o n o f o b j e c t s i n S c a l a . I t s e e ms : – O b j e c t a l g e b r a s c a n b e u s e f u l l y d u a l i z e d . F u t u r e w o r k : – O p t i m i z e p e r f o r m a n c e t o b e p r a c t i c a l l y u s e f u l . – D e v e l o p a c o n s i s t e n t a n d e a s y t o u s e d s l . – I n v e s t i g a t e d u a l i t y t o o b j e c t a l g e b r a s f o r m a l l y . 14

Further Materials. S l i d e s : h t t p : / / fi l e s . b - s t u d i o s . d e / h e s s p l - s l i d e s . p d f I C F P S R C P o s t e r : h t t p : / / fi l e s . b - s t u d i o s . d e / i c f p 2 0 1 4 - p o s t e r . p d f Mi x i n C o mp o s i t i o n : h t t p s : / / g i t h u b . c o m / b - s t u d i o s / Mi x i n C o m p o s i t i o n 15

EOS E n d o f S l i d e s , n o t h i n g t o s e e h e r e . 16

obj .extend Enables Dynamic Specialization by ... A A b b s s t t r r a a c c t t i i o o n n B B a a r r r r i i Fix : e e … m a k i n g u s e o f t h e “ fi r s t - c l a s s y - n e s s ” o f r r – extend N o v e l m e t h o d Fix i s a d d e d t o – O r i g i n a l c o a l g e b r a a n d i n i t i a l s t a t e a r e k e p t Fix i n s i d e t h e c l o s u r e o f b u t n e v e r r e v e a l e d . trait Fix[ F [_]] { def out : F [Fix[ F ]] ⇒ G [ S 2 ], state 2 : S 2 ): Fix[ F WithF G ] def extend [ G [_], S 2 ]( co 2 : S 2 } 17 def unfold[ F [_], S 1 ]( co 1 : S 1 => F [S 1 ], state 1 : S 1 ): Fix[ F ] = new Fix[ F ] {…}