Lucian: Dataflow and Object-orientation

Transcription

1 : Object-orientation Computer Lab, University of Cambridge BCTCS 09

2 : Language interoperation s and No one language is all things to all people/programs Some languages/paradigms are more appropriate for a certain class of program The best of both worlds... Language interoperation Multi-paradigm languages But how to interoperate and combine paradigms?

3 : s and Interoperates dataflow and object-orientation Declarative dataflow à la Interoperated with bject-orientated programming What do the two paradigms mean with respect to each other?

4 : LUCID, The dataflow programming language s and Wadge, W.W., and Ashcroft, E. A. LUCID, The dataflow programming language. Academic Press Professional, Inc., San Diego, CA, USA, A declarative dataflow language Iteration and variable update is declarative Lazy, demand driven

5 : Declarative iteration and assignment s and Procedural Multiple scattered statements x = 2... x = 0... loop:... x += 1... Single inseparable definition x = 2 fby 0 fby x + 1

6 : One of LUCID: s s and Declarative language of infinite streams Expressions return streams Operators are independent processes, connected with streams Programs are continuous Asynchronous

7 : LUCID operations as stream operations s and Constant s 1 = 1, 1, 1,... Arithmetic/boolean binary operators applied pointwise x + y = x 0 + y 0, x 1 + y 1,... Conditional construct (pointwise) { x0 if b 0 if b then x else y = y 0 otherwise,...

8 : LUCID operations as stream operations s and First operator - repeat first element (head) first x = x 0, x 0,... Next operator - skip first element (tail) next x = x 1, x 2,... Followed by - fby - (cons) x fby y = x 0, y 0, y 1...

9 : s and Classic example: Natural numbers Example n = 1 fby (n+1) Remember: x fby y = x 0, y 0, y 1...

10 : Interoperate dataflow with objects? s and How are objects and streams related? What are objects and streams formally?

11 : data structure s and Blackbox with hidden state X Observe a value A Transition to next state X Keep asking for a next state and observation

12 : Functional objects s and Blackbox with hidden state (private data/) Observe attributes Immutable Method calls return new object i.e. transform/transition to next object state Turn imperative OO into pseudo-functional OO using deep cloning

13 : Functional objects - Infinite tree s and

14 : and s s and generalise streams have more transformations and observations Therefore, can embed streams into OO.

15 : Single path of object tree = stream s and Can describe path (history) of an object s transformations using dataflow equations

16 : s and Underlying computational device: objects streams embedded into functional objects attribute = value method = next Dataflow objects: describe object transformations in a single definition Single definition is a stream of object snapshots

17 : Dataflow objects s and Procedural Multiple scattered statements x = new X... x.m1(...)... x.m2(...)... Single inseparable definition x = new X <<= m1(...) fby m2(...)

18 : s and Interoperation Has no class definitions of its own. Instantiate objects defined in some OO language. Some OO language class BankAccount: balance = 0 credit(x) =... debit(x) =... o = new BankAccount......

19 : Dataflow Object : s and Describe an object s life in a single definition Example new class new class <<= new class = BA 0, BA 0, BA 0,... where BA 0 is a newly instantiated BankAccount object.

20 : Dataflow Object : s and Give list of method calls after <<= Example ba = new BankAccount <<= c(2) fby d(6) fby id} ba = BA 0, BA 0.c(2), BA 0.c(2).d(6), BA 0.c(2).d(6),... where BA 0 is a newly instantiated BankAccount object.

21 : Dataflow Object : Attribute reference s and Example ba.balanace ba.balance = ba 0.balance, ba 1.balance, ba 2.balance,... Attribute at each object snapshot

22 : Dataflow Object : s and Apply a method pointwise as opposed to giving a stream of method calls Example ba = ba.credit(param) ba = ba 0.c(p 0 ), ba 1.c(p 1 ),... No longer a dataflow object, just a stream of object snapshots

23 : s and Example ba = new BankAccount <<= credit(10) fby debit(2) fby debit(20) fby id balance = ba.balance overdrawn? = ba.balance < 0 balance = 0, 10, 8, 12, 12,... overdrawn? = false, false, false, true, true,...

24 : s and is an OO-dataflow language extending Has a declarative dataflow view of objects Define object and its transformations in a single declaration embeds streams into objects i.e. interpreted with objects

25 : s and Equational reasoning on object behaviours Object versioning, and rollback Reactive, continuous programming with objects Hybrid programming: combining dynamic, reactive programs with discrete units of functionality: objects Benefits of IO and libraries from OO into Coarse grained parallelism

26 : s and Coalgebraic semantics for reasoning Invariant equations on object behaviour for further reasoning Typing rules for when a stream of snapshots is historical or not Real world OO is not well behaved: global side effects, aliasing Current implementation is old and broken in places Parallel implementation on stream processors

27 : s and eod