The Development of Oz / Mozart

1
The Development ofOz / Mozart

• Gert Smolka
• Saarland University

2
Overview

• Oz 1991-1995
• Mozart 1996-1999
• Alice 1999 - now

3
1991-1995

• Oz

4
1991 1995 (Oz)

• DFKI, Programming Systems Lab
• Project Hydra, 1991-1995, BMBF
• Project ACCLAIM, 1992-1995, EU BRA

5
Hydra proposal

• Research on
• Constraint logic programming
• Computation and deduction
• Knowledge representation and reasoning
• Concurrent objects
• Develop programming system for logic-based
  problem solving
• Language
• VM and compiler

6
Students and postdocs

• Ralph Treinen (1991-95)
• Peter van Roy (1994-96)
• Rolf Backofen
• Joachim Niehren

• Ralph Scheidhauer
• Michael Mehl
• Martin Henz
• Jörg Würtz
• Christian Schulte
• Tobias Müller
• Martin Müller
• Konstantin Popow

7
My background 1991

• Functional logic programming (Fresh)
• Cornell 1983-85
• Algebraic specification and rewriting (Obj)
• SRI, Kaiserslautern, 1985-1989
• Typed logic programming (TEL)
• Cornell, SRI, Kaiserlautern, IBM 1983-1989
• Feature constraints and residuation (Life)
• MCC, Kaiserslautern, IBM, Digital PRL, DFKI
• Knowledge representation (KL-One)
• IBM, DFKI

8
Important influences

• Life, residuation
• Hassan Ait-Kaci and Andreas Podelski
• Paris R. Lab of Digital, May 1990
• AKL, deep guards, WAM-technology
• Seif Haridi, Sverker Janson
• ICLP 1991 in Paris
• Acclaim EU project (1992-1995)
• Milners Pi-calculus (Oxford April 1992)

9
1992

• First-class procedures
• Compositional syntax
• Synchronous channel communication
• First version of Oz
• Prototype, demos

10
(No Transcript)
11
(No Transcript)
12
1993

• Object system
• GUI (Interviews), Browser
• Asynchronous channels
• Names
• Efficient prototype offered through ftp
• Oz Handbook in work

13
1994

• Search combinator
• Cells replace channels
• Threads considered first-class
• Definition of Kernel Oz
• Tcl/Tk replaces Interviews
• Finite domain constraints
• Project with Daimler (multi agent)

14
1995

• DFKI Oz 1.0 released
• PERDIO Proposal finished
• Explorer
• Oz-2 explicit concurrency
• SICS joins Oz development
• WOZ 95 in Martigny

15
(No Transcript)
16
Key qualities

• Formal model
• Kernel language
• Implementation
• Users
• Ambition
• Abandoned logic programming

17
Key concepts

• First-class procedures (FP)
• Search based on spaces
• Concurrency based on cells and logic variables
• Classes as modules

18
1996-1999

• Mozart

19
1996-1999 (Mozart)

• Proposal by Schulte, Smolka, van Roy
• Carried out in collaboration with SICS
• Mozart Consortium founded 1996
• SFB project NEP at Saarland University
• Leif Kornstaedt, Denys Duchier
• Oz-2 ? Oz-3, conservative extension
• Mozart 1.0 released January 1999

20
(No Transcript)
21
Extension of Oz with support for

• distributed programming
• persistent storage
• components

22
Inspiration from

• Cardellis Obliq (distribution)
• network transparency
• network awareness
• Java (components)
• load components from Internet when needed

23
Export / import of language structures

• Easy for stateless structures
• Replication
• Code is stateless
• Problematic for stateful structures
• must not be replicated
• must be referred to by proxies
• may move
• may die

24
Components

• System is graph of components starts with one
  component
• Further components are loaded from net only when
  needed (analog to Web)
• Component consists of
• a procedure executed upon installation
• URLs of required components

25
Code ? Functor ? Component

• Code
• text, creates structure when executed
• Functor
• stateless structure
• loaded from net to install component
• Component
• structure created by functor

26
Code

• Text
• Can be executed by interpreter
• Execution wrt to environment identifier ?
  structure
• Execution creates structure

27
Mozart compiler

• Offline interpreter
• Executes code that
• installs an environment
• creates a functor
• Stores functor in file system
• Translation of code into low-level code
  semantically not visible

28
Comparison with Java

• Class files are type checked code
• Functors are structures obtained by execution of
  codes
• Java compiler is type checker
• Mozart compiler is interpreter that executes code

29
Mozarts component system is programmed in Oz

• Pickles store functors
• By-need synchronization triggers loading of
  functors
• Futures provide safety (read-only views of logic
  variables)

30
(No Transcript)
31
1999-now

• Alice

32
Alice (since 1999)

• NEP Project, SFB 378
• Thorsten Brunklaus
• Leif Kornstaedt (now MS)
• Didier Le Botlan
• Andreas Rossberg
• Guido Tack

33
Oz designed as hybrid

• constraint programming platform
• general purpose programming language
• gt design conflicts and complexity
• gt refactorize into
• programming language constraint
  library

34
Gecode

• Constraint library (C)
• Bindings for different languages
• Done by Christian Schulte (KTH) and Guido Tack
  (Saarbrücken)

35
Abandon logic variables

• Can be bound everywhere
• Malicious consumer can bind tail variable of
  stream
• Break abstractions
• Unification not needed

36
Have futures instead

• Multilisp Halstead 1985
• Added to Mozart in 1998
• Refine logic variable into
• consumer end (future)
• producer end (promise)
• Data flow synchronization by-need
  synchronization

37
Functional kernel language

• Input-output-distinction
• implicit with relational kernel
• explicit with functional kernel
• Should be explicit because of
• clearness of programs
• efficiency of execution
• typing

38
Functional threads

• concur, byneed (unit ? a) ? a
• create thread executing arg. function
• return future
• future represents result of thread

39
Atomic cell exchange

• exchange a cell ? a ? a
• atomic read and write
• concur and exchange almost suffice to program
  concurrent data structures (e.g., locks, ports,
  channels)

40
Promises avoid polling

• promise unit ? a promise
• get a promise ? a
• put a promise ? a ? unit
• Can be implement with concur and exchange
• get with thread polling cell
• put with exchange on arbiter cell

41
Implementing promises with polling

• fun promise () let
• val value ref NONE
• fun poll () case !value of NONE gt poll ()
  SOME x gt x
• val future byneed poll
• val arbiter ref false
• in
• (arbiter, value, future)
• end
• fun get (_,_,x) x
• fun put (a,v,_) x if exchange a true then raise
  Error else v SOME x
• Rossberg 2004

42
Exceptions

• Futures are associated with threads
• Threads can fail with exceptions
• Failed futures
• Request of failed future propagates exception
• Avoids infinite blocking on failed future

43
Alice

• Standard ML
• functional threads
• components
• remote procedure calls
• dynamic types

44
Dynamic types

• pack atype ? a ? package
• unpack atype ? package ? a
• Used for take / offer
• At the level of module types
• higher-order module system

45
Alice System

• Mozart-based version is available
• Efficient stand-alone version soon to be
  released
• Based on SEAM, an architecture and library for
  the implementation of virtual machines

46
Thanks to everyone who has contributed to Oz,
Mozart and Alice
47
What I see as main contributions

• Oz
• new architecture for constraint search
• theory of feature tree constraints and relative
  simplification
• efficient threads with data flow synchronization
• Mozart
• Pickling of higher-order structures
• component system based on pickling and futures
• distributed store, offer/take-primitives

48

• Alice
• dynamic types