Lecture 1: Introduction to Oz Programming Language

Lecture 1: Introduction to Oz Programming Language Per Brand

Features of Oz • Oz is an object-oriented language, it provides state, abstract data types, classes, objects, and inheritance. • Oz provides the features of functional programming: a compositional syntax, first-class procedures, and lexical scoping. In fact, every Oz entity is first class, including procedures, threads, classes, methods, and objects. • Oz is a concurrent (constraint) language • users can create dynamically any number of sequential threads that can interact with each other. • executing a statement in Oz proceeds only when all realdataflow dependencies on the variables involved are resolved. • Oz is a (concurrent) logic programming language

Multi-paradigm • Object-oriented, functional, constraint programming paradigms in one package • Is this really possible ?? • Can I easily achieve in Oz all that I can in my single-paradigm language X?? • To a large degree yes • dynamically typed though • Cross-paradigm advantages also • Doesn’t this multi-paradigm make a big mess ?? • Well-integrated • Simple kernel language • higher-order makes the kernel simpler than most single paradigm languages

Example: Object-oriented declare class Counter %% class definition attr count meth init %% constructor method count<-0 %% attribute update end meth inc %% public method (returning void) count<-@count +1 %% attribute access by @ end meth get($) %% public method (returning value) @count %% implicit return end end Obj={New Counter init} %% object constructed {Obj inc} %% object used {Obj inc} {Show {Obj get($)}} %% we expect to see ‘2’

Observations • Syntax • keyword rather than special character approach • not enough special characters for all paradigms • No types • Oz is dynamically typed • What would happen on {Obj dec} ? • Where’s the distinction with public/private/protected etc?. • All methods in the example are public • Later we show how to achieve private/protected in language security model • Method of returning values look strange • Oz uses a more general concept than functions/object invocations returnvalues

Example: Functional declare fun{Double X} %% function definition 2 * X end fun{Map F L}%% parameter F is a function case L of X|Xs then {F X}|{Map F Xs}%% returns list [] nil then nil end {Show {Map Double [1 2 3 4]}} %% we expect to see ‘[2 4 6 8]’ %% Alternatively using anonymous function (lambda expression) {Show {Map fun{$ X} 2*X end [1 2 3 4]}}

Example: Concurrent Constraint Programming declare X A%% X and A are created unconstrained thread %% thread created caseXof[_]then %% thread waits on data-flow {Show triggered1(X)} end end thread caseXof[1]then %% thread waits on data-flow {Show triggered2(X)} end end {Delay 100} X=[A] %% X = [_] only first thread woken {Delay 100} A=1 %% X = [1] second thread woken %% we expect to see ‘triggered1([_])’ before ‘triggered2([1])’

Observations • Concurrency more and more important for networked applications. • With concurrency (threads) you need to synchronize • Two methods • locking (monitors, sentinels etc.) • blocks other threads • threads woken by unlocking (or notify) • natural method when dealing with atomicity in continually changing composite stateful data e.g. critical region in object method application • data-flow • threads blocked on data availability • simpler and more efficient (data vs lock and data) • requires monotonicity • i.e. after thread becomes runnable always runnable, though the thread may suspend later after activation

Concurrent Constraint Programming • Also gives powerful techniques for combinatorial search problems • Not covered in this course (www.mozart-oz.org) • see Chapter 12 of Oz-Tutorial • see Finite Domain Constraint Programming Tutorial • see Part I in Demo Applications

How Mozart/Oz will be presented • Non-stateful non-concurrent subset • Oz without stateful entities, e.g. objects • Stateless data structures • Single-assignment variables • Procedures • Practical programming • OPI - open programming interface • Mozart web site • Concurrency and State • Threads and data-flow synchronization • Objects • Ports (for message sending) • Libraries • Distribution in Mozart • Misc • Mozart VM • Comparison with other platforms/languages for distributed programming • Issues in distributed programming

Numbers • Oz supports integers and floats • Integers have infinite precision • Characters are integers in the range 0..255 with syntactic support • e.g. &t represents the character ‘t’ • Floats are different from integers and must have decimal points. • Other examples of floats are shown where ‘~’ is unary minus: ~3.141 4.5E3 ~12.0e~2 • .

Literals • Literals are divided into atoms and names. • An atom is symbolic entity that has an identity made up of a sequence of alphanumeric characters starting with a lower case letter, or arbitrary printable characters enclosed in quotes. • Example: • a foo '=' ':=' 'OZ 2.0' • 'Hello World’ • Important -sequences that begin with lower case letter are by default atoms • Atoms have an ordering based on lexicographic ordering. • Another category of elementary entities is name. • The only way to create a name is by calling the procedure: • {NewName X} • X is assigned a new name that is guaranteed to be worldwide unique

Boolean and Unit • A subtype of name is bool. • Consists of two names protected from being redefined by having the reserved keywords true and false. • There is also the type unit that consists of the single name unit. • This is used as synchronization token in many concurrent programs. local X Y B in X = foo {NewName Y} B = true {Show [X Y B]} % shows [foo <N> true]end

Records • Records are structured compound entities. • A record has a label and a fixed number of components or arguments. • There are also records with a variable number of arguments that are called open records. • Example: tree(key: I value: Y left: LT right: RT) • It has four arguments. • The label tree. • Each argument consists of a pair Feature:Field. • The features of the record is key, value, left, and right. • The corresponding fields are I, Y, LT, and RT.

Tuples • It is possible to omit the features of a record. • In Oz, this is called a tuple. • Example: • The following tuple has the same label and fields as the record shown before: • tree(I Y LT RT) • It is just a syntactic notation for the record: • tree(1:I 2:Y 3:LT 4:RT)

Lists • The category of lists does not belong to a single data type in Oz. They are rather conceptual structure. • A list is: • either the atom nil representing the empty list, or • is a tuple using the infix operator ´|´ and two arguments which are respectively the head and the tail of the list. 1|2|3|nil • Another convenient special notation for a closed list, i.e. list with determined number of elements is: [1 2 3] • One can also use the standard notation for lists: '|'(1 '|'(2 nil))

Infix Operator ´#´ • Another convenience is the infix operator ´#´. • Used to anonymously group terms a#b b#c#d • One can also use the standard notation : ’#'(a b) ’#'(a b c)

Operations on Records - 1 • To select a field of a record component, we use the infix operator ‘.’ local X Y in X = person(first:per last:brand)Y = person(per brand) {Show X.first} %% shows per {Show Y.2} %% shows brand end

Operations on Records - 2 • The arity of a record is a list of the features of the record sorted lexicographically. local X Y in X = person(last:brand first:per )Y = person(per brand) {Show {Arity X}} %% shows [first last] {Show {Arity Y}} %% shows [1 2] {Show {Label X}} %% shows person end

Strings • Strings are not a primitive type • Further notational variant is allowed for lists whose elements correspond to character codes. Lists written in this notation are called strings, e.g. • "OZ 2.0" • is the list • [79 90 32 50 46 48] • or equivalently • [&O &Z & &2 &. &0]

Virtual Strings • A virtual string is special a tuple that represents a string with virtual concatenation. • Virtual strings are used for I/O with files, sockets, and windows. • All atoms, except nil and #, numbers, strings, and #-labeled tuples can be used to compose virtual strings: 123#"-"#23#" is "#100 • represents the string "123-23 is 100"

Variable Declaration • In Oz computations are performed by a number of sequential threads. • Threads have access to a shared memory called the store. • Threads manipulate the store by reading, and adding information • Information is accessed through the notion of variables. • Oz variables are single-assignment variables. • A single assignment variable: • Initially it is introduced with unknown value, unbound • Later it might be assigned a value, in which case the variable becomes bound. • Once a variable is bound, it cannot be changed

Variable Introduction A thread executing the statement:local X Y Z in S end • Introduces three single assignment variables. • Executes S in the scope of these variables. Another form of declaration is:declare X Y Z inS • Makes X Y and Z visible globally in S,and all statements the follows S textually in the session (file) • Unless overridden again by another variable declaration of the same textual variables.

Binding a Variable local P F N A in P = person(fname:F lname:N add:A) F = per N = brand local T N in A = ’SICS’(town:T nat:N) T = kista N = sweden end end Store P F A N 4 unbound variables

Binding a Variable-2 Store Store local P F N A in P = person(fname:F lname:N add:A) F = per N = brand local T N in A = ’SICS’(town:F nat:N) T = kista C = sweden end end person P fname lname F add N A The record shows itself as a graph in the store

Binding a Variable-3 Store Store local P F N A in P = person(fname:F lname:N add:A) F = per N = brand local T N in A = ’SICS’(town:T nat:N) T = kista N = sweden end end person P fname lname F per add brand A SICS The outer variable N is no longer visible Lexical scoping town nat T N

Binding a Variable-4 Store Store local P F N A in P = person(fname:F lname:N add:A) F = per N = brand local T N in A = ’SICS’(town:T nat:N) T = kista N = sweden end end person P fname lname F per add N brand A SICS The inner variables T N are no longer visible The outer variable N is again visible What happens after the next end - what is visible then?? town nat kista sweden

Equality operator - 1 • The equality infix operator = X = 1 • Binds the unbound variable X to the integer 1. • Add this information to the store. • If X is already assigned the value 1, the operation is a no-op • If X is already bound to an incompatible value, a proper exceptionwill be raised. • local X in • X=1 • X=1 % no-op • end • local X in • X=1 • X=2 % exception • end

Equality operator - 2 • local X Y in • X=Y • X=1 • {Show X#Y} • end • X=Y when X and Y are both unbound • Binds the variables X and Y to each other, unifies them. • Add this information to the store. • If X is assigned the value 1, then Y is also bound to 1. • Show will show 1#1. • local X in • X=1 • X=2 % exception • end

Equality operator - over structures • X=Y • X and Y are unified or mutually constrained • Adds the minimal information to the store to make X and Y equal local X Y A B in X=f(A b) Y=f(a B) end X Y X Y f f f f b a a b a b

Equality operator - over structures • X=Y • If X and Y are non-unifiable or incompatible a proper exceptionwill be raised. • The exception is called Tell failure: • from constraint view we are trying to tell (add) a constraint to the store that would make it inconsistent • in the constraint view there are only two things you do with the store • tell • ask local X Y A B in X=f(A b) Y=f(a c) %% raises %% exception end

Anonymous variables • The syntax _ can be used for creating a single-assignment variable without a handle. • Even without a handle the single-assignment variable can be bound indirectly. local X in X=f(_ _) {Show X} %% shows X=f(_ _) X=f(a _) %% shows X=f(a _) X=f(a b) %% shows X=f(a b) end

Equality Test Operator == • The equality test operator == is the askpartner of the tell equality operator = • The thread executing cannot know the result of the test Y==1 and waits until it can be decided. • Later we shall see how useful this is local X Y in X=1 {Show X==1} %% shows true {Show X==2} %% shows false {Show X==f(a b) %% shows false {Show Y==1} %% doesn’t show anything end

Data Types with Structural Equality • The data types we have introduced so far, records, integers, floats, etc. have structural equality • 5 == 5.0 evaluates to false - floats and integers never equal • f(A B)==f(C D) evaluates to true iff A==B and C==D are true local X Y in {Show f(a b)==f(a b)} %% shows true {Show f(a b)==f(a c)} %% shows false {Show f(a b)==f(c d)) %% shows false {Show f(a _)==f(a b)} %% suspends!!! end

Lecture 1: Introduction to Oz Programming Language