Oz (programming language)
Oz is a multiparadigm programming language, developed in the Programming Systems Lab at Université catholique de Louvain, for programming language education. It has a canonical textbook: Concepts, Techniques, and Models of Computer Programming.
Oz was first designed by Gert Smolka and his students in 1991. In 1996, development of Oz continued in cooperation with the research group of Seif Haridi and Peter Van Roy at the Swedish Institute of Computer Science. Since 1999, Oz has been continually developed by an international group, the Mozart Consortium, which originally consisted of Saarland University, the Swedish Institute of Computer Science, and the Université catholique de Louvain. In 2005, the responsibility for managing Mozart development was transferred to a core group, the Mozart Board, with the express purpose of opening Mozart development to a larger community.
The Mozart Programming System is the primary implementation of Oz. It is released with an open source license by the Mozart Consortium. Mozart has been ported to Unix, FreeBSD, Linux, Windows, and macOS.
Language features
Oz contains most of the concepts of the major programming paradigms, including logic, functional, imperative, object-oriented, constraint, distributed, and concurrent programming. Oz has both a simple formal semantics and Oz is a concurrency-oriented language, as the term was introduced by Joe Armstrong, the main designer of the Erlang language. A concurrency-oriented language makes concurrency easy to use and efficient. Oz supports a canonical graphical user interface language QTk.In addition to multi-paradigm programming, the major strengths of Oz are in constraint programming and distributed programming. Due to its factored design, Oz is able to successfully implement a network-transparent distributed programming model. This model makes it easy to program open, fault-tolerant applications within the language. For constraint programming, Oz introduces the idea of computation spaces, which allow user-defined search and distribution strategies orthogonal to the constraint domain.
Language overview
Data structures
Oz is based on a core language with very few datatypes that can be extended into more practical ones through syntactic sugar.Basic data structures:
- Numbers: floating point or integer
- Records: for grouping data :
circle. Here the terms x,y, radius etc. are called features and the data associated with the features are the values. - Tuples: Records with integer features in ascending order:
circle. - Lists: a simple linear structure
'|') % as a record.
2| % with some syntactic sugar
2|4|6|8|nil % more syntactic sugar
% even more syntactic sugar
Those data structures are values, first class and dynamically type checked. Variable names in Oz start with an uppercase letter to distinguish them from literals which always begin with a lowercase letter.
Functions
Functions are first class values, allowing higher order functional programming:fun
if N =< 0 then 1 else N* end
end
fun
div % integers can't overflow in Oz
end
fun
case List of nil then 0
H|T then H+ % pattern matching on lists
end
end
Functions may be used with both free and bound variables. Free variable values are found using static lexical scoping.
Higher-order programming
Functions are like other Oz objects. A function can be passed as an attribute to other functions or can be returned in a function.fun % A general function
N*N
end
fun % F is a function here - higher order programming
case Xs
of nil then nil
X|Xr then |
end
end
%usage
%browses
Anonymous functions
Like many other functional languages, Oz supports use of anonymous functions with higher order programming. The symbol $ is used to denote these.In the following, the square function is defined anonymously and passed, causing
Since anonymous functions don't have names, it is not possible to define recursive anonymous functions.
Procedures
Functions in Oz are supposed to return a value at the last statement encountered in the body of the function during its execution. In the example below, the function Ret returns 5 if X > 0 and -5 otherwise.declare
fun
if X > 0 then 5 else ~5 end
end
But Oz also provides a facility in case a function must not return values. Such functions are called procedures. Procedures are defined using the construct "proc" as follows
declare
proc
if X > 0 then else end
end
The above example doesn't return any value, it just prints 5 or -5 in the Oz browser depending on the sign of X.
Dataflow variables and declarative concurrency
When the program encounters an unbound variable it waits for a value. For example, below, the thread will wait until both X and Y are bound to a value before showing the value of Z.thread
Z = X+Y
end
thread X = 40 end
thread Y = 2 end
The value of a dataflow variable cannot be changed once it is bound:
X = 1
X = 2 % error
Dataflow variables make it easy to create concurrent stream agents:
fun
if N Max then nil
else
N|
end
end
fun
case Stream
of nil then S
H|T then S|
end
end
local X Y in
thread X = end
thread Y = end
end
Because of the way dataflow variables work, it is possible to put threads anywhere in a program and guaranteed that it will have the same result. This makes concurrent programming very easy. Threads are very cheap: it is possible to have 100,000 threads running at once.
Example: Trial division sieve
This example computes a stream of prime numbers using the trial division algorithm by recursively creating concurrent stream agents that filter out non-prime numbers:fun
case Xs of nil then nil
X|Xr then Ys in
thread Ys = end
X|
end
end
Laziness
Oz uses eager evaluation by default, but lazy evaluation is possible. Below, the fact is only computed when value of X is needed to compute the value of Y.fun lazy
if N =< 0 then 1 else N* end
end
local X Y in
X =
Y = X + 1
end
lazy evaluation gives the possibility of storing truly infinite data structures in Oz. The power of lazy evaluation can be seen from the following code sample:
declare
fun lazy
case Xs#Ys
of # then
if X < Y then X|
elseif X>Y then Y|
else X|
end
end
end
fun lazy
case Xs
of nil then nil
X|Xr then N*X|
end
end
declare H
H = 1 |
The code above elegantly computes all the Regular Numbers in an infinite list. The actual numbers are computed only when they are needed.
Message passing concurrency
The declarative concurrent model can be extended with message passing via simple semantics:declare
local Stream Port in
Port =
% Stream is now 1|_
% Stream is now 1|2|_
...
% Stream is now 1|2|.. |n|_
end
With a port and a thread, asynchronous agents can be defined:
fun
Msg Out in
thread end
end
State and objects
It is again possible to extend the declarative model to support state and object-oriented programming with very simple semantics. To create a new mutable data structure called Cells:local A X in
A =
A := 1 % changes the value of A to 1
X = @A % @ is used to access the value of A
end
With these simple semantic changes, the whole object-oriented paradigm can be supported. With a little syntactic sugar, OOP becomes well integrated in Oz.
class Counter
attr val
meth init
val:=Value
end
meth browse
end
meth inc
val :=@val+Value
end
end
local C in
C =
end