Oz (programming language)

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 (real integer)
• Records: for grouping data : circle(x:0 y:1 radius:3 color:blue style:dots). Here the terms x,y, radius etc. are called features and the data associated with the features (in this case 0,1,3 etc.) are the values.
• Tuples: Records with integer features in ascending order: circle(1:0 2:1 3:3 4:blue 5:dots) .
• Lists: a simple linear structure

'|'(2 '|'(4 '|'(6 '|'(8 nil)))) % as a record.
2|(4|(6|(8|nil))) % with some syntactic sugar
2|4|6|8|nil % more syntactic sugar
[2 4 6 8] % even more syntactic sugar

Those data structures are values (constant), first class and dynamically type checked. Variable names in Oz start with an uppercase letter to distinguish them from literals[4] which always begin with a lowercase letter.

Functions[5] are first class values, allowing higher order functional programming:

fun {Fact N}
   if N =< 0 then 1 else N*{Fact N-1} end
end

fun {Comb N K}
   {Fact N} div ({Fact K} * {Fact N-K}) % integers can't overflow in Oz (unless no memory is left)
end

fun {SumList List}
   case List of nil then 0
   [] H|T then H+{SumList T} % 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.[6]

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 {Square N}  % A general function
   N*N
end

fun {Map F Xs}  % F is a function here - higher order programming
   case Xs
      of nil then nil
      [] X|Xr then {F X}|{Map F Xr}
   end
end

%usage
{Browse {Map Square [1 2 3]}}  %browses [1 4 9]

Like many other functional languages, Oz supports use of anonymous functions (i.e. functions which do not have a name) with higher order programming. The symbol $ is used to denote these.

In the following, the square function is defined anonymously and passed, causing [1 4 9] to be browsed.

{Browse {Map fun {$ N} N*N end [1 2 3]}}

Since anonymous functions don't have names, it is not possible to define recursive anonymous functions.

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 {Ret X}
   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.[7] Procedures are defined using the construct "proc" as follows

declare
proc {Ret X}
   if X > 0 then {Browse 5} else {Browse ~5} 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.

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
   {Browse Z}
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 {Ints N Max}
   if N == Max then nil
   else 
      {Delay 1000}
      N|{Ints N+1 Max}
   end
end

fun {Sum S Stream}
   case Stream
      of nil then S
      [] H|T then S|{Sum H+S T}
   end
end

local X Y in
   thread X = {Ints 0 1000} end
   thread Y = {Sum 0 X} end
   {Browse Y}
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.[8]

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 {Sieve Xs}
   case Xs of nil then nil
   [] X|Xr then Ys in
      thread Ys = {Filter Xr fun {$ Y} Y mod X \= 0 end} end
      X|{Sieve Ys}
   end
end

Oz uses eager evaluation by default, but lazy evaluation[9] is possible. Below, the fact is only computed when value of X is needed to compute the value of Y.

fun lazy {Fact N}
   if N =< 0 then 1 else N*{Fact N-1} end
end
local X Y in
  X = {Fact 100} 
  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 {Merge Xs Ys}
   case Xs#Ys
   of (X|Xr)#(Y|Yr) then
      if X < Y then X|{Merge Xr Ys}
      elseif X>Y then Y|{Merge Xs Yr}
      else X|{Merge Xr Yr}
      end
   end
end

fun lazy {Times N Xs}
   case Xs
   of nil then nil
   [] X|Xr then N*X|{Times N Xr}
   end
end

declare H
H = 1 | {Merge {Times 2 H} {Merge {Times 3 H} {Times 5 H}}}
{Browse {List.take H 6}}

The code above elegantly computes all the Regular Numbers[10] in an infinite list. The actual numbers are computed only when they are needed.

The declarative concurrent model can be extended with message passing via simple semantics:

declare
local Stream Port in
   Port = {NewPort Stream}
   {Send Port 1} % Stream is now 1|_ ('_' indicates an unbound and unnamed variable)
   {Send Port 2} % Stream is now 1|2|_ 
   ...
   {Send Port n} % Stream is now 1|2| .. |n|_
end

With a port and a thread, asynchronous agents can be defined:

fun {NewAgent Init Fun}
   Msg Out in
   thread {FoldL Msg Fun Init Out} end
   {NewPort Msg}
end

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 = {NewCell 0}
   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(Value)
      val:=Value
   end
   meth browse
      {Browse @val}
   end
   meth inc(Value)
      val :=@val+Value
   end
end

local C in
   C = {New Counter init(0)}
   {C inc(6)}
   {C browse}
end