7.3.2 Object Style

We now change from the Oz library style to that of object oriented programming. Cell objects are modelled by records of procedures that give access to a hidden library cell. The type of a cell object is:

unit(get     : -> value
     put     : value -> 
     exchange: value -> value
     ...)

Cell objects can be created by calling a creation function NewCellObject (the cell's class) while passing the initial value.

<Cell.oz>=

functor 
export 
   new:NewCellObject
define 
   fun{NewCellObject Init}
      C = {Cell.new Init}
      fun {Get} {Access C} end 
      proc{Put Value} {Assign C Value} end 
   in 
      unit(get:Get
           put:Put
           exchange:fun{$ New}  
                       Old
                    in  
                       {Exchange C Old New}  
                       Old  
                    end  
           access:Get
           assign:Put)    
   end 
end

This functor defines a module which exports the function NewCellObject at feature new.

The procedures of cell objects are defined by reduction to the corresponding library functions. Essentially, they do nothing else than applying the library functions to the concrete hidden cell.

But note that we also did some more changes and less systematic chages for our convenience. We turn the binary library procedure Cell.exchange into a function Exchange that inputs a value to be put into the cell and outputs the old of the cell.

Furthermore, we have also adapted the name of the functions for convenience. We use put and get as usual in the library rather than Access and Assign. Finally, we have turned Exchange into a function that inputs the new value and returns the old one. Note that the input argument New is often left variable in applications. We can now use bags by linking a compiled version of the module Bag.ozf that is made available at the following URL:

<URL>=

AuthorURL   = 'http://www.ps.uni-sb.de/~niehren' 
CourseDir   = '/Web/Vorlesungen/Oz-NL-SS01' 
CourseURL   = AuthorURL#CourseDir
PickleVersion= 'Version.3.2' 
DataDir      = '/vorlesung/Functors/'#PickleVersion
URL          = CourseURL#DataDir

This allows you to test the above implementation of cell objects.

<Test Cell.oz>=

declare <URL> 
declare CellURL = URL#'/Cell.ozf' 
declare [CellMod] = {Module.link [CellURL]}
declare C={CellMod.new 2002}
{Inspect {C.get}}
{C.put 4711}
{Inspect {C.put}}
declare NewVal OldVal = {C.exchange NewVal}
{Inspect OldVal}  
NewVal=OldVal-42
{Inspect {C.get}}