unit

#unit

##Syntax A compilationUnit is one of:

(a) [ importList ] mainProgram (b) unit moduleDeclaration (c) unit monitorDeclaration (d) unit classDeclaration

##Description A program can be divided up into units, each in a separate file. All of these files except the main program begin with the keyword unit. The unit contains the main program, a module, a monitor or a class.

##Example Here is stack module that is separated out into a file whose name is stack.

The main program, which is in another file, gains access to the stack by importing it. Here is the main program:

    unit        % The keyword unit begins each separate file
    module stack
        export push, pop
    
        var top : int := 0
        var contents : array 1 .. 100 of int
    
        procedure push ( i : int )
            top += 1
            contents ( top ) := i
        end push
    
        procedure pop ( i : int )
            i := contents ( top )
            top -= 1
        end pop
    end stack        import var stack    % Use the stack
    var n : int
    �
    stack . push ( n )
    �
    stack . pop ( n )

##Details In this example, the keyword var in the import list is required because the main program causes a change in the stack, by calling push and pop. The import lists of units that are modules, monitors and classes are used to gain access to further units.

If the stack were in a file with a different name, say stk.t, the import list would be rewritten to use an in clause, as follows:

A mainProgram is simply a program. See program.

    import var stack in "stk.t"

##See also module.html, monitor.html and class.html. See also export.html list, import.html list, inherit.html list, implement.html list and [implement_by.html](implement by) list.