VDM-10 Language Manual

Chapter 17. Top-level Specification (VDM-SL)
17.2 A Structured Specification
As an extension to the standard VDM-SL language, it is possible to structure an VDM-SL specification
using modules. In this section, the use of modules to create the top-level specification will
be described. With the structuring facilities offered by VDM-SL it is possible to:
• Export constructs from a module.
• Import constructs from a module.
• Rename constructs upon import.
• Define a state in a module.
17.2.1 The Layout of a Module
Before the actual facilities are described, the general layout of a module is described. A module
consists of three parts: a module declaration, an interface section, and a definitions section. It is
possible to leave out the definitions part in the early development of a module specification.
In the module declaration, the module is named. The name must be a unique module name
within the complete specification. The second part, the interface section, defines the relation of a
module with other modules and consists of a number of sections. These sections are:
• An imports section. In the imports section, all the constructs that are going to be used from
other modules are described. If constructs are going to be renamed it has to be done in the
imports section.
• An exports section. Here all the constructs that are going to be used in other modules are
defined. If no exports section is present the module cannot be used from other modules.
The third part of a module declaration, the definitions section, contains all the definitions of the
module. Thus, in general, the syntax of a module is:
Syntax:
module = ‘module’, identifier, interface,
[ module body ], ‘end’, identifier ;
module body = ‘definitions’, definition block, { definition block } ;
To illustrate the use of modules, the example flat top-level specification are rewritten with some
minor modifications. Some unimportant parts of the flat specification are left out for clarity.
153