VDM-10 Language Manual

Chapter 3
Data Type Definitions
As in traditional programming languages it is possible to define data types in the VDM languages
and give them appropriate names. Such an equation might look like:
✞
types
✡✝
Amount = nat
✆
Here we have defined a data type with the name “Amount” and stated that the values which
belong to this type are natural numbers (nat is one of the basic types described below). One
general point about the type system of the VDM languages which is worth mentioning at this point
is that equality and inequality can be used between any value. In programming languages it is
often required that the operands have the same type. Because of a construct called a union type
(described below) this is not the case for the VDM languages.
In this section we will present the syntax of data type definitions. In addition, we will show how
values belonging to a type can be constructed and manipulated (by means of built-in operators).
We will present the basic data types first and then we will proceed with the compound types.
3.1 Basic Data Types
In the following a number of basic types will be presented. Each of them will contain:
• Name of the construct.
• Symbol for the construct.
• Special values belonging to the data type.
• Built-in operators for values belonging to the type.
• Semantics of the built-in operators.
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