VDM-10 Language Manual

Chapter 3. Data Type Definitions
✡✝
cons : Expr
altn : Expr
✆
and let expr = mk Cond(mk Var("b",),mk Const(3),
mk Var("v",nil)) then:
is Cond(expr) ≡ true
is Const(expr.cons) ≡ true
is Var(expr.altn) ≡ true
is Infix(expr.test) ≡ false
Using union types we can extend the use of previously defined operators. For instance,
interpreting = as a test over bool | nat we have
1 = false ≡ false
Similarly we can take use union types for taking unions of sets and concatenating sequences:
{1,2} union {false,true} ≡ {1,2, false,true}
[’a’,’b’]ˆ[,] ≡ [’a’,’b’, ,]
In the set union, we take the union over sets of type nat | bool; for the sequence concatenation
we are manipulating sequences of type char | | .
3.2.7 The Object Reference Type (VDM++ and VDM-RT only)
The object reference type has been added as part of the standard VDM-SL types. Therefore there
is no direct way of restricting the use of object reference types (and thus of objects) in a way that
conforms to pure object oriented principles; no additional structuring mechanisms than classes are
foreseen. From these principles it follows that the use of an object reference type in combination
with a type constructor (record, map, set, etc.) should be treated with caution.
A value of the object reference type can be regarded as a reference to an object. If, for example,
an instance variable (see section 10) is defined to be of this type, this makes the class in which that
instance variable is defined, a ‘client’ of the class in the object reference type; a clientship relation
is established between the two classes.
An object reference type is denoted by a class name. The class name in the object reference
type must be the name of a class defined in the specification.
The only operators defined for values of this type is the test for equality (‘=’) and inequality
(‘’). Equality is based on references rather than values. That is, if o1 and o2 are two distinct
objects which happen to have the same contents, o1 = o2 will yield false.
Constructors Object references are constructed using the new expression (see section 6.13).
Operators
Operator Name Type
t1 = t2 Equality A * A → bool
t1 t2 Inequality A * A → bool
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