VDM-10 Language Manual

VDM-10 Language Manual
state designator = name
| field reference
| map or sequence reference ;
field reference = state designator, ‘.’, identifier ;
map or sequence reference = state designator, ‘(’, expression, ‘)’ ;
multiple assign statement = ‘atomic’, ‘(’ assign statement, ‘;’,
assign statement,
[ { ‘;’, assign statement } ] ‘)’ ;
Semantics: The assignment statement corresponds to a generalisation of assignment statements
from traditional high level programming languages. It is used to change the value of the
global or local state. Thus, the assignment statement has side-effects on the state. However,
in order to be able to simply change a part of the state, the left-hand side of the assignment
can be a state designator. A state designator is either simply the name of a global variable, a
reference to a field of a variable, a map reference of a variable, or a sequence reference of a
variable. In this way it is possible to change the value of a small component of the state. For
example, if a state component is a map, it is possible to change a single entry in the map.
An assignment statement has the form:
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✡✝
sd := ec
where sd is a state designator, and ec is either an expression or a call of an operation. The
assignment statement denotes the change to the given state component described at the righthand
side (expression or operation call). If the right-hand side is a state changing operation
then that operation is executed (with the corresponding side effect) before the assignment is
made.
Multiple assignment is also possible. This has the form:
✞
✡✝
atomic (sd1 := ec1;
...;
sdN := ecN
)
All of the expressions or operation calls on the right hand sides are executed or evaluated,
and then the results are bound to the corresponding state designators. The right-hand sides
are executed atomically with respect to invariant evaluation. However in the case of a multithreaded
concurrent VDM++ or VDM-RT model, execution is not necessarily atomic with
respect to task switching.
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