VDM-10 Language Manual

Chapter 11
The State Definition (VDM-SL)
If global variables are desired in a specification, it is possible to make a state definition. The
components of the state definition can be considered the collection of global variables which can
be referenced inside operations. A state in a module is initialised before any of the operation
definitions (using that state) in a module can be used by the interpreter.
Syntax:
state definition = ‘state’, identifier, ‘of’, field list,
[ invariant ], [ initialisation ], ‘end’, [ ‘;’ ] ;
invariant = ‘inv’, invariant initial function ;
initialisation = ‘init’, invariant initial function ;
invariant initial function = pattern, ‘==’, expression ;
Semantics: The state definition has the form:
✞
✡✝
state ident of
id1 : type1
...
idn : typen
inv pat1 == invpred
init pat2 == initpred
end
✆
A state identifier idn is declared of a specific type typen. The invariant invpred is a
boolean expression denoting a property which must hold for the state ident at all times.
initpred denotes a condition which must hold initially. It should be noticed that in order
to use the interpreter, it is necessary to have an initialisation predicate (if any of the operations
using the state are to be executed). In addition the body of this initialisation predicate
must be a binary equality expression with the name (which also must be used as the pattern)
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