VDM-10 Language Manual

VDM-10 Language Manual
The synchronization definition blocks of the class description provide the user with ways to override
the defaults described above.
Syntax: synchronization definitions = ‘sync’, [ synchronization ] ;
synchronization = permission predicates ;
Semantics: Synchronization is specified in VDM++ and VDM-RT using permission predicates.
15.1 Permission Predicates
The following gives the syntax used to state rules for accepting the execution of concurrently
callable operations. Some notes are given explaining these features.
Syntax: permission predicates = permission predicate, { ‘;’,
permission predicate } ;
permission predicate = ‘per’, name, ‘=>’, expression
| mutex predicate ;
mutex predicate = ‘mutex’, ‘(’, ‘all’, ‘)’
| ‘mutex’, ‘(’, name list ‘)’ ;
Semantics: Permission to accept execution of a requested operation depends on a guard condition
in a (deontic) permission predicate of the form:
per operation name => guard condition
The use of implication to express the permission means that truth of the guard condition (expression)
is a necessary but not sufficient condition for the invocation. The permission predicate
is to be read as stating that if the guard condition is false then there is non-permission.
Expressing the permission in this way allows further similar constraints to be added without
risk of contradiction through inheritance for the subclasses. There is a default for all
operations:
per operation name => true
but when a permission predicate for an operation is specified this default is overridden.
Guard conditions can be conceptually divided into:
• a history guard defining the dependence on events in the past;
• an object state guard, which depends on the instance variables of the object, and
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