VDM-10 Language Manual

VDM-10 Language Manual
(permutation is an auxiliary function taking two sequences which returns true iff one
sequence is a permutation of the other.)
13.16 The Duration Statement (VDM-RT)
Syntax: statement = . . .
| duration statement ;
duration statement = ‘duration’, ‘(’, numeral, ‘)’,
statement ;
Semantics: The duration statement is a runtime directive to the Toolbox interpreter telling it that
when incrementing the internal clock for the enclosed statement, the value (a natural number)
given in the duration statement should be used instead of the increment which would
normally be computed for that statement. Thus the duration statement provides a mechanism
to override the Toolbox’s default execution time computation.
Example: First a simple example:
✞
✡✝
while n < 10 do
duration(10) n := n + 1;
✆
In this example, assuming that this loop is not executed in the context of an enclosing duration
statement, on each iteration of the loop the Toolbox will increment its internal clock by
10 time units, rather than computing the amount of time required to execute the statement n
:= n + 1.
If duration statements are nested, the outermost one takes precedence and the remainder are
ignored. For instance
✞
✡✝
duration(30)
( n := 1;
while n < 10 do
duration(10) n := n + 1;
)
The outer duration statement takes precedence, so assuming this is not executed in the context
of an enclosing duration statement, the interpreter would increment its internal clock by
30 time units when executing this statement.
Note that nesting can occur due to operation calls. Consider the following example:
122
✆