VDM-10 Language Manual

Chapter 13. Statements
result of the complete recursive trap statement. Otherwise, the value is matched in order
against each of the patterns pati. When a match cannot be found, the exception is returned
as the result of the recursive trap statement. If a match is found, the corresponding statement
si is evaluated. If this does not raise an exception, the result value of the evaluation of si is
returned as the result of the recursive trap statement. Otherwise, the matching starts again,
now with the new exception value (the result of the evaluation of si).
Examples: In many programs, we need to allocate memory for a single operation. After the
operation is completed, the memory is not needed anymore. This can be done with the
always statement:
✞
✡✝
( dcl mem : Memory;
always Free(mem) in
( mem := Allocate();
Command(mem, ...)
)
)
In the above example, we cannot act upon a possible exception raised within the body statement
of the always statement. By using the trap statement we can catch these exceptions:
✞
✡✝
trap pat with ErrorAction(pat) in
( dcl mem : Memory;
always Free(mem) in
( mem := Allocate();
Command(mem, ...)
)
)
Now all exceptions raised within the always statement are captured by the trap statement.
If we want to distinguish between several exception values, we can use either nested trap
statements or the recursive trap statement:
✞
DoCommand : () ==> int
DoCommand () ==
( dcl mem : Memory;
always Free(mem) in
( mem := Allocate();
Command(mem, ...)
)
);
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