VDM-10 Language Manual

VDM-10 Language Manual
✡✝
Example : () ==> int
Example () ==
tixe
{ |-> return -1,
|-> DoCommand(),
err |-> return -2 }
in
DoCommand()
✆
In operation DoCommand we use the always statement in the allocation of memory, and all
exceptions raised are captured by the recursive trap statement in operation Example. An
exception with value results in a return value of -1 and no exception raised. If
the value of the exception is we try to perform the operation DoCommand again.
If this raises an exception, this is also handled by the recursive trap statement. All other
exceptions result in the return of the value -2.
13.12 The Error Statement
Syntax: statement = . . .
| error statement
| . . . ;
error statement = ‘error’ ;
Semantics: The error statement corresponds to the undefined expression. It is used to state explicitly
that the result of a statement is undefined and because of this an error has occurred.
When an error statement is evaluated the interpreter will terminate the execution of the specification
and report that an error statement was evaluated.
Pragmatically use of error statements differs from pre-conditions as was the case with undefined
expressions: use of a pre-condition means it is the caller’s responsibility to ensure that
the pre-condition is satisfied when the operation is called; if an error statement is used it is
the called operation’s responsibility to deal with error handling.
Examples: The operation SquareRoot on page 109 does not exclude the possibility that the
number to be square rooted might be negative. We remedy this in the operation SquareRootErr:
✞
SquareRootErr : real * real ==> real
SquareRootErr (r,e) ==
if r < 0
then error
118