VDM-10 Language Manual

Chapter 13. Statements
where expression e is the return value of the operation.
Examples: In the following example OpCall is an operation call whereas FunCall is a function
call. As the if statement only accepts statements in the two branches FunCall is
“converted” to a statement by using the return statement.
✞
✡✝
if test
then OpCall()
else return FunCall()
For instance in VDM++, we can extend the stack class from the previous section with an
operation which examines the top of the stack:
✞
✡✝
public Top : () ==> Elem
Top() ==
return (hd stack);
✆
✆
13.11 Exception Handling Statements
Syntax: statement = . . .
| always statement
| trap statement
| recursive trap statement
| exit statement
| . . . ;
always statement = ‘always’, statement, ‘in’, statement ;
trap statement = ‘trap’, pattern bind, ‘with’, statement, ‘in’,
statement ;
recursive trap statement = ‘tixe’, traps, ‘in’, statement ;
traps = ‘{’, pattern bind, ‘|->’, statement,
{ ‘,’, pattern bind, ‘|->’, statement }, ‘}’ ;
exit statement = ‘exit’, [ expression ] ;
Semantics: The exception handling statements are used to control exception errors in a specification.
This means that we have to be able to signal an exception within a specification. This
can be done with the exit statement, and has the form:
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