6.1. Declaring and throwing🔗

throws T in a procedure's signature says the procedure may finish by throwing a T. In the body, throw e raises e and abandons the rest of the procedure.

composite Exception {}
composite ArithmeticException extends Exception {}

procedure div(a: int, b: int) returns (r: int)
  throws (e: Exception)
  opaque
{
  if b == 0 then {
    var ae: ArithmeticException := new ArithmeticException;
    throw ae
  };
  r := a / b
};

throws is part of the signature — it changes what callers have to deal with — so it sits with returns, before opaque.