Eberon Implicit Type Narrowing

Original Oberon-07 CASE with type guards has notorious loophole. Implicit type narrowing is introduced as alternative solution without possible type violation problem.

The idea of implicit type narrowing is to make the compiler smart enough to comprehend that type testing (using IS) and following IF branch or logical conjunction (&) in expression narrows just tested variable type. The same type narrowing happens in the inverse case: logical not (~) for IS operation and following ELSE branch or logical disjunction (OR). Also compiler should guarantee that tested variable is not modified after type narrowing so there is no loopholes to corrupt type system by chance. That guarantee is easy to reach in case of In Place Variables because their scope is very local and they cannot be modified by local procedures. Type narrowing is also appling to procedure arguments because they cannot be modified. So if the example will use pb variable as in place variable then it will be compiled without addition type casts:

VAR
    pbVar: PBase;
BEGIN
    pb <- pbVar;
    IF pb IS PDerived THEN
        pb.derivedField := 123;
    END;

    ASSERT(~(pb IS PDerived) OR (pb.derivedField = 123));
END.

Type narrowing is also applies to Ternary operator, WHILE statement and its ELSIF branches.

Generic Message Bus

Implicit type narrowing is also used for VAR arguments to support generic message bus pattern.

TYPE
    Message = RECORD END;
    Derived1 = RECORD (Message) derivedField1: BOOLEAN END;
    Derived2 = RECORD (Message) derivedField2: BOOLEAN END;

PROCEDURE handleMessage(VAR msg: Message);
BEGIN
    IF msg IS Derived1 THEN
        ASSERT(msg.derivedField1);
    ELSIF msg IS Derived2 THEN
        ASSERT(msg.derivedField2);
    END;
END handleMessage;