🚀 go-pugleaf

A. Borchert writes:
> Here an example, following program is perfectly legal Oberon text
> but rejected by the Sparc Oberon System from ETH Zurich:
> 
> (* CLASS=CONFORMANCE *)
> MODULE t03;
>    TYPE c = RECORD a, b: INTEGER END;
>    PROCEDURE e;
>       TYPE b = POINTER TO c;
>          c = ARRAY 3 OF CHAR;
>       VAR p: b;
>    BEGIN (* e *)
>       p^[1] := "x";
>    END e;
> END t03.
> The compiler complains at the designator >>p^[1]<< that
> "indexed variable is not an array".

Section 4 (Declarations and Scope rules) of the Oberon-2 Report:
  The scope of an object x extends textually from the point of its declaration 
  to the end of the block (module, procedure, or record) to which the 
  declaration belongs and hence to which the object is local.

and in the same section:
  A type T of the form POINTER TO T1 (see 6.4) can be declared before
  the scope of T1. In this case, the declaration of T1 must follow in
  the same block to which T is local;

I think this is an ambiguity in the language definition.
The forward declaration rule should be explicitely restricted to
the cases where T1 is not visible at the point of the forward declaration.
Anyway, the intention is that the above program is neither legal
Oberon nor legal Oberon-2.


> MODULE RecAssignment;
>    TYPE T0 = RECORD END;
>         T1 = RECORD (T0) END;
>    VAR t1a, t1b: T1;
>    PROCEDURE AssignIt(VAR ta, tb: T0);
>    BEGIN
>       ta := tb; (* statically the same type and ... *)
>    END AssignIt;
> BEGIN
>    AssignIt(t1a, t1b); (* ... dynamically the same type *)
> END RecAssignment;
> 
> is perfectly legal in Oberon but not in Oberon-2. But, of course,
> this feature was never implemented in ETH compilers (tell me if
> I'm wrong here).

Both Oberon and Oberon-2 compilers will compile the above module.
The context condition "Te and Tv are record types and Te is an extension
of Tv and the dynamic type of v is Tv" cannot be fully checked at compile time
but compiles into a runtime check in both Oberon and Oberon-2.
The original Oberon-Report does not mention implicit runtime
checks (c.f. array indices, ...).

- Josef Templ
the usual disclaimer applies