[Oberon] Bit-fiddling: SETs and type casts in Oberon-07

[Hans Klaver]

Chris, Jörg and Florian,

Thanks for your answers. Now it makes more sense to me.

I looked into these matters a bit more. 

It appears that Wirth's OR compiler always allows casts of 'scalar' types (here meaning all Oberon basic types) with SYSTEM.VAL to a type of the same or smaller SYSTEM.SIZE (narrowing casts), without warning. Widening casts (to a type with larger SYSTEM.SIZE) are only possible via cast to BYTE and INTEGER. 

I suppose this is (as Florian wrote) because widening casts lead to 'undefined behaviour'. Well, I think the behaviour in cases of SYSTEM.VAL(SET, v) is only partially undefined: the part of the SET one is interested in (elements 0 .. 8*SYSTEM.SIZE(v)-1) is quite well defined as long as it is guaranteed that element 0 of the SET is the least significant bit of the variable; the undefined part, the elements > 8*SYSTEM.SIZE(v)-1, should just be disregarded.

The other two Oberon-07 compilers I checked (Astrobe for Cortex-M3 and OBNC) allow all casts of the basic types to and from each other, narrowing or widening; the Astrobe compiler gives a warning 'type cast !' with any use of SYSTEM.VAL.

Chris wrote:

> The Oberon-07 report states:
> 
> EXCL(v, x) is equivalent to v := v - {x}.  (* where v is a variable not an expression *)
> 
> Hence trying to write EXCL(SYSTEM.VAL(SET, ch), 5) would be equivalent to writing:
> 
> SYSTEM.VAL(SET, ch) := SYSTEM.VAL(SET, ch) - {5} 
> 
> To me, that is as wrong as trying to write something like:
> 
> ORD(ch) := ORD(ch) + 1
> 
> or INC(ORD(ch))


Concerning this I found out that only OBNC requires a variable as first parameter of EXCL and INCL, and disallows SYSTEM.VAL in its place; Wirth's OR and the Astrobe compiler have no problem with SYSTEM.VAL as first parameter of these procedures. So for instance they accept the following (Astrobe with a warning 'type cast !'):
  
  VAR i: INTEGER;
  ...
  EXCL(SYSTEM.VAL(SET, i), 5)
  RETURN i

This is in accordance with the language report: there it says that SYSTEM.VAL has the function of 'identity' (earlier described as 'x interpreted as of type T'); use of this function here can be equated with use of a variable.

So I think that OBNC is a bit too restrictive in disallowing SYSTEM.VAL as first parameter of EXCL and INCL.

Regards,

Hans