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

[Hans Klaver]

Hi all,

While trying to make an Oberon-07 version of the classic predeclared function CAP I noticed some differences in the behaviour of the two compilers I use regularly.

One compiler can compile this:

  PROCEDURE CAP1 (ch: CHAR): CHAR;
  (* 
    Capitalizes ASCII lower-case letters while leaving capital letters unchanged. 
    Beware: digits and other characters *are* changed.
  *)
    VAR chSet: SET;
  BEGIN 
    chSet := SYSTEM.VAL(SET, ch);  (* cast CHAR to SET *)
    EXCL(chSet, 5)                 (* clear bit 5 *)       
  RETURN SYSTEM.VAL(CHAR, chSet)   (* cast SET back to CHAR *)
  END CAP1;

The other compiler does not swallow the above version (casting of CHAR to SET not allowed), but does compile the following:

  PROCEDURE CAP2 (ch: CHAR): CHAR;
  (* 
    Same semantics as CAP1.
  *)
    VAR i: INTEGER;
  BEGIN
    i := SYSTEM.VAL(BYTE, ch);     (* cast CHAR to BYTE and assign to INTEGER *)
    EXCL(SYSTEM.VAL(SET, i), 5)    (* clear bit 5 *)
  RETURN SYSTEM.VAL(CHAR, i)       (* cast INTEGER directly back to CHAR *)
  END CAP2;

The interesting thing is that the first compiler compiles CAP1 but does not compile CAP2 (its EXCL procedure always needs a variable as first parameter).

It would be nice (imho) to have a compiler that combines the behaviours of the two, i.e. accepts a cast of CHAR to SET, and doesn't need a variable as the first parameter of EXCL. Then it would have no trouble with the next simplest version of CAP:

  PROCEDURE CAP0 (ch: CHAR): CHAR;
  (*
    Neither compilers can cope with this one.
  *)
  BEGIN
    EXCL(SYSTEM.VAL(SET, ch), 5)   (* cast CHAR to SET and clear bit 5 *)
  RETURN ch
  END CAP0;


My questions are: 
- the behaviour of which of the two above mentioned existing compilers do you like best?
- do you know an Oberon-07 compiler that can compile CAP0, and what do you think of it?

Regards,

Hans Klaver