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

[thutt at harp-project.com]

Hans Klaver writes:
 > 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).

EXCL is a built-in procedure.  It doesn't return a value, but updates
the first argument directly.  So, because the compiler must store the
result, the first argument must be a variable.

You can do what you want using the set operators in an expression.

--
Pickled beets.  Pickled beaks.  What's the difference?