[Oberon] Negative integer literals in Oberon

[dave at brownsmeet.com]

Hi Oleg,

Re 1 - Personally I really dislike '90909090H - 100000000H' because it 
is more confusing to read than the alternatives.

Instead of SYSTEM.VAL(INTEGER, 90909090H) you may prefer 
SHORT(90909090H). It's a bit more concise, avoids the dependency on 
SYSTEM, and works whatever the 32 bit integer type is called (i.e. 
INTEGER or LONGINT).

(If you are adding support for LONGINT then you will almost certainly be 
adding support for SHORT() and LONG()).

---

Re 2 - Yes, this fixes 32 bit hex literals. But it doesn't fix 16 or 8 
bit hex literals. I think these should be considered too. It's also 
difficult to explain the use of the H vs L suffix - the description in 
the component pascal documentation is correct but very confusing.

---

Re 3 - Yes, in this case (assuming you have added support for 64 bit 
integers) all arithmetic is 64 bit, and assignments to smaller int 
variables are simply clipped at the time they are stored. You don't need 
SHORT() or LONG().

If you do this I'd like to see a compilation option to do range checking 
at run time on every store to < 64 bits.

---

Re 4 - Oberon already has casting under another name (type guard) for 
extended types - with the type name in  brackets following the variable 
name. Maybe keep the syntax similar.


-- Dave.


On 2020-05-01 11:26, Oleg N. Cher wrote:
> Dear Dave,
> 
> After all, the question: is the literal 90909090H considered 64-bit
> unsigned or 32-bit signed? We have four solutions:
> 
> 1. Consider it as a 64-bit literal, by context. And if we need to
> declare it as a 32-bit literal, we'll write it like this:
> 
>   myint := 90909090H - 100000000H;
> 
> If we don't like writing code this way, we'll write it differently:
> 
>   myint := SYSTEM.VAL(INTEGER, 90909090H);
> 
> , quite in the spirit of the Oberon paradigm. So, Artur, this is what
> I suggest you do in your compiler.
> 
> 2. Consider it as a 32-bit literal. And for 64-bit use the postfix
> modifier like 0FFFFFFFFL, as in Component Pascal. Ofront+ also
> implements this option (in -C mode).
> 
> 3. Pretend that we have all integer literals of the same size. In this
> case, all the hexadecimal digits allowed as a bitfield value of the
> literal. This way is implemented in most Oberon-07 compilers, which is
> justified by the lack of integer arithmetic of different bits in them.
> (BYTE is a non-arithmeric type in Oberon-07, and its size does not
> used in operations, always casted to INTEGER).
> 
> 4. Explicitly specifying the constant type, as there was in Turbo 
> Pascal:
> 
> CONST MyConst = INTEGER(90909090H);
> 
> As in Active Oberon, Delphi and Turbo Pascal. But then it is necessary
> to allow the same (note, non-system) casting in expressions - and
> rushed.
> 
> 
> I would suggest two ways. The non-system way:
> 
>   CONST MyConst = 90909090H - 100000000H;
> 
> This will be our payment for the fact that there are no unsigned types
> in Oberon. This is a simplification that I think is very useful, but
> has a cost. So now we have all integer constants as signed numbers.
> 
> And the system way. Ofront+ supports:
> 
>   myint := SYSTEM.VAL(INTEGER, 90909090H);
> 
> And in constants too:
> 
>   CONST MyConst = SYSTEM.VAL(INTEGER, 90909090H);
> 
> The disadvantage here is the compatibility problem, because old
> sources may contain literals that rely on the size of 32 bits. But
> this is mitigated by the fact that Ofront+ supports 5 Oberon dialects,
> and we can choose any of it at our discretion.
>