Don wrote:
Andrei Alexandrescu wrote:
Don wrote:
Andrei Alexandrescu wrote:
One fear of mine is the reaction of throwing of hands in the air
"how many integral types are enough???". However, if we're to judge
by the addition of long long and a slew of typedefs to C99 and
C++0x, the answer is "plenty". I'd be interested in gaging how
people feel about adding two (bits64, bits32) or even four (bits64,
bits32, bits16, and bits8) types as basic types. They'd be bitbags
with undecided sign ready to be converted to their counterparts of
decided sign.
Here I think we have a fundamental disagreement: what is an 'unsigned
int'? There are two disparate ideas:
(A) You think that it is an approximation to a natural number, ie, a
'positive int'.
(B) I think that it is a 'number with NO sign'; that is, the sign
depends on context. It may, for example, be part of a larger number.
Thus, I largely agree with the C behaviour -- once you have an
unsigned in a calculation, it's up to the programmer to provide an
interpretation.
Unfortunately, the two concepts are mashed together in C-family
languages. (B) is the concept supported by the language typing rules,
but usage of (A) is widespread in practice.
In fact we are in agreement. C tries to make it usable as both, and
partially succeeds by having very lax conversions in all directions.
This leads to the occasional puzzling behaviors. I do *want* uint to
be an approximation of a natural number, while acknowledging that
today it isn't much of that.
If we were going to introduce a slew of new types, I'd want them to
be for 'positive int'/'natural int', 'positive byte', etc.
Natural int can always be implicitly converted to either int or uint,
with perfect safety. No other conversions are possible without a cast.
Non-negative literals and manifest constants are naturals.
The rules are:
1. Anything involving unsigned is unsigned, (same as C).
2. Else if it contains an integer, it is an integer.
3. (Now we know all quantities are natural):
If it contains a subtraction, it is an integer [Probably allow
subtraction of compile-time quantities to remain natural, if the
values stay in range; flag an error if an overflow occurs].
4. Else it is a natural.
The reason I think literals and manifest constants are so important
is that they are a significant fraction of the natural numbers in a
program.
[Just before posting I've discovered that other people have posted
some similar ideas].
That sounds encouraging. One problem is that your approach leaves the
unsigned mess as it is, so although natural types are a nice addition,
they don't bring a complete solution to the table.
Andrei
Well, it does make unsigned numbers (case (B)) quite obscure and
low-level. They could be renamed with uglier names to make this clearer.
But since in this proposal there are no implicit conversions from uint
to anything, it's hard to do any damage with the unsigned type which
results.
Basically, with any use of unsigned, the compiler says "I don't know if
this thing even has a meaningful sign!".
Alternatively, we could add rule 0: mixing int and unsigned is illegal.
But it's OK to mix natural with int, or natural with unsigned.
I don't like this as much, since it would make most usage of unsigned
ugly; but maybe that's justified.
I think we're heading towards an impasse. We wouldn't want to make
things much harder for systems-level programs that mix arithmetic and
bit-level operations.
I'm glad there is interest and that quite a few ideas were brought up.
Unfortunately, it looks like all have significant disadvantages.
One compromise solution Walter and I discussed in the past is to only
sever one of the dangerous implicit conversions: int -> uint. Other than
that, it's much like C (everything involving one unsigned is unsigned
and unsigned -> signed is implicit) Let's see where that takes us.
(a) There are fewer situations when a small, reasonable number
implicitly becomes a large, weird numnber.
(b) An exception to (a) is that u1 - u2 is also uint, and that's for the
sake of C compatibility. I'd gladly drop it if I could and leave
operations such as u1 - u2 return a signed number. That assumes the
least and works with small, usual values.
(c) Unlike C, arithmetic and logical operations always return the
tightest type possible, not a 32/64 bit value. For example, byte / int
yields byte and so on.
What do you think?
Andrei
