アンドレイ☆アレクサンドレスク先生大喜び？の巻

I've had a talk with Walter today, and two interesting things transpired.

First off, Walter pointed out that I was wrong about one conversion rule (google value preservation vs. type preservation). It turns out that everything that's unsigned and less than int is actually converted to int, NOT unsigned int as I thought. This is the case in C, C++, and D.

Second, as of today Walter devised a very crude heuristic for typechecking narrowing conversions:

(a) a straight assignment x = y fails if y is wider than x.

(b) however, x = e compiles for more complex expressions EVEN if there is potential for loss of precision.

Now enter polysemy. With that, we can get the right rules in place and minimize false positives. An expression will yield a polysemous value with the as-C-does-it type as its principal type. The secondary type is a carefully computed narrower type that is the tightest actual type.

If you just say auto or use the value with overloaded functions etc., it's just like in C - the as-in-C type will be in vigor. But if you specifically ask for a narrower type, the secondary type enters in effect.

Examples:

uint u1 = ...;
ushort us1 = ...;
auto a = u1 & us1; // fine, a is uint
ushort b = u1 & us1; // fine too, secondary type kicks in

long l1 = ...;
auto c = u1 / l1; // c is long
int d = u1 / l1; // fine too, secondary type kicks in

We need to think this through for complex expressions etc. Walter and I are quite excited that this will take care of a significant portion of the integral conversions mess (in addition to handling literals, constants, and variables within a unified framework).

The plan is to deploy polysemous integrals first without changing the rest of the conversion rules. At that point, if the technique turns out to enjoy considerable success, Walter agreed to review and possibly stage in the change I suggested to drop the implicit signed -> unsigned conversion. With that, I guess we can claim victory in the war between spurious vs. too lax conversions.

I'm very excited about polysemy. It's entirely original to D, covers a class of problems that can't be addressed with any of the known techniques (subtyping, coercion...) and has a kick-ass name to boot. Walter today mentioned he's still not sure I hadn't made "polysemy" up. Indeed, Thunderbird and Firefox are suggesting it's a typo - please "add to dictionary" :o).

Andrei

えーとー・・・後で考える．