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digitalmars.D - Inherited mutability, freeze, thaw and more in Rust

reply "bearophile" <bearophileHUGS lycos.com> writes:
Another post from the Rust language blog:


 From the post:

This is a powerful idea but quite beyond Rust’s type system, 
and I am interested in exploring solutions that lead to similar 
expressiveness while avoiding the quagmire of dependent types.<

Adding dependent types to Rust increases the language complexity, but it also offers very good capabilities that only very few languages like ATS have. A first part, "More background: inherited mutability" is about a problem that I have discussed a little in this newsgroup. So it seems it wasn't a so naive and useless topic. Rust immutability seems better thought out/designed than D immutability, but Rust design is not finished yet. Rust type system is much more powerful than D type system, but maybe it's also a little harder to understand. In Rust even if a struct (record) field is not mutable, you are allowed to replace the contents of a mutable variable that contains one of such struct instances. It means it accepts code similar to this one, because inherits mutability: struct Foo { immutable int x; int y; } void main() { Foo f = Foo(5, 10); f = Foo(20, 30); } This is quite handy, because in D once a struct has an immutable field, you can't do a lot with it, you can't reassign, etc. A D class instance with one immutable field doesn't have such problems: Doing this is of course not accepted in Rust: void main() { immutable Foo f = Foo(5, 10); f = Foo(20, 30); } Bye, bearophile
Jul 26 2012
parent "Chris Cain" <clcain uncg.edu> writes:
On Thursday, 26 July 2012 at 14:54:59 UTC, bearophile wrote:
 struct Foo {
     immutable int x;
     int y;
 void main() {
     Foo f = Foo(5, 10);
     f = Foo(20, 30);

There's problems with this which would have significant consequences in D. It would destroy the ability to reason about code and make many different possible optimizations. A quick example of one of the problems that would be raised by this: int calc(immutable int * input) { auto result = input * 7; // ... lots of other things result += input + otherThing; return result; } void main() { Foo f = Foo(5, 10); auto bar = task!calc(&f.x); bar.executeInNewThread(); // do some things... f = Foo(20, 30); // do some other things... writeln(bar.yieldForce()); // Oops. } In this case, the problem is pretty clear. Even though calc takes an immutable int pointer, we have no idea whether calc will use 5 or 20 in its calculation. It may even **change while calc is running!** Furthermore, the compiler might generate code that *is not equivalent to the program as written* as a side effect to that unpredictable state of affairs. For instance, it might choose to cache the int in a register and therefore wouldn't change while calc is running ... which is clearly not equivalent, but *should* be correct if we assume that the input couldn't change ... which is what immutable normally guarantees. Now Rust may have other features/limitations which make this not a problem (and even correct), but it could have far reaching consequences for D.
Jul 26 2012