On Monday, 15 October 2018 at 18:46:45 UTC, Manu wrote:
1. shared should behave exactly like const, except in addition
to inhibiting write access, it also inhibits read access.
How is this significantly different from now?
-----------------
shared int i;
++i;
Error: read-modify-write operations are not allowed for shared
variables. Use core.atomic.atomicOp!"+="(i, 1) instead.
-----------------
There's not much one can do to modify a shared value as it is.
Current situation where you can arbitrarily access shared
members
undermines any value it has.
Unless I'm missing something, I can't arbitrarily do anything
with shared members right now.
Shared must assure you don't access
members unsafely, and the only way to do that with respect to
data
members, is to inhibit access completely.
Or use atomic operations.
Assuming this world... how do you use shared?
https://github.com/atilaneves/fearless
or
https://dlang.org/phobos/core_atomic.html
From there, it opens up another critical opportunity; T* ->
shared(T)*
promotion.
I don't think that works. See below.
If you write a lock-free queue for instance, and all the
methods are
`shared` (ie, threadsafe), then under the current rules, you
can't
interact with the object when it's not shared, and that's fairly
useless.
Not really:
-----------
struct FancyQueue(E) {
// ...
void pushBack(this T)(E element) {
static if(is(T == shared)) {
// lock mutex or whatever is needed
auto safeThis = () @trusted { return cast(FancyQueue)
this; }();
} else {
// no need to lock anything
alias safeThis = this;
}
// profit
}
}
-----------
Usable if shared or not.
Assuming the rules above: "can't read or write to members", and
the understanding that `shared` methods are expected to have
threadsafe implementations (because that's the whole point),
what are the risks from allowing T* -> shared(T)* conversion?
int i;
tid.send(&i);
++i; // oops, data race
All the risks that I think have been identified previously
assume that you can arbitrarily modify the data.
Do you have any examples of arbitrarily modifying shared data? I
can't think of any.
That's insanity... assume we fix that... I think the promotion
actually becomes safe now...?
I don't think so, no.
