I'll just point out that the C++ name for this is "Proxy classes".
Maybe, for the sake of reducing confusion, it might be a good idea to
adopt that.
Shachar
On 12/03/18 15:59, Simen Kjærås wrote:
There's been a discussion[0] over in D.learn about a library-only
implementation of properties. One of the ideas mentioned there is rvalue
types - types that automatically decay into a different type when passed
to a function or assigned to a variable. They are useful for property
wrappers like this, and when you want to perform a series of operations
on something before performing some final step.
An example of the latter is `a ^^ b % c` for BigInts, where the naïve
way would be horribly inefficient, and there's a much better way of
doing it, which requires more knowledge of the operations involved. If
`a ^^ b` returned an rvalue type that either decays to a regular BigInt
or acts as the LHS in `tmp % c`, it would have the necessary information
and be able to do the right thing.
Another use case is a chain of operations, e.g. fluent initialization:
Widget.create()
.width(35)
.height(960)
.data(readData())
.Done();
Where in current D the Done() step needs to be explicit, an rvalue type
would automatically call Done when the result is assigned to a variable
or passed to a function.
The problem with such a set of types, of course, is that
`typeof(functionThatReturnsRvalueType())` will be different from
`typeof((){ auto t = functionThatReturnsRvalueType(); return t;}())`,
and that bleeds into documentation. It may also be confusing that
`return a ^^ b % c;` is much faster than `auto tmp = a ^^ b; return tmp
% c;`.
An example of how they would work:
struct ModularExponentiationTemporary {
BigInt lhs, rhs;
@rvalue // Or however one would mark it as such.
alias get this;
BigInt get() {
return pow(lhs, rhs);
}
BigInt opBinaryRight(string op : "%")(BigInt mod) {
return modularPow(lhs, rhs, mod);
}
}
unittest {
BigInt b = 4;
BigInt e = 13
BigInt m = 497;
// b ^^ e returns a ModularExponentiationTemporary,
// and its opBinaryRight() is immediately invoked.
auto fast = b ^^ e % m;
assert(is(typeof(fast) == BigInt));
assert(fast== 445);
// b ^^ e returns a ModularExponentiationTemporary,
// and its get() method is immediately invoked.
auto slowTmp = b ^^ e;
auto slow = slowTmp % m;
assert(is(typeof(slowTmp == t) == BigInt));
assert(is(typeof(slow) == BigInt));
assert(slow == 445);
}
Is this an interesting concept? Are there other use cases I haven't
covered? Can this be done with existing language features? Are there
problems I haven't foreseen?
--
Simen
[0]: https://forum.dlang.org/post/[email protected]
