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On Monday, 28 November 2022 at 09:27:11 UTC, Walter Bright wrote:
>I've written most of a DIP for one. Should I:
I really wish that instead of adding more language features, we would improve the language so that things like this are implementable in the language.
I'm not sure about the argument that not everyone might use the same type if it's implemented as a custom type. Even if one were to argue that not everyone uses Phobos, the implementation can go into Druntime, which is an extension of the compiler, which solves this problem.
The example in the DIP about null pointers is wrong. It can still be useful to distinguish between non-null pointers, null pointers, and some third state such as "no pointer". Nullable!(Nullable!(Nullable!int)) foo is perfectly valid.
We don't even need a standardized NonNull type or trait or whatever. Leverage existing D features instead:
template nullIsValid(T)
{
static if (is(T == struct))
enum nullIsValid = {
try
{
union U
{
ubyte[T.sizeof] bytes = 0;
T t;
}
U u;
assert(u.t); // call invariant
return true; // no exception was thrown - null is a valid state
}
catch (Throwable e)
return false; // exception was thrown - null is an INVALID state
}();
else
enum nullIsValid = true;
}
or something like that. Currently it doesn't work because you can't call struct invariants directly, and can't catch assert(false) in CTFE. But, these are fixable and examples of things that would improve metaprogramming at a fundamental, building-block level and enable building more cool high-level types and data structures, not just sum types.
