Is there any reason why in can't work with dynamic arrays?

[Adam D. Ruppe]

On Tuesday, 17 September 2019 at 18:10:40 UTC, Brett wrote:
> Alternatively, have a keys for a DA:
>
> then one can do
>
> if (y in x.keys)

Indeed.

Actually, fun fact: you can do that in your own code. Write a 
function `keys` that returns a struct implementing 
`opBinaryRight`.

Like so:

```
import std.stdio;

struct KeyRange {
	size_t max;
	bool opBinaryRight(string op : "in")(size_t v) {
		return v >= 0 && v < max; // the < 0 never passes but meh 
clearer code
	}
}

KeyRange keys(T)(T[] t) { return KeyRange(t.length); }
alias keys = object.keys; // also bring in the runtime library's 
keys for AA; this line lets both work together in the same module 
without name conflicts


void main()
{
	int[] x = [0, 2];
	int y = 1;
	writeln(y in x.keys); // works!
	writeln(y+5 in x.keys); // this too
}
```

The language features used there are UFCS (allowing x.keys), 
operator overloading (opBinaryRight), and overload set merging 
(the alias line with the comment). Searching those terms on this 
website should give more information if you'd like to learn more.