brunal opened a new issue, #9458:
URL: https://github.com/apache/arrow-rs/issues/9458
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
We cannot fully type (on index & value types) a run-end-encoded array value.
| What | Value | Reference |
|-|-|-|
| Array | `dyn Array` | `&dyn Array` |
| RunArray | `RunArray<I: RunArrayIndexType>` | `&RunArray<I>` |
| TypedRunArray | zilch! | `TypedRunArray<'a, I, V>` |
We can have a value describing a RunArray with a known index type. And we
can take a reference to it.
But for a RunArray with known index and value types, we can only have a
reference type, not a value type. This means that we cannot have a function
return a fresh, fully described run array, with a known value type. We can only
return a partially-typed RunArray, and add a comment saying what the values
are. For example, consider the fictional:
```
/// Returns a RunArray whose values are `A`.
fn encode_array<A: Array, I: RunArrayIndexType>(array: A) -> RunArray<A> {
todo!()
}
```
It would be nice to return a `TypedRunArray<I, A>`. But that's not possible.
We can only add a comment, and let the user cast as needed.
**Describe the solution you'd like**
I'd rather see TypedRunArray a value type.
An issue is that an Array cannot both be a `struct RunArray` and a `struct
TypedRunArray` so there's some type design to do.
Additionally, it would be a backwards-imcompatible change. TypedRunArray
might have to stay as-is. Maybe introduce `TypedRunArrayValue`, and one can
make a `TypedRunArray<'_>` from either a `&RunArray` or a `&TypedRunArrayValue`.
**Additional context**
Another defect of the current `TypedRunArray<I, V>` is that `V` is not
constrained to `impl Array`. Not sure that's intended.
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