alamb opened a new issue #876:
URL: https://github.com/apache/arrow-rs/issues/876
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
I want to cast a null constant in DataFusion to an array (so the expression
evaluation logic is general). Sometimes this requires casting.
While this type of casting is definitely a corner case, it is required for
completeness and will unify our handling of null constants in DataFusion
**Describe the solution you'd like**
I want the cast kernel to support casting to/from NullArray for all data
types
Using `Int8` as example, I want this code to work
```rust
// should be able to cast Int8Array to Null array
let input: ArrayRef = Arc::new(vec![Some(1), Some(2),
None].into_iter().collect::<Int8Array>());
let cast = arrow::compute::cast(&input, &DataType::Null).expect("should
work");
let expected = new_null_array(&DataType::Null, 3);
assert_eq!(&cast, &expected);
```
Today it errors with:
```
thread 'main' panicked at 'should work: CastError("Casting from Int8 to Null
not supported")', src/main.rs:15:62
```
And also I want to cast from `Null` to `Int8`:
```rust
// Likewise, should be able to cast NullArray to Int8Array
let input = new_null_array(&DataType::Null, 3);
let cast = arrow::compute::cast(&input, &DataType::Int8).expect("should
work");
let expected = new_null_array(&DataType::Int8, 3);
assert_eq!(&cast, &expected);
// which is equivalent to
let expected: ArrayRef = Arc::new(vec![None, None,
None].into_iter().collect::<Int8Array>());
assert_eq!(&cast, &expected);
```
Which errors like this:
```
thread 'main' panicked at 'should work: CastError("Casting from Null to Int8
not supported")', src/main.rs:26:62
```
**Describe alternatives you've considered**
N/A
**Additional context**
See https://github.com/apache/arrow-datafusion/issues/1179 and
https://github.com/apache/arrow-datafusion/issues/1184
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