[GitHub] [arrow-rs] alamb opened a new issue #558: Make compute kernels easier to use by taking `&ArrayRef` and downcasting internally

[GitBox <gi...@apache.org>]

alamb opened a new issue #558:
URL: https://github.com/apache/arrow-rs/issues/558


   **Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
   I would like to do arithmetic on temporal arrys like `TimestampNanosecondArray`
   
   https://docs.rs/arrow/4.4.0/arrow/compute/kernels/arithmetic/fn.add.html
   
   The current compute kernels have the following signatures:
   
   ```rust
   pub fn add<T>(
       left: &PrimitiveArray<T>, 
       right: &PrimitiveArray<T>
   ) -> Result<PrimitiveArray<T>> 
   ...
   ```
   
   So if I have two `ArrayRefs` (aka `Arc<dyn Array>`) I am forced to downcast them to specific known types like
   ```rust
   let left = left.as_any().downcast_ref<TimestampNanosecondArray>().unwrap();
   let right = right.as_any().downcast_ref<DurationArray>().unwrap();
   
   let result = compute::add(left, right).unwrap();
   ```
   
   
   However, the cast kernel is much easier to use as it does this type dispatch / downcasting internally for me:
   
   ```rust
   let cast_result = compute::cast(&left, DataType::Int32)`
   ```
   
   **Describe the solution you'd like**
   I would like for the `add` (and other compute kernels) to take a `&ArrayRef` as argument and then do any type matching / downcasting necessary internally, so I, as a user, don't have to think about it
   
   **Describe alternatives you've considered**
   An alternate is to take an `&dyn Array` and avoid the need for wrapping the array in an `Arc` -- however, given the prevalence of `ArrayRef` / `Arc` in Arrow APIs already I would personally prefer a uniform interface with the kernels like `cast` which already take `&ArrayRef`
   
   **Additional context**
   There is additional discussion and context at https://github.com/apache/arrow-rs/issues/527#issuecomment-880649326


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[GitHub] [arrow-rs] alamb commented on issue #558: Make compute kernels easier to use by taking `&ArrayRef` and downcasting internally

[GitBox <gi...@apache.org>]

alamb commented on issue #558:
URL: https://github.com/apache/arrow-rs/issues/558#issuecomment-880670906


   Or maybe the signature could be something like 
   
   ```rust
   fn add(left: A, right: A) -> ArrayRef 
   where
     A: AsRef<dyn Array>
   ```
   
   So that we could take either an `&ArrayRef` or `&dyn Array` directly


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