[PR] Simple Functions Preview [datafusion]

[via GitHub]

findepi opened a new pull request, #14668:
URL: https://github.com/apache/datafusion/pull/14668

   This is currently a preview PR for 
https://github.com/apache/datafusion/issues/12635 to share status of the work.
   
   Based on `add_one` function, this creates `add_one_udf` (by convention, 
configurable):
   
   ```rust
   #[excalibur_function]
   fn add_one(a: u64) -> u64 {
       a + 1
   }
   ```
   
   The generated `ScalarUDFImpl` is fully functional, with signature derived 
from the Rust-level signature of the decorated function:
   
   ```rust
   
   #[test]
   fn simple_function_signature() {
       let udf = add_one_udf();
       assert_eq!(udf.name(), "add_one");
   
       assert_eq!(
           udf.signature(),
           &Signature::coercible(
               vec![TypeSignatureClass::Native(Arc::new(NativeType::UInt64))],
               Volatility::Immutable
           )
       );
       let return_type = udf.return_type(&[DataType::UInt64]).unwrap();
       assert_eq!(return_type, DataType::UInt64);
   }
   
   #[test]
   fn simple_function_invoke_array() {
       let udf = add_one_udf();
   
       let invoke_args = 
vec![ColumnarValue::Array(Arc::new(UInt64Array::from(vec![
           1000, 2000, 3000, 4000, 5000,
       ])))];
       let ColumnarValue::Array(result_array) = udf
           .invoke_with_args(ScalarFunctionArgs {
               args: invoke_args,
               number_rows: 5,
               return_type: &DataType::UInt64,
           })
           .unwrap()
       else {
           panic!("Expected array result");
       };
   
       assert_eq!(
           &*result_array,
           &UInt64Array::from(vec![1001, 2001, 3001, 4001, 5001])
       );
   }
   
   #[test]
   fn simple_function_invoke_scalar() {
       let udf = add_one_udf();
   
       let invoke_args = 
vec![ColumnarValue::Scalar(ScalarValue::UInt64(Some(1000)))];
       let ColumnarValue::Array(result_array) = udf
           .invoke_with_args(ScalarFunctionArgs {
               args: invoke_args,
               number_rows: 1,
               return_type: &DataType::UInt64,
           })
           .unwrap()
       else {
           panic!("Expected array result");
       };
   
       assert_eq!(&*result_array, &UInt64Array::from(vec![1001]));
   }
   ```
   


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