findepi commented on issue #12635:
URL: https://github.com/apache/datafusion/issues/12635#issuecomment-2401892724
I experimented with this on the way from DataFusion meetup in Belgrade.
i came up with something like this
#### function author would write this
row-oriented function logic
```rust
// hand-written
pub struct MyGcd {}
impl MyGcd {
pub fn call(x: i64, y: i64) -> Result<i64> {
datafusion::functions::math::gcd::compute_gcd(x, y)
}
}
```
#### a macro would generate
generated (syntactically derived)
```rust
impl datafusion::logical_expr::ScalarUDFImpl for MyGcd {
fn as_any(&self) -> &dyn Any {
self
}
fn name(&self) -> &str {
"MyGcd"
}
fn signature(&self) -> &Signature {
todo!() // TODO here we should use logical typyes
}
fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
todo!() // TODO here we should use logical typyes
}
fn invoke(&self, args: &[ColumnarValue]) -> Result<ColumnarValue> {
lifted_invoke::<MyGcd>(self, args)
}
}
impl Invocable for MyGcd {
const ARGUMENT_COUNT: u8 = 2;
type ArgumentTypeList = (i64, (i64, ()));
type OutArgType = ();
type FormalReturnType = Result<i64>;
fn invoke(&self, regular_args: Self::ArgumentTypeList, _out_arg: &mut
Self::OutArgType) -> Self::FormalReturnType {
let (a, (b, ())) = regular_args;
MyGcd::call(a, b)
}
}
```
All the heavy lifting happens inside `lifted_invoke` (source
https://gist.github.com/findepi/f1b0b84fc5f2002cb3d31b19a5bb983e#file-library-rs)
Notes on API design:
- `type ArgumentTypeList = (i64, (i64, ()));` -- this form of list
presentation yields to template programming
- `OutArgType` + `FormalReturnType` -- we need both to support plain
returning functions (numerics) and out args (for returning allocatable stuff
like strings or structs)
- `FormalReturnType = Result<i64>;` -- `lifted_invoke` supports functions
fallible and infallible functions, as well as nullable return values (via
`Option<T>`)
Note:
- for continuing work on this would be best to have logical types defined
(https://github.com/apache/datafusion/issues/12622). @notfilippo could we
perhaps have a logical types stub in `main` at some point?
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