support user-defined types

[wangrunji0408]

Sometimes user want to return a complex struct in their functions. Currently user has to encode the struct type in the function signature, and return struct fields as a tuple. For instance:

#[function("key_value(varchar) -> struct<key:varchar,value:varchar>")]
fn key_value(kv: &str) -> Option<(&str, &str)> {
    kv.split_once('=')
}

This inline approach only works well for simple struct types. It will soon run into trouble when the struct type becomes very complex.

Therefore, we should allow users define their types separately using struct. We will introduce a derive macro to reflect the code and generate code for retrieving from and storing into arrow arrays.

The designed user interface would be like:

#[derive(StructType)]
struct KeyValue<'a> {
    key: &'a str,
    value: &'a str,
}

#[function("key_value(varchar) -> KeyValue")]
fn key_value(kv: &str) -> Option<KeyValue<'_>> {
    let (key, value) = kv.split_once('=')?;
    Some(KeyValue { key, value })
}

cc @TennyZhuang

[wangrunji0408]

implemented in arrow-udf v0.2