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Arroyo user-defined functions (UDFs) can be written in Rust, with performance that is comparable to built-in functions. UDFs are defined as Rust functions, annotated with the #[udf] attribute from the arroyo_udf_plugin crate. The parameters and return type of the UDF are determined by the definition of the function. All types must be valid SQL data types. For String and Binary types (TEXT BYTEA in SQL), UDFs use the reference type (&str and &[u8]) for arguments and the owned types (String and Vec<u8>) Here’s an example of a simple UDF that squares an integer: 
use arroyo_udf_plugin::udf;

#[udf]
fn my_square(x: u64) -> u64 {
  x * x
}
For more general details about UDFs, see the UDF overview, and for a complete example of how to use UDFs to solve a real-world problem of custom format parsing, see the UDF tutorial.

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Nullability

SQL values are generally allowed to be null. How null values interact with your UDF is controlled via the type signature of the UDF parameters and return types. If a parameter is an Option type (for example Option<i64>), then it will be called with all inputs, even if they are NULL. If the parameter is not an Option type (for example i64), then it will only be called with non-NULL inputs. Similarly, if the return type is an Option type, then the output type is nullable, otherwise it is non-nullable. In table form:
InputParameter typeReturn typeCalled onNullability
NullableTTNon-null valuesNullable
NullableOption<T>TAll valuesNon-null
NullableTOption<T>Non-null valuesNullable
NullableOption<T>Option<T>All valuesNullable
Non-nullTTAll valuesNon-null
Non-nullOption<T>TAll valuesNon-null
Non-nullTOption<T>All valuesNullable
Non-nullOption<T>Option<T>All valuesNullable

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Dependencies

UDFs can depend on external crates. To add dependencies, you can define a special comment in the UDF definition like this: 
/*
[dependencies]
regex = "1.10.2"
*/

use arroyo_udf_plugin::udf;
use regex::Regex;

#[udf]
fn my_regex_matches(s: &str) -> bool {
    let re = Regex::new(r"test").unwrap();
    re.is_match(s)
}
Internally, the contents of the [dependencies] comment are used to generate a Cargo.toml file for the UDF. See the Cargo.toml reference for more details on the syntax. Dependencies may also include environment variables, which will be substituted at compile time: 
/*
[dependencies]
my-repo = {
  git = "https://{{ GITHUB_USER }}:{{ GITHUB_TOKEN }}@github.com/{{ GITHUB_ORG }}/my-repo.git"
}
*/