Macro export_functions 

macro_rules! export_functions {
    ($(($FUNC:ident, $DOC:expr, $($arg:tt)*)),*) => { ... };
    (single $FUNC:ident, $DOC:expr, @config) => { ... };
    (single $FUNC:ident, $DOC:expr, @config $arg:ident,) => { ... };
    (single $FUNC:ident, $DOC:expr, @config $($arg:ident)*) => { ... };
    (single $FUNC:ident, $DOC:expr, $arg:ident,) => { ... };
    (single $FUNC:ident, $DOC:expr, $($arg:ident)*) => { ... };
}

macro that exports a list of function names as:

1. individual functions in an expr_fn module
2. a single function that returns a list of all functions

Equivalent to

pub mod expr_fn {
    use super::*;
    /// Return encode(arg)
    pub fn encode(args: Vec<Expr>) -> Expr {
        super::encode().call(args)
    }
 ...
/// Return a list of all functions in this package
pub(crate) fn functions() -> Vec<Arc<ScalarUDF>> {
    vec![
      encode(),
      decode()
   ]
}

Exported functions accept:

• Vec<Expr> argument (single argument followed by a comma)
• Variable number of Expr arguments (zero or more arguments, must be without commas)
• Functions that require config (marked with @config prefix)

Note on configuration construction paths:

• The convenience wrappers generated for @config functions call the inner constructor with ConfigOptions::default(). These wrappers are intended primarily for programmatic Expr construction and convenience usage.
• When functions are registered in a session, DataFusion will call with_updated_config() to create a ScalarUDF instance using the session’s actual ConfigOptions. This also happens when configuration changes at runtime (e.g., via SET statements). In short: the macro uses the default config for convenience constructors; the session config is applied when functions are registered or when configuration is updated.