Macro node_fn 

macro_rules! node_fn {
    ($f:expr) => { ... };
}

Convenience macro to wrap a sync closure for use with add_node.

use langgraph::prelude::*;

graph.add_node("doubler", node_fn!(|input, _config| {
    let n = input.as_i64().unwrap_or(0);
    Ok(json!(n * 2))
}));