fasteval 0.2.4

// usage:  cargo run --release --example ns_cachedcallbacknamespace

fn main() -> Result<(), fasteval::Error> {
    let mut num_lookups = 0;
    let val = {
        let cb = |name:&str, _args:Vec<f64>| -> Option<f64> {
            num_lookups += 1;
            match name {
                "x" => {
                    // Pretend that it is very expensive to calculate this,
                    // and that's why we want to use the CachedCallbackNamespace cache.
                    for _ in 0..1000000 { /* do work */ }  // Fake Work for this example.
                    Some(2.0)
                }
                _ => None,
            }
        };
        let mut ns = fasteval::CachedCallbackNamespace::new(cb);

        fasteval::ez_eval("x * (x + 1)", &mut ns)?
    };
    assert_eq!(val, 6.0);
    assert_eq!(num_lookups, 1);  // Notice that only 1 lookup occurred.
                                 // The second 'x' value was cached.

    Ok(())
}