units

Units of Measure for Rust. Easy to use, type-safe and customizable.

Usage (Example)

#[macro_use] extern crate units;

// It is recommended to put units inside a separate module
pub mod my_units {
    // Here we define our unit system with three units
    units! {
        MyUnits {
            Meter[m],
            Second[s],
            Mile[mile]
        }
    }
}

// Import the unit constants and also the dimensionless unit `one`
use my_units::f64::{one, m, s, mile};

fn main() {
    let km = 1000.*m; // Define the `km` unit as 1000 meter
    let h = 60.*60.*s; // Define an hour as 60 * 60 seconds
    // These units could also have been defined as separate base units,
    // in order to further reduce the risk to mix them up.

    // Let's calculate how far a car has moved if it has accelerated
    // uniformly from 0 to 100 km/h in 12 seconds
    let initial_speed = 0.*km/h;
    let final_speed = 100.*km/h;
    let time = 12.*s;
    let acceleration = (final_speed - initial_speed)/time;
    let result = 0.5 * acceleration * time * time; // s = a/2 * t^2
    // Here we use debug formatting, which will automatically print the base dimensions
    println!("{:?}", result); // This will print `166.66666666666669 m`

    // Let's convert the result to miles
    let meter_per_mile = 1609.344*m/mile; // This has unit `m/mile`
    let result_in_miles = result / meter_per_mile; // This has unit `mile`
    // Here we get a dimensionless value by eliminating the unit, then use deref (*) to extract the raw f64.
    println!("{} miles", *(result_in_miles/mile)); // This will print `0.103561865372889 miles`

    // Now we want to know how long a ball will fall until it reaches the
    // floor if dropped from a height of 10 meters
    let height = 1.5*m;
    let g = 9.81*m/s/s; // Use a gravitational constant of 9.81 m/s^2
    let time = (2. * height / g).sqrt(); // sqrt() takes care of the units
    // Print the result in milliseconds and round to 2 digits
    let ms = 0.001*s; // 1 ms = 0.001 s
    println!("The ball falls for {:.2} ms until it hits the ground.", *(time/ms));
    // The above will print `The ball falls for 553.00 ms until it hits the ground.`
}

Use cargo run --example basics to run this example.

There is an alternative, safer syntax for assigning units to values using function call operator overloading, but this is currently only available in Rust nightly. See examples/basics.rs for both versions.

How does it work?

The macro invocation shown above will generate a struct MyUnits<U,T=f64>, where the first type parameter is a special marker to get the dimension right (it contains exponents for every base unit, encoded as type-level integers). Meter, Second and Mile will be type aliases for this marker type with the correct exponents.

The second type parameter denotes the wrapped numeric type (defaults to f64). The autogenerated child modules my_units::f64 and my_units::f32 then both contain the constants m, s and mile, each of the correct dimension and wrapping the value 1.0. For example, f64::m is of type MyUnits<Meter, f64>.

Additionally, the type One and the constant one are provided for dimensionless values, and only those can be unwrapped using the Deref trait (this works automatically for method calls, but sometimes needs to be made explicit using the * prefix operator, see the example above).