Diman

Diman is a library for zero-cost compile time unit checking.

use diman::si::f64::{Length, Time, Velocity};

fn get_velocity(x: Length, t: Time) -> Velocity {
    x / t
}

let v1 = get_velocity(Length::kilometers(36.0), Time::hours(1.0));
let v2 = get_velocity(Length::meters(10.0), Time::seconds(1.0));

assert_eq!(v1, v2);

Let's try to assign add quantities with incompatible units:

use diman::si::f64::{Length, Time};

let time = Time::seconds(1.0);
let length = Length::meters(10.0);
let sum = length + time;

This results in a compiler error:

let sum = length + time;
                   ^^^^
= note: expected struct `Quantity<_, Dimension { length: 1, time: 0, mass: 0, temperature: 0, current: 0, amount_of_substance: 0, luminous_intensity: 0 }>`
        found struct `Quantity<_, Dimension { length: 0, time: 1, mass: 0, temperature: 0, current: 0, amount_of_substance: 0, luminous_intensity: 0 }>`

Disclaimer

Diman is implemented using Rust's const generics feature. While min_const_generics has been stabilized since Rust 1.51, Diman uses more complex generic expressions and therefore requires the two currently unstable features generic_const_exprs and adt_const_params.

Moreover, Diman is in its early stages of development and APIs will change.

If you cannot use unstable Rust for your project or require a stable library, consider using uom or dimensioned, both of which do not require any experimental features and are much more mature libraries in general.

Features

Invalid operations between physical quantities (adding length and time, for example) turn into compile errors.
Newly created quantities are automatically converted to an underlying base representation. This means that the used types are quantities (such as Length) instead of concrete units (such as meters) which makes for more meaningful code.
Systems of units and quantities can be user defined via the unit_system! macro. This gives the user complete freedom over the choice of quantities and makes them part of the user's library, so that arbitrary new methods can be implemented on them.
f32 and f64 float storage types (behind the f32 and f64 feature gate respectively).
Vector storage types via glam (behind the glam-vec2, glam-vec3, glam-dvec2 and glam-dvec3 features).
Serialization and Deserialization via serde (behind the serde feature gate, see the official documentation for more info).
HDF5 support using hdf5-rs (behind the hdf5 feature gate).
Quantities implement the Equivalence trait so that they can be sent via MPI using mpi (behind the mpi feature gate).
Random quantities can be generated via rand (behind the rand feature gate, see the official documentation for more info).

Design

Diman aims to make it as easy as possible to add compile-time unit safety to Rust code. Physical quantities are represented by the Quantity<S, D> struct, where S is the underlying storage type (f32, f64, ...) and D is the dimension of the quantity. For example, in order to represent the SI system of units, the dimension type would look as follows:

#![feature(adt_const_params)]
use diman::dimension;

#[dimension]
pub struct Dimension {
    pub length: i32,
    pub time: i32,
    pub mass: i32,
    pub temperature: i32,
    pub current: i32,
    pub amount_of_substance: i32,
    pub luminous_intensity: i32,
}

Addition and subtraction of two quantities is only allowed when the D type is the same. During multiplication of two quantities, all the entries of the two dimension are added. This functionality is implemented automatically by the #[diman] macro. Note: This macro is currently not a derive macro for the Mul/Div traits because the dimension multiplication and division are required to be const. While this is possible, it would require using yet another unstable feature const_trait_impl.

Using the above Dimension type, we can define our own quantity type and a corresponding set of physical quantities and dimensions using the unit_system macro:

#![allow(incomplete_features)]
#![feature(generic_const_exprs, adt_const_params)]
use diman::unit_system;
use diman::dimension;

#[dimension]
pub struct Dimension {
    pub length: i32,
    pub time: i32,
}

unit_system!(
    Quantity,
    Dimension,
    [
        def Length = { length: 1 },
        def Time = { time: 1 },
        def Velocity = Length / Time,
        unit (meters, "m") = Length,
        unit (kilometers, "km") = 1000.0 * meters,
        unit (seconds, "s") = 1.0 * Time,
        unit hours = 3600 * seconds,
        unit meters_per_second = meters / seconds,
        unit kilometers_per_hour = kilometers / hours,
        constant MY_FAVORITE_VELOCITY = 1000 * meters_per_second,
    ]
);

use f64::{Length, Time, Velocity, MY_FAVORITE_VELOCITY};

fn fast_enough(x: Length, t: Time) {
    let vel = x / t;
    if vel > MY_FAVORITE_VELOCITY {
        println!("{} is definitely fast enough!", vel.in_kilometers_per_hour());
    }
}

fast_enough(Length::kilometers(100.0), Time::hours(0.3));

This will define the Quantity type and implement all the required traits and methods. Here, def defines Quantities, which are concrete types, unit defines units, which are methods on the corresponding quantities and constant defines constants. The macro also accepts more complex definitions such as def EnergyRatePerVolume = (Energy / Time) / Volume. The definitions do not have to be in any specific order.

The Quantity type

The macro will automatically implement numerical traits such as Add, Sub, Mul, and various other methods of the underlying storage type for Quantity<S, ...>. Quantity should behave just like its underlying storage type whenever possible and allowed by the dimensions. For example:

Addition of Quantity<Float, D> and Float is possible if and only if D is dimensionless.
Quantity implements the dimensionless methods of S, such as abs for dimensionless quantities.
It implements Deref to S if and only if D is dimensionless.
Debug is implemented and will print the quantity in its representation of the "closest" unit. For example Length::meters(100.0) would be debug printed as 0.1 km. If printing in a specific unit is required, conversion methods are available for each unit (such as Length::in_meters).
.value() provides access to the underlying storage type of a dimensionless quantity.
.value_unchecked() provides access to the underlying storage type for all quantities if absolutely required. This is not unit-safe since the value will depend on the unit system!
Similarly, new quantities can be constructed from storage types using Quantity::new_unchecked. This is also not unit-safe.

Some other, more complex operations are also allowed:

use diman::si::f64::{Length, Volume};
let x = Length::meters(3.0);
let vol = x.cubed();
assert_eq!(vol, Volume::cubic_meters(27.0))

This includes squared, cubed, sqrt, cbrt as well as powi.

Quantity products and quotients

Sometimes, intermediate types in computations are quantities that don't really have a nice name and are also not needed too many times. Having to add a definition to the unit system for this case can be cumbersome. This is why the Product and Quotient types are provided: