Flight-School / Rate

A type-safe ratio of Foundation unit quantities

Rate

A ratio of two related quantities, expressed in terms of an amount of numerator unit per single denominator unit.

This functionality is discussed in Chapter 5 of Flight School Guide to Swift Numbers.

Requirements

• Swift 4.0+

Installation

Swift Package Manager

Add the Rate package to your target dependencies in Package.swift:

import PackageDescription

let package = Package(
  name: "YourProject",
  dependencies: [
    .package(
        url: "https://github.com/Flight-School/Rate",
        from: "1.0.0"
    ),
  ]
)

Then run the swift build command to build your project.

Carthage

To use Rate in your Xcode project using Carthage, specify it in Cartfile:

github "Flight-School/Rate" ~> 1.0.0

Then run the carthage update command to build the framework, and drag the built Rate.framework into your Xcode project.

Usage

One of the shortcomings of the Foundation Unit and Measurement APIs is the inability to dynamically declare compound units. This can make it difficult to perform dimensional analysis and other multi-step calculations.

The Rate structure allows you to express the ratio between two units in a type-safe manner. Multiplying a measurement with one unit type by a rate whose denominator is that same unit type causes those types to cancel out, resulting in a measurement with the numerator type.

For example, volume over time multiplied by time yields volume:

let flowRate = Rate<UnitVolume, UnitDuration>(value: 84760,
                                               unit: .cubicFeet,
                                                per: .seconds)
let oneDay = Measurement<UnitDuration>(value: 24, unit: .hours)

(flowRate * oneDay).converted(to: .megaliters) // 207371ML

License

MIT

Contact

Mattt (@mattt)