Units

Units is a small Go package that implements types, constants, converter functions and some mathematics for the following physical types:

• Distance
• Velocity
• Angle
• Angular velocity

The concept is very similar to that of the time.Duration type in the Go standard library.

Why would I want to use the units package?

When operating with programmed variables that hold physical types, it is sometimes hard to see what kinds of data a variable holds, and what kind of unit it is stored in. Does it hold a velocity or a distance? Given it is a velocity, does it store the value in meters per second, millimeteres per second, or in some other kind of unit? British miles per second perhaps? Or maybe in lightyears per minute? Is the angle measured in degrees or radians?

By using the units package, all variables with a certain physical type (e.g. distance, angle or velocity), is stored in the same format with the same unit. Even though the numbers are always stored in the same way, you can specify them anyway you want, and represent them anyway you want. E.g. even though an angle is stored as milliradians, you can specify it as degrees, and display it as gradians.

Because Go only allows explicit type casts, makes sure you are warned if you try to store one physical type in a variable that holds another physical type. E.g. it is hard to use a variable storing a velocity in a function that takes a distance, by a mistake.

For best practice, types from the units package should only be converted to float64 values by use of the converter functions (unless you have a good reason not to). Likewise, when declaring a variable of a certain physical type, you would be wise to multiply it with one of the constants (of the same type) provided by the units package.

Examples

Import and const definitions:

import(
	"fmt"
	"time"
	"github.com/smyrman/units"
)

// Define some shorcuts (optional)
const(
	// The following has type time.Duration:
	sec   = time.Second
	msec  = time.Millisecond

	// The following has type units.Distance:
	m     = units.Meter
	cm    = units.Centimeter
	mm    = units.Millimeter

	// The following has type units.Velocity:
	kmph  = units.KilometerPerSecond
	mpsec = units.MeterPerSecond

	// The following has type units.Angle:
	deg = units.Degree
	rad = units.Radian
)

Distance to velocity example:

// For 'd', the type is units.Distance, the underlying type is flot64, and
the underlying value is 6000.0:
d := 6*m

// For 't', the type is time.Duration, the underlying type is int64, and
the underlying value is 2e9:
t := 2*sec

// For 'v', the type will be units.Velocity, the underlying type is float64, and
the underlying value will be 3000.0:
v := dist.DivideWithDuration(t)

// v.MetersPerSecond() returns the float64 value 3.0.
fmt.Printf("v = %.2f\n", v.MetersPerSecond())

Origin

The units package was originally developed for use in the robot Loke (eng:Loki), that participated in the Eurobot competition in France in 2012 (http://eurobot-ntnu.no).