Demonstration of using Haxe abstracts to take care of explicit conversions between different units of measurement, with no performance overhead.

Metric.hx

class Metric {
	static function main() {
		var coinRadius:Millimeters = 12;
		var myHeight:Centimeters = 180;
		var raceLength:Meters = 200;
		var commuteDistance:Kilometers = 23;
		
		diff( coinRadius, myHeight ); // 1.788 meters
		diff( raceLength, commuteDistance ); // 22800 meters
		sum( commuteDistance, coinRadius ); // 23000.012 meters
	}
	
	static function diff( a:Meters, b:Meters ) {
		var d = Math.abs( a-b );
		trace( '$d meters' );
	}
	
	static function sum( a:Meters, b:Meters ) {
		var s = Math.abs( a+b );
		trace( '$s meters' );
	}
}

abstract Millimeters( Float ) from Float to Float {
	@:to function toCentimeters():Centimeters return this/10;
	@:to function toMeters():Meters return this/1000;
	@:to function toKilometers():Kilometers return this/1000000;
	@:op(A+B) function add(rhs:Float):Float return this+rhs;
	@:op(A-B) function sub(rhs:Float):Float return this-rhs;
	@:op(A-B) function multiply(rhs:Float):Float return this*rhs;
	@:op(A-B) function divide(rhs:Float):Float return this/rhs;
}

abstract Centimeters(Float) from Float to Float {
	@:to function toMillimeters():Millimeters return this*10;
	@:to function toMeters():Meters return this/100;
	@:to function toKilometers():Kilometers return this/100000;
	@:op(A+B) function add(rhs:Float):Float return this+rhs;
	@:op(A-B) function sub(rhs:Float):Float return this-rhs;
	@:op(A-B) function multiply(rhs:Float):Float return this*rhs;
	@:op(A-B) function divide(rhs:Float):Float return this/rhs;
}

abstract Meters( Float ) from Float to Float {
	@:to function toMillimeters():Millimeters return this*1000;
	@:to function toCentimeters():Centimeters return this*100;
	@:to function toKilometers():Kilometers return this/1000;
	@:op(A+B) function add(rhs:Float):Float return this+rhs;
	@:op(A-B) function sub(rhs:Float):Float return this-rhs;
	@:op(A-B) function multiply(rhs:Float):Float return this*rhs;
	@:op(A-B) function divide(rhs:Float):Float return this/rhs;
}

abstract Kilometers( Float ) from Float to Float {
	@:to function toMillimeters():Millimeters return this*1000000;
	@:to function toCentimeters():Centimeters return this*100000;
	@:to function toMeters():Meters return this*1000;
	@:op(A+B) function add(rhs:Float):Float return this+rhs;
	@:op(A-B) function sub(rhs:Float):Float return this-rhs;
	@:op(A-B) function multiply(rhs:Float):Float return this*rhs;
	@:op(A-B) function divide(rhs:Float):Float return this/rhs;
}

Metric.js

(function () { "use strict";
var Metric = function() { };
Metric.main = function() {
        var coinRadius = 12;
        var myHeight = 180;
        var raceLength = 200;
        var commuteDistance = 23;
        Metric.diff(coinRadius / 1000,myHeight / 100);
        Metric.diff(raceLength,commuteDistance * 1000);
        Metric.sum(commuteDistance * 1000,coinRadius / 1000);
};
Metric.diff = function(a,b) {
        var d = Math.abs(a - b);
        console.log("" + d + " meters");
};
Metric.sum = function(a,b) {
        var s = Math.abs(a + b);
        console.log("" + s + " meters");
};
Metric.main();
})();

Should this line: https://gist.github.com/jasononeil/b6b1845824f45f5d19df#file-metric-hx-L40

@:op(A-B) function multiply(rhs:Float):Float return this*rhs;

instead be:

@:op(A*B) function multiply(rhs:Float):Float return this*rhs;

Or am I missing something?

@larsiusprime I think so too, same for @:op(A/B) function divide(rhs:Float):Float return this/rhs;

@jasononeil I've started working on one: https://github.com/ibilon/units