14.7. Unit Conversions

AppleScript provides a number of classes whose sole purpose is to allow you to perform measurement unit conversions. They are implemented as classes so that you can use the as operator to perform the conversion; that is, the conversion is really a coercion.

Because of this implementation, the way you have to speak in order to perform a conversion ends up looking fairly silly. You can't say 3 feet; you have to coerce 3 (a number) to the feet class, by saying 3 as feet. Now you coerce to the desired class; suppose this is yards. But now you have a value of the yards class. You can't do anything with it, so you have to coerce it to a number (or a string). So, for example:

on feetToYards(ft)
    return ft as feet as yards as number
end feetToYards
feetToYards(3) -- 1.0

The implemented units are themselves a mixed lot. Many important units, such as acres and hectares, aren't implemented at all. Accuracy of some of the conversions has also been called into question, but this is said to be fixed in Tiger. Table 14-1 provides a list.

Table 14-1. Conversion unit classes

meters	inches	feet	yards
miles	kilometers	centimeters	square meters
square feet	square yards	square miles	square kilometers
liters	gallons	quarts	cubic meters
cubic centimeters	cubic feet	cubic inches	cubic yards
kilograms	grams	ounces	pounds
degrees Celsius	degrees Fahrenheit	degrees Kelvin

A better list of conversion units is built into Mac OS X by way of the Unix tool units. Here's a way to use it:

on convert(val, unit1, unit2)
    set text item delimiters to " "
    set conv to do shell script ({"units", unit1, unit2} as string)
    return val * (word 1 of paragraph 1 of conv as real)
end convert
convert(4, "feet", "meters") -- 1.2192