F# has Units of Measure

One of the great things about F# it that it allows you to annotate values with Units of Measure. (Remember the Mars Climate Orbiter burning up because all subcontractors used metric except Lockheed Martin?) F# will do limited testing to make sure your units are consistent. Units of Measure doesn't really understand the concepts of length, volume, force, or currency, but used wisely can be a great help. A little example:

[<Measure>]
type cm
[<Measure>]
type inch
val cmPerInch : float<cm/inch> = 2.54

> let length1 = 1.0<cm>
let length2 = 1.0<inch>;;

val length1 : float<cm> = 1.0
val length2 : float<inch> = 1.0

> let length3 = length1 + length2;;

  let length3 = length1 + length2;;
  ------------------------^^^^^^^

stdin(41,25): error FS0001: The unit of measure 'inch' does not match the unit of measure 'cm'
> let length3 = length1 + length2 * cmPerInch;;

val length3 : float<cm> = 3.54