Don't forget F#'s units of measure! Which by the way are pretty cool, as the compiler automatically infers new units of measure for you:

    > [<Measure>] type km;
    > [<Measure>] type hour;
    > let distance = 60.0<km>;
    > let time = 0.5<hour>;
    > let avgSpeed = distance / time ;;
    val avgSpeed : float<km/hour> = 100.0

(And of course forbids physically illegal stuff such as adding values with different dimensions)