Shape union in F#

UnionTypes.fs

module UnionTypes

// Learn more about F# at http://fsharp.org

type Shape =
    | Circle of float
    | Square of float
    | Triangle of float * float
    | Rectangle of float * float

let GetArea shape =
    match shape with
    | Circle(radius) -> 3.14 * radius * radius
    | Square(width) -> width * width
    | Triangle(``base``, height) -> ``base`` * height / 2.0
    | Rectangle(width, height) -> width * height