a simple example of pattern matching in F#

pattern-matching.fs

type Shape =
    | Circle of float
    | Rectangle of double*double
    | EquilateralTriangle of double

let pi = 3.141592654

let area shape =
    match shape with
    | Circle r -> r*r*pi
    | Rectangle (h,w) -> h*w
    | EquilateralTriangle side -> (sqrt 3.0)/4.0*side*side

let circle = Circle(15.0)
printfn "The area is %f" (area circle)