F#利用Discriminated Union实现函数重载

在F#中不可以像C++一样进行函数重载，但是通过Discriminated Union，可以实现函数重载的效果。

http://blog.ploeh.dk/2013/10/21/replace-overloading-with-discriminated-unions/

type Period =
    | Year of int
    | Month of int * int
    | Day of int * int * int
 
module Dates =
    let InitInfinite (date : DateTime) =
        date |> Seq.unfold (fun d -> Some(d, d.AddDays 1.0))
 
    let In period =
        let generate dt predicate =
            dt |> InitInfinite |> Seq.takeWhile predicate
        match period with
        | Year(y) -> generate (DateTime(y, 1, 1)) (fun d -> d.Year = y)
        | Month(y, m) -> generate (DateTime(y, m, 1)) (fun d -> d.Month = m)
        | Day(y, m, d) -> DateTime(y, m, d) |> Seq.singleton
 
    let BoundariesIn period =
        let getBoundaries firstTick (forward : DateTime -> DateTime) =
            let lastTick = forward(firstTick).AddTicks -1L
            (firstTick, lastTick)
        match period with
        | Year(y) -> getBoundaries (DateTime(y, 1, 1)) (fun d -> d.AddYears 1)
        | Month(y, m) -> getBoundaries (DateTime(y, m, 1)) (fun d -> d.AddMonths 1)
        | Day(y, m, d) -> getBoundaries (DateTime(y, m, d)) (fun d -> d.AddDays 1.0)