Active Pattern (1) : Single-case Active Pattern

(In learning from "Programming F# 3.0, 2nd Edition")

Single-case Active Pattern is defined as a special function enclosed (| |).

> open System;;
> let (|WhatDayOfWeek|) (year, month, day) =
-     System.DateTime(year,month,day).DayOfWeek;;

val ( |WhatDayOfWeek| ) : year:int * month:int * day:int -> DayOfWeek

> let isWeekEnd year month day =
-     match (year, month, day) with
-     | WhatDayOfWeek System.DayOfWeek.Sunday
-     | WhatDayOfWeek System.DayOfWeek.Saturday
-         -> true
-     | WhatDayOfWeek _
-         -> false;;

val isWeekEnd : year:int -> month:int -> day:int -> bool

> isWeekEnd 2013 3 15;;
val it : bool = false
> isWeekEnd 2013 3 16;;
val it : bool = true