an example of using an active pattern as a function argument

xmasGreetings.fsx

open System
let xmas = DateTime.Parse("December 25")
let (|ChristmasEve|ChristmasDay|Twelvetide|NearlyThere|TisTheSeason|NotChristmas|) (d : DateTime) =
  match (d - xmas).Days with
  | 0  -> ChristmasDay d.Year
  | -1 -> ChristmasEve d.Year
  | t when t > 0 && t < 12 -> Twelvetide t
  | t when t > -7  && t < 0 -> NearlyThere
  | t when t > -25 && t < 0 -> TisTheSeason
  | _ -> NotChristmas

let (|Twelvetide|) (Twelvetide n) =
  match n with
  | 1 -> "Partridge in a pear tree"
  | 2 -> "Turtle doves"
  | 3 -> "French hens"
  | 4 -> "Calling Birds"
  | 5 -> "Gold Rings"
  | 6 -> "Geese a-Laying"
  | 7 -> "Swans a-Swimming"
  | 8 -> "Maids a-Milking"
  | 9 -> "Ladies Dancing"
  | 10 -> "Lords a-Leaping"
  | 11 -> "Pipers Pipin"
  | 12 -> "Drummers Drumming"

let greetings = function
| ChristmasEve y | ChristmasDay y -> "Merry Christmas!"
| NearlyThere                     -> "Avoid the malls"
| TisTheSeason                    -> "Be Jolly!"
| NotChristmas                    -> "Bahumbug"
| Twelvetide s -> s

let rec from (d : DateTime) = seq {
  yield d; yield! from <| d.AddDays(1.)
}
from (DateTime.Today)
|> Seq.takeWhile (function NotChristmas -> false | _ -> true)
|> Seq.iter (greetings >> printfn "%s")