Multiple-Case Active Pattern, Decomposition 2

pattern_match2_fsharp_blog_12.fsx

open System

let (|Spring|Summer|Fall|Winter|) (date : DateTime) =
    match date.Month, date.Day with
    | 1, d | 2,  d         | 3,  d when d < 21 -> Winter
    | 3, d | 4,  d | 5, d  | 6,  d when d < 21 -> Spring
    | 6, d | 7,  d | 8, d  | 9,  d when d < 21 -> Summer
    | 9, d | 10, d | 11, d | 12, d when d < 21 -> Fall
    | _                                        -> Winter

NOTE: This has a subtle bug as discussed in this blog post (in the UPDATE: Aug-11-2020) section: http://www.jason-down.com/2017/01/24/f-pattern-matching-part-2-active-patterns/