当某些东西不属于特定类型时如何进行模式匹配

Not

type Color = | Red | Green | Blue

let (|NotRed|_|) = function
    | Red -> None
    | color -> Some color

let rec print = function
    | (NotRed head) :: tail -> 
        printfn "%A is Not Red" head
        print tail
    | _ :: tail -> print tail
    | _ -> ()

print [Red; Green; Blue]

Green is Not Red
Blue is Not Red

_

match x with
| Y _ :: tail -> ()
| _ :: tail -> // List.head(x) is definitely not a Y

match x with
| (Y _ | (Z (1 | 2 | 3 | 4)) :: tail -> ()
| _ :: tail -> // Excludes all "Y x" and "Z n when n in 1,2,3,4"