f# - 当某物不属于特定类型时如何进行模式匹配

Question

我们都习惯于在某些是特定类型的情况下进行模式匹配，例如，

match x with
| Y(x) :: tail -> ... // assumes List.head(x) is of type Y(x)

但是当某些东西不属于特定类型时，我如何才能匹配大小写呢？例如，

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

谢谢!

Answer 1

虽然没有直接支持Not，但您可以使用 partial active pattern .

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]

输出

Green is Not Red
Blue is Not Red