有沒有辦法將調用嵌套到F＃活動模式？

Question

有沒有將調用嵌套到活動模式的方法？

事情是這樣的：

type Fnord = 
| Foo of int 

let (|IsThree|IsNotThree|) x = 
    match x with 
    | x when x = 3 -> IsThree 
    | _ -> IsNotThree 

let q n = 
    match n with 
    | Foo x -> 
    match x with 
    | IsThree -> true 
    | IsNotThree -> false 
    // Is there a more ideomatic way to write the previous 
    // 5 lines? Something like: 
// match n with 
// | IsThree(Foo x) -> true 
// | IsNotThree(Foo x) -> false 

let r = q (Foo 3) // want this to be false 
let s = q (Foo 4) // want this to be true 

或者是跟其他比賽對手的首選方式去？

Answer 1

它的工作原理。你只是把圖案倒過來。

type Fnord = 
| Foo of int 

let (|IsThree|IsNotThree|) x = 
    match x with 
    | x when x = 3 -> IsThree 
    | _ -> IsNotThree 

let q n = 
    match n with 
    | Foo (IsThree x) -> true 
    | Foo (IsNotThree x) -> false 

let r = q (Foo 3) // want this to be true 
let s = q (Foo 4) // want this to be false