.net - 将标识符模式与 `as` 模式相结合

Question

如何将标识符模式的结果复制到 as 模式以制作元组？

我的问题很困惑，所以我创建了一个例子，我想打印这个人的信息，他是老师还是学生:

type Person =
    | Teacher of name: string * age: int * classIds: int list
    | Student of name: string

let printTeacher (name, age, classIds) =
    printfn "Teacher: %s; Age: %d; Classes: %A" name age classIds

let print = function
    | Teacher (name, age, classIds) -> printTeacher (name, age, classIds)
    | Student name -> printfn "Student: %s" name

匹配模式很长且重复:

| Teacher (name, age, classIds) -> printTeacher (name, age, classIds)

所以我尝试使用 as 让它更短模式，但失败:

| Teacher ((_, _, _) as teacher) -> printTeacher teacher

因为上面 teacher有 Person类型，而不是 string*int*int list .在不改变 printTeacher 的情况下，我应该怎么做才能获得更短的模式？类型签名 string*int*int list -> unit ?

Answer 1

我能想到的一种方法是更改​​ Teacher 的定义构造函数:

type Person =
    | Teacher of items: (string * int * int list)
    | Student of name: string

let printTeacher (name, age, classIds) =
    printfn "Teacher: %s; Age: %d; Classes: %A" name age classIds

let print = function
    //| Teacher (name, age, classIds) -> printTeacher (name, age, classIds) // Still works
    | Teacher items -> printTeacher items
    | Student name -> printfn "Student: %s" name

通过更改 Teacher要采用显式元组，您可以按名称引用它，但另一种方式仍然有效。

但是，您失去了为元组项目命名的功能。

如果您不想或不能更改您的类型定义，另一种方法是为教师构造函数引入一个事件模式:

type Person =
    | Teacher of name: string * age: int * classIds: int list
    | Student of name: string

// Active pattern to extract Teacher constructor into a 3-tuple.
let (|TeacherTuple|_|) = function
| Teacher (name, age, classIds) -> Some (name, age, classIds)
| _ -> None

let printTeacher (name, age, classIds) =
    printfn "Teacher: %s; Age: %d; Classes: %A" name age classIds

let print = function
    | TeacherTuple items -> printTeacher items
    | Student name -> printfn "Student: %s" name
    // To make the compiler happy. It doesn't know that the pattern matches all Teachers.
    | _ -> failwith "Unreachable."