F#:解构绑定有歧视的联合 [英] F#: Destructuring bind with a discriminated union

问题描述

open System

let x = (1, 2)
let (p, q) = x
printfn "A %A" x
printfn "B %A %A" p q

let y = Some(1, 2)
try
  let None = y
  ()
with
  | ex -> printfn "C %A" ex
let Some(r, s) = y
printfn "D %A" y
// printfn "E %A %A" r s

• http://ideone.com/cS9bK0
• http://ideone.com/cS9bK0

当我取消最后一行的注释时，编译器将拒绝抱怨的代码

When I uncomment the last line, the compiler rejects the code complaining

/home/rRiy1O/prog.fs(16,19):错误FS0039:未定义值或构造函数'r'
/home/rRiy1O/prog.fs(16,21):错误FS0039:未定义值或构造函数s

/home/rRiy1O/prog.fs(16,19): error FS0039: The value or constructor 'r' is not defined
/home/rRiy1O/prog.fs(16,21): error FS0039: The value or constructor 's' is not defined

在解构let时不允许使用枚举吗?

Is it not allowed to use enumerations in destructuring let?

但是首先，即使当我注释掉最后一行时...我在这里做什么 am ?这是输出:

But first, even when I comment out the last line... what am I doing here? Here's the output:

A (1, 2)
B 1 2
D Some (1, 2)

更新

记录下来，这是固定版本:

Update

For the record, here's the fixed version:

open System

let x = (1, 2)
let (p, q) = x
printfn "A %A" x
printfn "B %A %A" p q

let y = Some(1, 2)
try
  let (None) = y
  ()
with
  | ex -> printfn "C %A" ex
let (Some(r, s)) = y
printfn "D %A" y
printfn "E %A %A" r s

• http://ideone.com/7qL9mH
• http://ideone.com/7qL9mH

输出:

A (1, 2)
B 1 2
C MatchFailureException ("/home/MBO542/prog.fs",10,6)
D Some (1, 2)
E 1 2

完美.

推荐答案

尝试破坏y的方式:

let Some(r, s) = y

您实际上是在定义一个名为Some的函数，该函数具有以元组形式传递的两个参数r和s.

You are actually defining a function named Some, with two arguments, r and s, passed in tupled form.

为正确地进行销毁，您需要添加括号:

For correct destructuring, you need to add parentheses:

let (Some (r, s)) = y

偶然地，在try块内发生了同样的事情:行let None = y创建了一个名为None且等于y的新值.

Incidentally, same thing happens inside the try block: the line let None = y creates a new value named None and equal to y.

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