F# - 可变数据

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F# 中的变量是不可变的，这意味着一旦变量绑定到值，就无法更改。它们实际上被编译为静态只读属性。

以下示例演示了这一点。

例子

let x = 10
let y = 20
let z = x + y

printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

let x = 15
let y = 20
let z = x + y

printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

当您编译并执行该程序时，它显示以下错误消息 -

Duplicate definition of value 'x'
Duplicate definition of value 'Y'
Duplicate definition of value 'Z'

可变变量

有时您需要更改存储在变量中的值。为了指定在程序的后续部分可以更改声明和分配的变量的值，F# 提供了mutable关键字。您可以使用此关键字声明和分配可变变量，您将更改其值。

mutable关键字允许您在可变变量中声明和赋值。

您可以使用let关键字为可变变量分配一些初始值。但是，要为其分配新的后续值，您需要使用<-运算符。

例如，

let mutable x = 10
x <- 15

下面的例子将阐明这个概念 -

例子

let mutable x = 10
let y = 20
let mutable z = x + y

printfn "Original Values:"
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

printfn "Let us change the value of x"
printfn "Value of z will change too."

x <- 15
z <- x + y

printfn "New Values:"
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

当您编译并执行该程序时，它会产生以下输出 -

Original Values:
x: 10
y: 20
z: 30
Let us change the value of x
Value of z will change too.
New Values:
x: 15
y: 20
z: 35

可变数据的使用

在数据处理中，尤其是记录数据结构中，经常需要和使用可变数据。以下示例演示了这一点 -

open System

type studentData =
   { ID : int;
      mutable IsRegistered : bool;
      mutable RegisteredText : string; }

let getStudent id =
   { ID = id;
      IsRegistered = false;
      RegisteredText = null; }

let registerStudents (students : studentData list) =
   students |> List.iter(fun st ->
      st.IsRegistered <- true
      st.RegisteredText <- sprintf "Registered %s" (DateTime.Now.ToString("hh:mm:ss"))

      Threading.Thread.Sleep(1000) (* Putting thread to sleep for 1 second to simulate processing overhead. *))

let printData (students : studentData list) =
   students |> List.iter (fun x -> printfn "%A" x)

let main() =
   let students = List.init 3 getStudent

   printfn "Before Process:"
   printData students

   printfn "After process:"
   registerStudents students
   printData students

   Console.ReadKey(true) |> ignore

main()

当您编译并执行该程序时，它会产生以下输出 -

Before Process:
{ID = 0;
IsRegistered = false;
RegisteredText = null;}
{ID = 1;
IsRegistered = false;
RegisteredText = null;}
{ID = 2;
IsRegistered = false;
RegisteredText = null;}
After process:
{ID = 0;
IsRegistered = true;
RegisteredText = "Registered 05:39:15";}
{ID = 1;
IsRegistered = true;
RegisteredText = "Registered 05:39:16";}
{ID = 2;
IsRegistered = true;
RegisteredText = "Registered 05:39:17";}