F# - 可變資料

F# 中的變數是不可變的，這意味著一旦變數繫結到一個值，就不能更改。它們實際上編譯為靜態只讀屬性。

下面的例子演示了這一點。

示例

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

編譯並執行程式時，會顯示以下錯誤訊息：

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

可變變數

有時需要更改儲存在變數中的值。為了指定在程式的後面部分可以更改已宣告和賦值變數的值，F# 提供了mutable關鍵字。可以使用此關鍵字宣告和賦值可變變數，其值將發生更改。

mutable關鍵字允許你宣告和賦值可變變數的值。

可以使用let關鍵字為可變變數賦值初始值。但是，要為其賦值新的後續值，需要使用<-運算子。

例如：

let mutable x = 10
x <- 15

下面的例子將闡明這個概念：

示例

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

編譯並執行程式時，將產生以下輸出：

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

可變資料的用途

可變資料經常在資料處理中需要並使用，尤其是在記錄資料結構中。下面的例子演示了這一點：

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()

編譯並執行程式時，將產生以下輸出：

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";}