F#中的变量是不可变的,这意味着变量一旦绑定到值,就无法更改,它们实际上被编译为静态只读属性。
下面的示例演示了这一点。
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
编译并执行程序时,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,为了指定程序后面部分中声明和分配的变量的值可能有变化,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分配值。
let mutable x=10 x <- 15
可变变量示例
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
编译并执行程序时,将产生以下输出-
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
可变数据使用
可变数据通常是需要的,并在数据处理中使用,尤其是在记录数据结构中。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()
编译并执行程序时,将产生以下输出-
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";}