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Variables in F# are immutable, which means once a variable is bound to a value, it can’t be changed. They are actually compiled as static read-only properties.
The following example demonstrates this.
Example
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
When you compile and execute the program, it shows the following error message −
Duplicate definition of value ''x'' Duplicate definition of value ''Y'' Duplicate definition of value ''Z''
Mutable Variables
At times you need to change the values stored in a variable. To specify that there could be a change in the value of a declared and assigned variable in later part of a program, F# provides the mutable keyword. You can declare and assign mutable variables using this keyword, whose values you will change.
The mutable keyword allows you to declare and assign values in a mutable variable.
You can assign some initial value to a mutable variable using the let keyword. However, to assign new subsequent value to it, you need to use the <- operator.
For example,
let mutable x = 10 x <- 15
The following example will clear the concept −
Example
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
When you compile and execute the program, it yields the following output −
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
Uses of Mutable Data
Mutable data is often required and used in data processing, particularly with record data structure. The following example demonstrates this −
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()
When you compile and execute the program, it yields the following output −
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";}
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