By default, all of the data types and values in F# are immutable, which means that they cannot be changed throughout a program's execution.
The simplest mutable variables in F# are declared using the mutable keyword. You can assign an initial value to a mutable variable by using the let keyword in the same way as you would define a value.
let mutable x = 1
x <- x + 1
The <- operator is used to assign a mutable variable a new value. The mutable keyword is frequently used with record types to create mutable records, as shown in the following example
type productData =
{ ID : int;
mutable IsAvailable : bool;
mutable Description : string; }
let getProduct id =
{ ID = id;
IsAvailable = false;
Description = null; }
let availableProducts (products : productData list) =
products |> List.iter(fun st ->
st.IsAvailable <- true
st.Description <- sprintf "In Stock "
Threading.Thread.Sleep(1000) (* Putting thread to sleep for 1 second to simulate processing overhead. *))
let printData (products : productData list) =
products |> List.iter (fun x -> printfn "%A" x)
let main() =
let products = List.init 4 getProduct
printfn "Before Product Availability:"
printData products
printfn "After Product Availability:"
availableProducts products
printData products
Console.ReadKey(true) |> ignore
main()
It will print the following data on the console.
Before Product Availability:
{ ID = 0
IsAvailable = false
Description = null }
{ ID = 1
IsAvailable = false
Description = null }
{ ID = 2
IsAvailable = false
Description = null }
{ ID = 3
IsAvailable = false
Description = null }
After Product Availability:
{ ID = 0
IsAvailable = true
Description = "In Stock " }
{ ID = 1
IsAvailable = true
Description = "In Stock " }
{ ID = 2
IsAvailable = true
Description = "In Stock " }
{ ID = 3
IsAvailable = true
Description = "In Stock " }