Immutability

The default behavior of F# when using the let keyword is an immutable value. The difference between this and a variable is that the value is not possible to change. With this, there are the following benefits:

• It encourages a more declarative coding style
• It discourages side effects
• It enables parallel computation

The following is an example implementation of the String.Join function using a mutable state:

// F# implementation of String.Join
let join (separator : string) list = 
    // create a mutable state
    let mutable result = new System.Text.StringBuilder()
    let mutable hasValues = false
        
    // iterate over the incoming list
    for s in list do
        hasValues <- true
        result
            .Append(s.ToString())
            .Append(separator)
            |> ignore
            
    // if list hasValues remove last separator
    if hasValues then
        result.Remove(result.Length - separator.Length, separator.Length) |> ignore

    // get result...