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...