The mutable variable 'i' is used in an invalid way.?

Question

I am attempting to write some simple code in F#, and i get this error:

Error   1   The mutable variable \'i\' is used in an invalid way. Mutable variables ma         


        

Answer 1

You can't refer to mutables inside a closure, and that includes such constructs as seq{} and async{} blocks.

You could write

let printProcess = async {
        let i = ref 1
        while true do
            System.Console.WriteLine(!i)
            i := !i + 1
    }

See this blog for some discussion.