RandomAccessWriter.fsx

type RowPointer = int
type Writer = Writer of (RowPointer -> RowPointer)
let run (f:Writer) = match f with | Writer f -> f

type Cursor() =
    member this.Bind(m: Writer, f: unit -> Writer) =
        Writer(fun rp ->
            let res = f() |> run
            let newRp = rp |> run m
            res newRp)
    member this.Return (x:Unit) = Writer (fun a -> a)
let cursor = Cursor()

let write content =
    Writer (fun (row:RowPointer) ->
        printfn "WRITING ROW %d: %s" row content
        row + 1)

let x = cursor {
    do! (write "Hello 1")
    do! (write "Hello 2")

    [1..5] 
    |> List.map (fun v ->
        do! (write "Hello 2")
        )
    |> ignore

    return ()
}
(run x) 0