F# unfold without tuples

unfoldf.fsx

// Seq.unfold (https://msdn.microsoft.com/en-us/library/ee340363.aspx)
// works with a generator function that gets a state and returns
//   Some (element, state) to generate another iterator, or
//   None to stop the iteration.
// The problem is that tuples are allocated every single iteration.
//
// The solution below uses instead a generator function that gets
// a state and an iterator function. The iterator function can be called
// to yield a new element and set a new state for the next iterator. If the
// generator function returns a unit, i.e. (), the iteration stops.
//
// See example at the end.

let unfoldf<'T, 'State> (f: 'State -> ('T -> 'State -> unit) -> unit)
                        (state: 'State) =
    let mutable _state = state
    let mutable _element = None
    let iterFunc (e: 'T) (s: 'State) =
      _element <- Some e
      _state <- s

    f _state iterFunc
    seq {
      while _element.IsSome do
        yield _element.Value
        _element <- None
        f _state iterFunc
    }

// Example:
// Start with 0 as the initial state and yield its string representation,
// incrementing the state until it reaches 100.
let iter =
  unfoldf (fun state iter ->
             if state < 100
             then iter (sprintf "%d" state) (state + 1)
             else () // this part can be omitted
          ) 0

for x in iter do printfn "%s" x