Recursive implementation of a zip function in F#
Based upon my comment on that answer I've toyed a little to make a recursive sequence expression with roughly the same performance (on my computer at least) as Seq.zip

let zipRec (s1: _ seq) (s2: _ seq) =
  use e1 = s1.GetEnumerator ()
  use e2 = s2.GetEnumerator ()

  let rec loop hasMoved1 hasMoved2 = seq {
    if hasMoved1 &amp;&amp; hasMoved2 then
      yield e1.Current, e2.Current
      yield! loop (e1.MoveNext ()) (e2.MoveNext ())
  }

  loop (e1.MoveNext ()) (e2.MoveNext ())


without the recursive requirement it would be simpler this way

let zipRec (s1: _ seq) (s2: _ seq) = seq {
  use e1 = s1.GetEnumerator ()
  use e2 = s2.GetEnumerator ()

  while e1.MoveNext () &amp;&amp; e2.MoveNext () do
    yield e1.Current, e2.Current
}