at

at.fsx

//let rec at (pos: int) (l: 'a list) : 'a Option = 
//    if pos - 1 > l.Length - 1 then
//        None
//    else if pos = 1 then
//        Some(l.Head)
//    else
//        at (pos - 1) l.Tail

let rec at pos = function
    | [] -> None
    | x::xs -> if pos = 1 then Some x else at (pos - 1) xs

Some "c"  = (at 3 [ "a" ; "b"; "c"; "d"; "e" ]);;
Some "e" = at 5 [ "a" ; "b"; "c"; "d"; "e" ];;
None = at 3 [ "a" ];;