A simple append function like this (in F#):

let rec app s t =
   match s with
      | [] -> t
      | (x::ss) -> x :: (app ss t)

will crash when s becomes big, since the function is not tail recursive. I noticed that F#'s standard append function does not crash with big lists, so it must be implemented differently. So I wondered: How does a tail recursive definition of append look like? I came up with something like this:

let rec comb s t =
   match s with
      | [] -> t
      | (x::ss) -> comb ss (x::t)
let app2 s t = comb (List.rev s) t 

which works, but looks rather odd. Is there a more elegant definition?