How can I implement a tail-recursive list append?

Question

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

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

will

Answer 1

From a quick glance at the F# sources, it seems the tail is internally mutable. A simple solution would be to reverse the first list before consing its elements to the second list. That, along with reversing the list, are trivial to implement tail recursively.