missing standard library functions for the Nix language

stdlib.nix

with builtins;
{

  every = n: xs:
    # take every `n`-th element from the list `xs`
    if n < 2 then xs else
    if xs == [] then [] else
    if length (tail xs) < n - 1 then [ (head xs) ] else
    [ (head xs) ] ++ every (apply-n-times tail xs n) n
  ;

  apply-n-times = n: f: arg: 
    if n < 1
      then arg
      else apply-n-times (n - 1) f (f arg)
  ;
  
  # the following does not work:
  #
  # iterate = f: x:
  #   [ x ] ++ iterate f (f x)
  # ;
  #
  # it produces a segmentation fault when evaluating a finite subset:
  #
  # head (iterate (x: x + 1) 0)

}