triggers "error: Found non-type binding for list[U]"

map

fn map[T, U](&list[T] ls, fn(&T t) -> U f) -> list[U] {
  fn p(&T t, &list[U] u) -> &list[U] {
    ret cons[U](f(t), u);
  }
  
  ret list.foldl[T, U](ls, nil, p); 
}