counts.elm

counts : List comparable -> List (comparable, Int)
counts items =
  let
    counts' items =
      case items of
        [] -> []
        
        (x, nx) :: [] -> [(x, nx)]
        
        (x, nx) :: (y, ny) :: rest ->
          if x == y then counts' ((x, nx + ny) :: rest)
          else (x, nx) :: counts' ((y, ny) :: rest)
  in
    List.sort items
    |> List.map (\x -> (x, 1))
    |> counts'