Algoritmo de Lunh para calcular CPF e CNPJ. (também segue o mesmo algoritmo reescrito em Haskell. A versão em Haskell tem uma aparência ligeiramente mais simples devido a recursos como guardas, listas de compreensão e ciclos...)

Lunh.elm

import Html exposing (text, p, div)


-- based on https://en.wikipedia.org/wiki/Luhn_algorithm
lunh : ( Int -> Int ) -> Int -> List Int -> List Int
lunh f len nums =
  let
    cycles = ceiling (toFloat len / (toFloat (List.length nums)))
    cycle nums len =
      nums
        |> List.repeat cycles
        |> List.concat
        |> List.take len
        
    cycle1to9 = cycle [2 .. 9]
    lunhSum len nums =
      nums
        |> List.reverse
        |> List.map2 (*) (cycle1to9 len)
        |> List.sum
  in
    if List.length nums == len then nums
    else nums
     |> lunhSum len
     |> \n -> (f n) :: List.reverse nums
     |> List.reverse
     |> lunh f len

-- Mod 11 rule based on https://pt.wikipedia.org/wiki/D%C3%ADgito_verificador#M.C3.B3dulo_11
mod11 : Int -> Int
mod11 total =
  total
   |> \n -> n % 11
   |> \n -> (if n < 2 then 0 else 11 - n)

-- CPF is a Lunh (module 11) of 11 digits
generateCPF : List Int -> List Int
generateCPF = lunh mod11 11

-- CNPJ is a Lunh (module 11) of 14 digits
generateCNPJ : List Int -> List Int
generateCNPJ = lunh mod11 14

main =
  div []
  [
    p [] 
    -- Let's try some CPF and put on the screen
    [ [1,1,1, 2,2,2, 3,3,3]
       |> generateCPF
       |> toString
       |> text
    ]
    , p []
    -- And some CNPJ
    [ [1,8,9,5,7,0,9,0,0,0,0,1]
       |> generateCNPJ
       |> toString
       |> text
    ]
  ]

lunh.hs

{-

Haskell version BASED on my Elm implementation

-}

-- based on https://en.wikipedia.org/wiki/Luhn_algorithm
lunh :: ( Int -> Int ) -> Int -> [Int] -> [Int]
lunh f len nums
  | length nums == len = nums
  | otherwise
    = let
      lunhSum len nums = sum [ y*x | (x,y) <- zip (cycle1to9 11) (reverse nums) ]
      cycle' nums len = take len (cycle nums)
      cycle1to9 = cycle' [2 .. 9]
    in lunh f len
      . reverse
      . (\n -> (f n) : reverse nums)
      . lunhSum len
      $ nums

-- Mod 11 rule based on https://pt.wikipedia.org/wiki/D%C3%ADgito_verificador#M.C3.B3dulo_11
mod11 :: Int -> Int
mod11 total
   = (\n -> if n < 2 then 0 else 11 - n)
   $ total `mod` 11

-- CPF is a Lunh (module 11) of 11 digits
generateCPF :: [Int] -> [Int]
generateCPF = lunh mod11 11

-- CNPJ is a Lunh (module 11) of 14 digits
generateCNPJ :: [Int] -> [Int]
generateCNPJ = lunh mod11 14

main =
  putStrLn . show . generateCPF $ [1, 1, 1, 2, 2, 2, 3, 3, 3]

file extensions confused me for a sec :P

Thanks, I fix it.