This is a rewrite of the example chapter 2 of Elm by Example that is compatible with Elm 0.15 and 2.1.0 of the core libraries.

aaarrrggghhh.elm

module Fibonacci where

import List exposing ((::), head, map2, reverse, tail)
import Maybe exposing (andThen)

fibonacci : Int -> List Int
fibonacci n =
  let fibonacci' n acc =
        if n <= 2
            then acc
            else
              case ((head acc), ((tail acc) `andThen` head)) of
                (Just n', Just n'') -> fibonacci' (n-1) ((n'+n'') :: acc)
                _                   -> []
    in fibonacci' n [1,1] |> reverse