Stack.elm

import Html exposing (..)
import String
import Lazy.List as LL exposing (LazyList)

type alias Stack = List String


push : String -> Stack -> Stack
push tok stack = 
 (tok :: stack)


pop : Stack -> (Maybe String, Stack)
pop stack = 
 case stack of
   hd :: tl -> (Just hd, tl)
   _ -> (Nothing, [])

stackIterator : Stack -> LazyList String
stackIterator stack =
 LL.iterate (\(mhd, tl) -> pop tl) (pop stack)
  |> LL.map fst
  |> LL.takeWhile (\a -> a /= Nothing)
  |> LL.map (Maybe.withDefault "") -- Default just for theoretical completeness

reverseString: String -> String
reverseString incoming = 
 let 
   stringStack = incoming 
    |> String.split "" 
    |> List.foldl push [] 
 in 
   -- How to use pop() here?
   LL.foldr String.append "" (stackIterator stringStack)


main : Html 
main = 
 "Hello World!"
 |> reverseString
 |> toString 
 |> text