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, Maybe Stack)
pop stack = 
 let 
   top = List.head stack 
 in 
 (top, List.tail stack)

stackIterator : Stack -> LazyList String
stackIterator stack =
  let nextOf stack =
    case pop stack of
      (Just h, Just t) -> LL.cons h (nextOf t)
      (Just h, Nothing) -> LL.cons h LL.empty
      (Nothing, _) -> LL.empty
  in
    nextOf stack

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