output.txt

$ elm repl
---- elm-repl 0.17.1 -----------------------------------------------------------
 :help for help, :exit to exit, more at <https://github.com/elm-lang/elm-repl>
--------------------------------------------------------------------------------
> import Main
> import Combine
> Combine.parse Main.comment "foo"
TypeError: Cannot read property 'ctor' of undefined

ProducesTypeError.elm

import Combine exposing (..)
import Combine.Char exposing (..)
import String
import Char

flattenStringList : List String -> String
flattenStringList =
  List.foldr (++) ""

comment : Parser String
comment =
  map flattenStringList <| many cContent

foo : Parser String
foo =
  string "foo"

cContent : Parser String
cContent =
  (map String.fromChar cText) `or` comment

cText : Parser Char
cText =
  char 'a'

ProducesTypeErrorWithUI.elm

import Combine exposing (..)
import Combine.Char exposing (..)
import String
import Char
import Html exposing (..)
import Html.App as Html
import Html.Events exposing (onClick, onInput)
import Html.Attributes exposing (..)

-- MODEL
type alias Model =
  { email : String -- user input
  , validity : Maybe Bool -- Nothing before we checked it
  , validationErrors : List String
  }
  
model = {
    email = ""
  , validity = Nothing
  , validationErrors = []
  }

flattenStringList : List String -> String
flattenStringList =
  List.foldr (++) ""

-- UPDATE

type Msg = SetEmail String | Validate

update : Msg -> Model -> Model
update msg model =
  case msg of
    SetEmail e -> { model | email = e }
    Validate   -> { model | validity = Just (validateEmail model.email) }


validateEmail : String -> Bool
validateEmail e =
  case parse cContent e of
    (Ok  _, _) -> True
    (Err _, _) -> False

-- VIEW
showMaybeValidity : Maybe Bool -> Html Msg
showMaybeValidity mev =
  case mev of
    Nothing -> div [] []
    Just ev ->
      case ev of
        True -> div [] [ text "valid" ]
        False -> div [] [ text "invalid" ]

view : Model -> Html Msg
view model =
  div []
    [
      input [ type' "text"
            , placeholder "user@example.org"
            , onInput SetEmail
      ] []
    , p [] []
    , button [ onClick Validate ] [ text "validate" ]
    , showMaybeValidity model.validity ]

foo : Parser String
foo =
  string "foo"

cContent : Parser String
cContent =
  cText `or` cContent

cText : Parser String
cText =
  string "a"

main =
  Html.beginnerProgram { model = model, view = view, update = update}

WorksAsDesigned.elm

import Combine exposing (..)
import Combine.Char exposing (..)
import String
import Char

flattenStringList : List String -> String
flattenStringList =
  List.foldr (++) ""

comment : Parser String
comment =
  map flattenStringList <| many cContent

foo : Parser String
foo =
  string "foo"

cContent : Parser String
cContent =
  (map String.fromChar cText) `or` foo

cText : Parser Char
cText =
  char 'a'