recursive fibo demo app

Fibo.elm

-- Elm 0.19.0
import Html exposing (..)
import Html.Events exposing (..)
import Html.Attributes exposing (..)
import Browser

main =
  Browser.sandbox { init = init, view = view, update = update }

type alias Model
  = { input: Int
    , output: Int
    , previousInput: Int
    }

init =
  { input = 0
  , output = 0
  , previousInput = 0
  }

type Msg
  = SendInput String
  | Submit

isValid model =
  model.input < 40

update msg model =
  case msg of
    SendInput newInput ->
      case String.toInt newInput of
        Just n ->
          { model | input = n }

        Nothing ->
          { model | input = 0 }

    Submit ->
      if isValid model then
        { model | output = fibo model.input, previousInput = model.input }
      else
        { model | output = 0, previousInput = 0 }

view model =
  div [] [ makeOutput model
         , br [] []
         , input [ placeholder "n", onInput SendInput ] []
         , button [ onClick Submit ] [ text "submit" ]
         ]

makeOutput model =
  if isValid model then
    text ("fibo(" ++ (String.fromInt model.previousInput) ++ ") = " ++ (String.fromInt model.output))
  else
    text "Input must be less than 40!"

fibo n =
  case n of
    0 ->
      0

    1 ->
      1

    _ ->
      fibo (n-1) + fibo(n-2)