Elm: Download Json with error handling

main.elm

import Http
import Markdown
import Html exposing (Html, div, text)
import Task exposing (Task, andThen)
import Json.Decode as Json exposing (..)

type alias ValWithErr = Result String Int

main : Signal Html
main =
  Signal.map view readme.signal

-- set up mailbox
--   the signal is piped directly to main
--   the address lets us update the signal
readme : Signal.Mailbox ValWithErr
readme =
  Signal.mailbox (Result.Ok 0)

report : ValWithErr -> Task x ()
report val =
  Signal.send readme.address val

get : Task Http.Error ValWithErr
get = Http.get dec1 readmeUrl

-- Decoder for get
dec1 : Decoder ValWithErr
dec1 = Json.map Result.Ok ("tag" := int)

-- Error handling specifics
safeGet : Task x ValWithErr
safeGet =
  get `Task.onError` (\err -> Task.succeed (errorMapper err))

errorMapper : Http.Error -> ValWithErr
errorMapper err =
    case err of
        Http.UnexpectedPayload s -> Result.Err s
        otherwise -> Result.Err "http error"

-- VIEW
view : ValWithErr -> Html
view msg =
    case msg of
        Result.Ok i  -> div [] [ text <| toString i ]
        Result.Err m -> div [] [ text m ]

-- get the readme *and then* send the result to our mailbox
port fetchReadme : Task Http.Error ()
port fetchReadme =
    safeGet `Task.andThen` report


-- the URL of Json
-- {"tag": 123} or {"tag": 1x23}

readmeUrl : String
readmeUrl =
  "http://127.0.0.1:5000/micro.json"