How can I get the error message out of Http.Error?

Question

I'm attempting to complete an exercise in the Elm 0.17 tutorial for HTTP. If fetching a gif fails, I would like to let the user know why it failed with an error message.

I've modified my model to be:

type alias Model =
  { topic : String
  , gifUrl : String
  , errorMessage : String
  }

And fetch fail in update to be:

FetchFail error ->
  (Model model.topic model.gifUrl (errorMessage error), Cmd.none)

Where the function errorMessage is as follows:

errorMessage : Http.Error -> String
errorMessage error =
  case error of
    Http.Timeout ->
      "Timeout"
    Http.NetworkError  ->
      "Network Error"
    Http.UnexpectedPayload _ ->
      "UnexpectedPayload"
    Http.BadResponse _ _ ->
      "BadResponse"

The above function seems like unnecessary boiler plate to me. Is there a way that I can directly convert Http.Error into a string?

Answer 1

You can convert anything to a string using toString. That will give you nearly the same output as your case statements:

toString Timeout == "Timeout"
toString NetworkError == "NetworkError"
toString (UnexpectedPayload "Error") == "UnexpectedPayload \"Error\"" 
toString (BadResponse 500 "Error") == "BadResponse 500 \"Error\""

Just replace the call to errorMessage with toString and you can get rid of errorMessage entirely:

FetchFail error ->
  (Model model.topic model.gifUrl (toString error), Cmd.none)