Runtime exception when empty header name is given

[ymtszw]

• elm 0.19.0
• elm/http@2.0.0
• Chrome 71.0.3578.80

Steps to reproduce:

• Put header "" "something" in headers list for any Http request

module Main exposing (main)

import Browser
import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)
import Http

type alias Model =
    {}

init : () -> ( Model, Cmd Msg )
init _ =
    ( {}
    , Http.request
        { method = "GET"
        , headers = [ Http.header "" "something" ]
        , url = "https://www.example.com"
        , body = Http.emptyBody
        , expect = Http.expectWhatever (always NoOp)
        , timeout = Nothing
        , tracker = Nothing
        }
    )

type Msg
    = NoOp

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        NoOp ->
            ( model, Cmd.none )

view : Model -> Html Msg
view model =
    div [] []

main : Program () Model Msg
main =
    Browser.element
        { init = init
        , view = view
        , update = update
        , subscriptions = always Sub.none
        }

Result (runtime exception)

Uncaught SyntaxError: Failed to execute 'setRequestHeader' on 'XMLHttpRequest': '' is not a valid HTTP header field name.

On the other hand, empty "value" does not cause exception.

Expected behavior/Proposal

What I actually wanted was, conditionally adding/not adding a Http.header based on a variable like so:

...
, headers =
    [ case fooBar of
        Foo str ->
            Http.header "foo" str
        Bar ->
            Http.header "" ""
    , ...
    ]
...

but as it turned out it ends up in a runtime exception.

• At least we should not leak runtime exception, so Http.header "" "something" should either:
  • not compile
  • not try to setRequestHeader() at runtime (effectively none for header)

The latter could be useful since it can be used paired with if or case inside Lists, like I wanted to do.

As for my intent, for now we can just wrap it in a List for the same effect:

...
, headers =
    case fooBar of
        Foo str ->
            [ Http.header "foo" str ]
        Bar ->
            []
    , ...
    ]
...