Bug in elm-lang/core `Dict.merge`?

README.md

This is an example of what I believe is a bug in core's Dict.merge implementation.

To reproduce

• clone the repo
• elm package install elm-lang/html
• elm reactor
• visit http://localhost:8000/Main.elm

Expected outcome

Dict should contain both u1 and u2 keys.

Actual outcome

Dict only contains u1 key.

Discussion

Am I missing something here? It's the first time I've used Dict.merge. I note that there are no tests in core for this function

What am I doing now?

I've pulled the function in locally to the module and am adding debugging statements to see if I can determine where things go south.

Main.elm

module Main exposing (..)

import Html.App as App
import Html exposing (Html, text)
import Dict exposing (Dict)


type alias Model =
    ()


type alias Msg =
    ()


existsInBoth : comparable -> List a -> List a -> Dict comparable (List a) -> Dict comparable (List a)
existsInBoth key leftValue rightValue dict =
    Dict.insert key (leftValue ++ rightValue) dict


magicMerge : Dict comparable (List a) -> Dict comparable (List a) -> Dict comparable (List a)
magicMerge left right =
    Dict.merge Dict.insert existsInBoth Dict.insert left right Dict.empty


init : ( Model, Cmd Msg )
init =
    ( (), Cmd.none )


update : Msg -> Model -> ( Model, Cmd Msg )
update _ _ =
    ( (), Cmd.none )


subscriptions : Model -> Sub Msg
subscriptions _ =
    Sub.none


view : Model -> Html Msg
view model =
    let
        -- this is input that behaves as desired...not important here, but perhaps as justification for the use of merge in the first place
        -- result =
        --     magicMerge (Dict.empty |> Dict.insert "a" [ 1 ]) (Dict.empty |> Dict.insert "a" [ 2 ] |> Dict.insert "b" [ 3 ])
        result =
            magicMerge (Dict.empty |> Dict.insert "u1" [ 1 ])
                (Dict.empty |> Dict.insert "u2" [ 1 ])
    in
        text <| toString <| result


main : Program Never
main =
    App.program
        { init = init
        , update = update
        , subscriptions = subscriptions
        , view = view
        }