keyword.elm

{-|

    import Json.Decode exposing (..)
    import Json.Decode.Pipeline exposing (..)

    type FooOrBar
        = Foo String
        | Bar Int

    oneOf
        [ succeed Foo
            |> keyword "type" "foo"
            |> required "foo_value" string
        , succeed Bar
            |> keyword "type" "bar"
            |> required "bar_value" int
        ]

-}
keyword : String -> String -> Decoder a -> Decoder a
keyword key value decoder =
    field key string
        |> andThen
            (\v ->
                if v == value then
                    decoder

                else
                    fail
                        ("expected "
                            ++ key
                            ++ " to be "
                            ++ value
                            ++ " but got "
                            ++ v
                        )
            )