Elm: Type alias for JSON with "type" property

Yes, type is a reserved keyword and cannot be used as a field name in a record.

In Elm and Haskell, the most common thing to do in your case seems to be to append a single quote, so it becomes type', and your type definition becomes

type alias Foo =
  { id: String
  , type': String
  }

This has its origin in the prime symbol from mathematics. It may look odd at first but it is valid syntax.

You can then use the following Json Decoder to translate the JSON into Foo:

fooDecoder =
  Json.object2
    Foo
    ("id" := Json.string)
    ("type" := Json.string)

Notice that the exact field name in Elm does not need to match the JSON field name.

Rarely will you find a language that lets you use keywords as variable names unescaped. This behavior is not unique to Elm.

You can now escape the keyword with an underscore.

ex.

type alias Foo =
  { id: String
  , type_: String
}

In oscpad, I'm using a "type" field in my json interface over websockets. But I don't have a field called type in an elm record. I just look for a "type" field when I'm parsing JSON. My code looks like this:

jsSpec : JD.Decoder Spec
jsSpec =
  ("type" := JD.string) `JD.andThen` jsCs

jsCs : String -> JD.Decoder Spec
jsCs t =
  case t of
    "button" -> SvgButton.jsSpec `JD.andThen` (\a -> JD.succeed (CsButton a))
    "slider" -> SvgSlider.jsSpec `JD.andThen` (\a -> JD.succeed (CsSlider a))
    "label" -> SvgLabel.jsSpec `JD.andThen` (\a -> JD.succeed (CsLabel a))
    "sizer" -> jsSzSpec `JD.andThen` (\a -> JD.succeed (CsSizer a))
    _ -> JD.fail ("unkown type: " ++ t)