Document decoding recursive structures

[mlms13]

This may very well already work perfectly. If that's the case, we should just document it and maybe add a test. If it doesn't work, we should make it possible.

The Elm example uses nested comments as their recursive structure. They have a special helper function (lazy), but that might be because their decoders have a newtype wrapper, while ours are functions (and might be more naturally lazy as a result?).

[mlms13]

I haven't added it to the docs yet, but it's definitely possible without modifications:

module Comment = {
  type t = {
    text: string,
    author: string,
    timestamp: Js.Date.t,
    likes: int,
    replies: list(t),
  };

  let make = (text, author, timestamp, likes, replies) => {
    text,
    author,
    timestamp,
    likes,
    replies,
  };

  let rec decode = json =>
    Decode.Pipeline.(
      pure(make)
      |> field("text", string)
      |> field("author", string)
      |> field("timestamp", date)
      |> field("likes", intFromNumber)
      |> field("replies", list(decode))
      |> run(json)
    );
};

[mlms13]

I'm going to wait for 2.x to document this, because it's easiest to understand the need for this in the context of record types, and record decoding will change quite a bit with the deprecation of Pipeline and the addition of letops.