Allow string (de)serialization

[seliopou]

Supporting string (de)serialization for types that do not support yojson functions may help library authors make use of ppx_deriving_yojson. This could use the existing convention of labeling a type with [@encoding string] to do string serialization instead of yojson serialization for large numbers. For example consider the (pseudo) code below:

module A : sig
  (* This module comes from package a *)
  type t
  val to_string : t -> string
  val of_string : string -> t
end

module B : sig
  (* This module comes from package b *)
  type t [@@deriving yojson]
end

module C : sig
  (* Somebody's trying to write this module in package c, with a and b as dependencies *)
  type t =
    | A of (A.t [@encoding `string])
    | B of B.t
    [@@deriving yojson]
end

The effect here would be that C.to_yojson would encode a B variant normally, but would encode an A variant with the leading "A" tag as usual, while using A.to_string to encode the value that variant contains.

I was looking into implementing this but thought to solicit feedback before proceeding much more. Thoughts?

[hcarty]

This would be quite useful. int and float versions as well. I've used each of these in manual form when working with external libraries.

[seliopou]

To add to my original comment (which I made a few edits to for readability), the most recent time I came across this was while I was using the ipaddr library. Specifically I wanted to serialize an Ipaddr.V4.Prefix.t. Under the hood this is an int32 * int, but for serialization purposes the x.x.x.x/x would satisfactory, if not preferable.

Of course doing the type aliasing and writing the (de)serializers manually would be very simple, but this use-case may warrant special support from the library.

[whitequark]

I would accept a PR implementing that.