Add extra deriving option for constant variant constructors

[patricoferris]

A very common pattern is to want to encode string values as a polymorphic (or not) variant with constant constructors. For example:

module Member = struct 
  type t = [ `Student | `Professor ][@@deriving yaml]
end 

type t = { name : string; member : Member.t }[@@deriving yaml]

The standard [@@deriving yaml] deriver has to deal with the possibility of non-constant constructors, so these constant ones are encode as Student: [] which is annoying because then we have:

let person = { name = "Alice"; member = `Student } 

encoding to the following yaml:

name: Alice
member: 
  Student: []

---

What would be nice is to opt into some string only version that generates the following code:

let of_yaml = function
    | `String "Student" -> Ok `Student
    | `String "Professor" -> Ok `Professor
    | _ -> Error (`Msg "Unknown value ")

let to_yaml = function
  | `Student -> `String "Student"
  | `Duplicate -> `String "Professor"

And additionally an extra little [@value string] to override the default of using the constructor name verbatim.

[desirekaleba]

Hi @patricoferris, Can I be assigned to this issue please?

[pitag-ha]

Hi @desirekaleba, nice that you want to contribute to OCaml! Do you already have OCaml up and running on your computer?

[desirekaleba]

Hi @pitag-ha, Yes I have Ocaml up and running on my computer. I followed this to achieve that.

[pitag-ha]

Hi @pitag-ha, Yes I have Ocaml up and running on my computer. I followed this to achieve that.

Cool! So yes: perfect to start working on an issue now! Which one do you prefer? This one here or the one on ppx_yojson that you've expressed interest in?

[desirekaleba]

I prefer starting with this one here.

[pitag-ha]

Cool. Don't hesitate to ask, if you have any questions.

[desirekaleba]

Hi @pitag-ha,

I used the previous days to skill up my OCaml skills. I am looking at the issue description but I don't fully understand how to deal with it. Can this tool be installed via opam so that I can first have a look at the current behavior?

[pitag-ha]

Hi @desirekaleba,

Yes, the deriver can be installed via opam:

opam install ppx_deriving_yaml

Also, I was just about to explain you how to load the deriver into the OCaml toplevel (in OCaml, we say toplevel to what in other languages is called REPL), but I've just tried myself and there might be a bug. I'll have a closer look soon and let you know. In the meanwhile, you can just set up a project with dune init exe <some_name> and try the deriver in there. Have you already had a look at OCaml up and running?

[desirekaleba]

Hi @pitag-ha,

Yes, I have Ocaml up and running. I also took some time to go through A First Hour with OCaml, although this did not only take an hour.

I now have a clear understanding of Ocaml. I can start a new project, write some code, build it and run the executable.

I am now looking for a way to first test this tool to fully understand how it works.

[patricoferris]

Hi both :))

I just opened PR #30 -- this adds a little contributing guide and also "expect-style tests". The tests in test/expect takes files that uses the deriver and then outputs the generated OCaml code. This might make it easier to start understanding what exactly the deriver does and also help when changing the output to add new features.

[patricoferris]

Also @pitag-ha

Also, I was just about to explain you how to load the deriver into the OCaml toplevel (in OCaml, we say toplevel to what in other languages is called REPL), but I've just tried myself and there might be a bug.

I think there might be (see #22). If you have utop installed (opam install utop) then you should be able to load it in there via dune by executing dune utop at the root of the project.

utop # type t = { name : string } [@@deriving yaml];;
type t = { name : string; }
val to_yaml : t -> [> `O of (string * [> `String of string ]) list ] = <fun>
val of_yaml :
  [> `O of (string * Yaml.value) list ] -> (t, [> `Msg of string ]) result =
  <fun>

[patricoferris]

@pitag-ha just to follow-up, I don't think this is a ppx/ppxlib problem, just a yaml one avsm/ocaml-yaml#60