ppx_deriving_qcheck cannot derive a generator from a recursive type declaration with containers

[Invizory]

Minimal reproducible example:

type t = Box of t list [@@deriving qcheck]

ppx_deriving_qcheck produces the following generator with unbound gen:

let gen = QCheck.Gen.map (fun gen0 -> Box gen0) (QCheck.Gen.list gen)

This trick (given -rectypes is enabled) make it derive the following generator:

type 'a open_t = Box of 'a list
and t = t open_t
[@@deriving qcheck]

let rec gen_open_t gen_a =
  QCheck.Gen.map (fun gen0 -> Box gen0) (QCheck.Gen.list gen_a)
and gen_sized n = gen_open_t (gen_sized (n / 2))
let gen = QCheck.Gen.sized gen_sized

However, this generatior is non-terminating.

[jmid]

Thanks for the report!

Ping @vch9 - I think we are not handling type constructors correctly in is_rec_typ

[vch9]

Thanks for the report again.

Definitely a problem in is_rec_type, t should not be considered recursive (in the latter). I will try to have a look

[vch9]

I think the deriver fails on a very much simpler example: type t = t list. I did not consider the rectypes flag as I just discovered it 😅.

It creates:

let gen = QCheck.Gen.list gen
let arb = QCheck.make @@ gen

Should we be smarter than:

let rec gen = QCheck.Gen.list gen
let arb = QCheck.make @@ gen

?

[jmid]

IMO using rectypes is a cornercase that we don't necessarily have to support (I read the report as trying to use it as a workaround).

I suggest prioritising the reported case type t = Box of t list of "vanilla OCaml usage" where it seems we don't identify an occurrence of t as recursive when it is a type parameter to a type constructor such as list (or option or result ...) 🤔

In trying out a potential workaround I just experienced the following error variant:

utop # #require "ppx_deriving";;
utop # #require "ppx_deriving_qcheck";;
utop # type t = Box of foo
       and foo = t list [@@deriving qcheck];;
Error: Unbound value n

[7h3kk1d]

I'm not sure but I think this is related. I'm having an issue where I can't derive a generator in a mutually recursive datatype with equivalent constructors:

[@deriving qcheck]
type conflicting_constructor =
  | A
  | B(conflicting_constructor2)
and conflicting_constructor2 =
  | B(conflicting_constructor)
  | C;

Error: This expression has type
       conflicting_constructor QCheck.Gen.t =
         Random.State.t -> conflicting_constructor
     but an expression was expected of type
       conflicting_constructor2 QCheck.Gen.t =
         Random.State.t -> conflicting_constructor2
     Type conflicting_constructor is not compatible with type
       conflicting_constructor2