[BUG][OCaml] Dependency cycle when generating Stripe's API with the OCaml generator

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The OCaml generator currently generates models in separate files.

This is creating dependency cycles when there's a mutual dependency between two types.

Here's an example from the Stripe API:

(* In src/models/issuing_transaction_authorization.ml *)
type t = {
    ...
    transactions: Issuing_transaction.t list;
    ...
} [@@deriving yojson { strict = false }, show ];;

(* In src/models/issuing_transaction.ml *)
type t = {
    ...
    authorization: Issuing_transaction_authorization.t option [@default None];
    ...
} [@@deriving yojson { strict = false }, show ];;

Building the generated project gives (among other errors):

Error: Dependency cycle between:            
   _build/default/.openapi.objs/openapi__Issuing_transaction_authorization.impl.all-deps
-> _build/default/.openapi.objs/openapi__Issuing_transaction.impl.all-deps
-> _build/default/.openapi.objs/openapi__Issuing_transaction_authorization.impl.all-deps
-> required by
   _build/default/.openapi.objs/byte/openapi__Issuing_transaction_authorization.cmi
-> required by
   _build/install/default/lib/openapi/openapi__Issuing_transaction_authorization.cmi
-> required by _build/default/openapi.install
-> required by alias install
-> required by alias default

openapi-generator version

openapi-generator-cli 6.0.0
  commit : 69f79fb
  built  : 2022-05-26T02:54:15Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

https://github.com/stripe/openapi/blob/master/openapi/spec3.json

Generation Details

After downloading the spec above, I ran:

openapi-generator-cli generate -i spec3.json -g ocaml

Steps to reproduce

After generating the project, I ran dune build.

Related issues/PRs

Not exactly related to the issue above, but I also hit https://github.com/OpenAPITools/openapi-generator/issues/8397, which causes the generation of "empty" types when generating a project for the Stripe API, e.g.

(* In src/models/business_profile_specs_support_url.ml *)
type t = {
} [@@deriving yojson { strict = false }, show ];;

let create () : t = {
}

Which fails compilation with

File "src/models/business_profile_specs_support_url.ml", line 9, characters 0-1:
9 | } [@@deriving yojson { strict = false }, show ];;
    ^
Error: Syntax error

Suggest a fix

I'm not familiar with openapi-generator implementation, but if possible, a fix for it would be to generate all the models in the same file and use recursive modules.

The example above could look like this:

(* In src/models.ml *)
module rec Issuing_transaction_authorization = struct
    type t = {
        ...
        transactions: Issuing_transaction.t list;
        ...
    } [@@deriving yojson { strict = false }, show ];;
end
and Issuing_transaction = struct
    type t = {
        ...
        authorization: Issuing_transaction_authorization.t option [@default None];
        ...
    } [@@deriving yojson { strict = false }, show ];;
end

Reproduce the issue with the Stripe spec using openapi-generator-cli generate -i spec3.json -g ocaml, then run dune build on the generated project. Start by tracing how the OCaml generator writes the separate files under src/models/ and how mutually dependent types are represented. Done means the generated project builds without the reported dependency cycle.