cryptosense / ppx_factory Goto Github PK

As soon as #13 gets merged ppx_factory will silently ignore derivations from abstract types in order to be compatible with ppx_import.

This is because ppxlib doesn't expose the tool_name required to only adopt this behaviour when called from ocamldep.

Once this is available to the deriver, we should bring back the error in other contexts.

There's an existing issue about this in ppxlib: ocaml-ppx/ppxlib#61

We need to be able to derive default values from variant types.
We should at least handle non parametrized variant types.

In the case of parametrized variant types we'd ideally like to be able to detect whether a generic default can be derived and do so or fail otherwise.

Ie in this example code

type ('a, 'b) t =
  | A of 'a
  | B of 'b
[@@deriving default]

type ('a, 'b) u =
  | A of 'a
  | B of 'b
  | C
[@@deriving default]

We should be able to derive the following default for u

let default = C

but fail on t

We should derive a default for (a, b) result types if we can derive a default for at least one of a or b.

Atm [@@deriving factory] doesn't work with ppx_import.

Ppx-es are called twice, first with ocamldep to resolve dependencies of a module, including dependencies of the generated code, and then with the actual compiler.

Because ppx_import needs bits of information from .cmi files, when called with ocamldep it can't expand completely and thus only expands into an abstract type. For example the following code:

type t = [%import: A.t]

is expanded as:

type t = A.t

during the first phase and as:

type t = A.t = ...

during the second.

Because we don't derive factories for abstract types, using both ppx_import and ppx_factory will make the build fail during that first phase.

In order for it to work, we'd ideally need to have a special case when called from ocamldep so that abstract types are silently "skipped" and only trigger an error during that second phase.

See ocaml/dune#1861 for details.

I'm not super familiar with the polymorphic variants AST representation and there seem to be quite a lot of cases but at least handling the basic ones should do.

The fact that default is a value makes it static. In some cases, like generating random values and incrementing IDs, it would be practical to customize default so that it can return a different value for a given factory function (I guess it could even be the default behavior, but it's not my issue here).

This can be worked around by creating factory functions manually but I think this defeats the purpose of this PPX.

So, would it make sense to be able to define default as function?

let default () =
  ...