Having flag attributes is cumbersome with current API

[panglesd]

With current API, one needs to create an "empty payload" pattern:

let p = Ast_pattern.(pstr nil)

then register the attribute with a unit continuation:

let attr = Attribute.declare name context p ()

And finally, when getting it, turn the unit option type into a bool type:

match Attribute.get attr x with
    | Some () -> true
    | None -> false

All of this could be simplified with the following API addition:

val declare_flag : string -> 'a Context.t -> ('a, unit) t
val has_flag : ('a, unit) t -> ?mark_as_seen:bool -> 'a -> bool

(reported by @sim642 in ocaml-ppx/ppx_deriving#263 (comment))

[dianaoigo]

@panglesd i would like to work on this issue

[panglesd]

Yes, sure!

Do not hesitate to ask for clarification if something in the issue is obscure!

[dianaoigo]

@panglesd i have forked and cloned the project . looking at the project, would it be a good idea to make the suggested changes in the src/ast_pattern.ml file? I am still learning and understanding more about the ocaml ecosystem.

[panglesd]

Attributes are part of the OCaml syntax, as in:

type t = {
  a : int [@foo "an attribute named foo whose payload is a string"]
}

Ast_pattern is a module to build "pattern-matches of the AST". It is used in several places in ppxlib, mostly to extract payloads in extension nodes and attributes. The documentation on Ast_pattern is available here.

ppxlib allows "specifying" attributes and their payloads, from a name, a context and an ast pattern. This is the declare function. From a declared attribute, one can then extract the value in the payload, using the ast-pattern: this is get.

Here, the issue is about adding support for a common case, where the attributes have no payload, the only thing we are interested in is whether they are present or not. For this, I propose to add the declare_flag and has_flag function. According to you, in which module in the API should they belong?

[dianaoigo]

@panglesd i don't know why i haven't been able to see till i restarted my machine today. At some point i thought you were ignoring me. I was actually caught between the ast_pattern and attrribute model i just wasn't sure which. for now i believe attribute module would be the best way since it's where declaration and retrieval of attributes is done.

[dianaoigo]

@panglesd since i restarted my machine i am having serious problems. now when i run _dune build _ i get the error
File "dune-project", line 6, characters 1-5:
6 | (cram enable)
^^^^
Error: Unknown field cram
and when i try reinstalling the dependencies using _opam install --deps-only --with-test .
_ i get the error
[ERROR] Package conflict!

• Missing dependency:
  • ppxlib < 0.1.0
    no matching version

No solution found, exiting
i am using ubuntu

[dianaoigo]

@pitag-ha i am having some problems. i restarted my machine after almost a week and it must have corrupted everything. now i can't even build or test as shown in the comment above

[panglesd]

Hello @dianaoigo !
First, if you ever feel that I am ignoring you, please ping me. It could happen that I miss one gh notification.
This was not the case here: I thought you were busy working on this or some other project since you did not reply to my clarifications.

The error message when running dune build seems to indicate that dune has a too low version. Reinstalling the dependencies was a good move, but the error you get is strange (I think I already had it once, I think it is due to ppxlib-bench.opam).

You could try:

• Rerunning your install command, after having removed ppxlib-bench.opam (or only this line: https://github.com/ocaml-ppx/ppxlib/blob/main/ppxlib-bench.opam#L17)
• Install dependencies by hand, see the list of them https://github.com/ocaml-ppx/ppxlib/blob/main/ppxlib.opam#L22..L35

Let me know if that unblocks the situation!

[panglesd]

Also, it is strange that this happened after a machine reboot... Maybe you changed the opam switch, or your environment is not up to date with the switch you have chosen? (eval $(opam env))

[dianaoigo]

I uninstalled everything dune, opam ocaml, reinstalled and updated. somehow that solved the issue. also learnt that any dune version below 2.7 will throw some crazy errors. I have been working on a different ocaml project maybe that was the cause. Could you offer a little more insight on how to proceed?

[panglesd]

@dianaoigo I already gave all the generic information I could in #404 (comment) and #404 (comment), in order to help you more I would need a more detailed question. For instance, I need to know what is clear and what is unclear to you, to answer the right thing. There is a nice blog post about asking good questions here that explains that very well!

[sim642]

This should be closed by #408 I think.

[panglesd]

Indeed, thanks!