@this with async keyword

[nojaf]

I'm struggling to combine @this with an async keyword to return a promise:

type pluginContext
type changeEvent

type p = {
  watch:  @this (pluginContext, string, changeEvent) => promise<unit>
}

let p1 = {
   watch: @this async (pc, name, ev) => {
    ()
  } 
}

Type Errors
[E] Line 9, column 22:
This has type:
    Js_OO.Callback.arity3<(pluginContext, string, changeEvent) => unit>
  But the record field watch is expected to have type:
    Js_OO.Callback.arity3<(pluginContext, string, changeEvent) => promise<unit>>
  
  The incompatible parts:
    unit vs promise<unit>

Playground

My syntax could be off, but I wonder if we support this.
@cristianoc do you have any ideas?

[cristianoc]

Perhaps the @this transformation is breaking async functions. It definitely predates the existence of async.
Looking at the generated parse tree might give some indication.

[nojaf]

Oh interesting, this is part of compiler/frontend/bs_builtin_ppx.ml.
Thanks for the pointer

[nojaf]

It get transformed to:

  structure_item 
    Pstr_value Nonrec
    [
      <def>
        pattern 
          Ppat_var "p1" 
        expression 
          Pexp_record
          [
            "watch" 
              expression 
                Pexp_apply
                expression 
                  Pexp_ident "Js_OO.unsafe_to_method" 
                [
                  <arg>
                  Nolabel
                    expression 
                      Pexp_constraint
                      expression 
                        Pexp_record
                        [
                          "I3" 
                            expression 
                              Pexp_fun
                              arity:3
                              Nolabel
                              None
                              pattern 
                                Ppat_var "pc" 
                              expression 
                                Pexp_fun
                                Nolabel
                                None
                                pattern 
                                  Ppat_var "name" 
                                expression 
                                  Pexp_fun
                                  Nolabel
                                  None
                                  pattern 
                                    Ppat_var "ev" 
                                  expression 
                                    attribute  "res.braces"
                                      []
                                    Pexp_construct "()" 
                                    None
                        ]
                        None
                      core_type 
                        Ptyp_constr "Js_OO.Callback.arity3" 
                        [
                          core_type 
                            Ptyp_any
                        ]
                ]
                transformed_jsx: false
          ]
          None
    ]

Should that be wrapped in

 Pexp_apply
      expression 
        Pexp_ident "Primitive_promise.unsafe_await" 

@cristianoc any thoughts? Or which other syntax can I inspect to view what I'm after?

[cristianoc]

@nojaf see this: #7702

Does the example look reasonable? It would be good to complete it to a full example where a method is actually used, to check that it is called as expected.
I'm not sure that a method constructed with @this can be called directly, but it might be supposed to be only used for bindings.

A separate question: is there some way to use another mechanism instead? After all, it seems that all that's required is a way to access this in some way.

[nojaf]

Hi @cristianoc, thanks, this looks great!

I will test this in my own code to verify it works as expected.
I'm working on a rollup plugin: https://rollupjs.org/plugin-development/#plugin-context
Inside the various build hooks, this should be the plugin context.