passing a callback without generating an arrow function

[joprice]

I'm binding to the sqlite3 library and have a member that takes a callback:

 abstract member all: sql: string * ``params``: obj * ?callback: (exn option -> ResizeArray<obj> -> unit) ->

This callback gets a this of type Statement, which I'd like to be able to interact access via jsThis<Statement>, but can't seem to get fable to generate anything other than an arrow function at the call site. Is it possible via an annotation or helper to force a function to not be an arrow?

[MangelMaxime]

I can't remember ways of forcing flat arrows with Fable.

One way of working around your problem, is by using Emit:

type MyType =
    [<Emit(
    """
    $0.all(function(error, rows) {
        return $1(this, error, rows)
    })
    """)>]
    abstract member all2: callback: (obj -> exn option -> ResizeArray<obj> -> unit) -> unit

let myType = unbox<MyType> null


myType.all2(fun sqlThis error rows ->
    printfn "Hello"
)

it will generates something like:

myType.all(function(error, rows) {
    return ((sqlThis, error, rows) => {
        toConsole(printf("Hello"));
    })(this, error, rows)
});

This is not the prettiest code but it should work 🤞

Another similar workaround to respect the original SQLite API is to create an helper to generate the flat function:

type MyType =
    // abstract member all: callback: (exn option -> ResizeArray<obj> -> unit) -> unit
    abstract member all2: callback: (exn option -> ResizeArray<obj> -> unit) -> unit

module Adapter =

    let all (callback : obj -> exn option -> ResizeArray<obj> -> unit) =
        emitJsStatement (callback) """
function(error, rows) {
    return $0(this, error, rows)
}
        """
    
let myType = unbox<MyType> null

myType.all2(Adapter.all (fun sqlThis error rows ->
    printfn "Hello"
))

Personally, I would used the second option like that you can stay closer to the original API and the user opt-in using flat functions when needed.