If julia functions which was brought to R throws an error, no real error is thrown, but an error object is returned #216

[schlichtanders]

I created an issue at JuliaCall however after a first inspection, I think it is actually an RCall issue.

Concretely something in the following is not raising julia errors when run on R. Also the stacktrace is not reported.

RCall.jl/src/callback.jl

Lines 122 to 147 in 69c22dd

	"""
	Wrap a callable Julia object `f` an a R `ClosSxpPtr`.
	Constructs the following R code
	function(...) .External(juliaCallback, fExPtr, ...)
	"""
	function sexp(::Type{RClass{:function}}, f)
	fptr = protect(sexp(RClass{:externalptr}, f))
	body = protect(rlang_p(Symbol(".External"),
	juliaCallback,
	fptr,
	Const.DotsSymbol))
	nprotect = 2
	local clos
	try
	args = protect(sexp_arglist_dots())
	nprotect += 1
	lang = rlang_p(:function, args, body)
	clos = reval_p(lang)
	finally
	unprotect(nprotect)
	end
	clos
	end

Helpful steps to provide error support

Looking into JuliaCall's error handling, there are two places:

1. on JuliaCall julia side the following try catch converts a julia error to an R object https://github.com/Non-Contradiction/JuliaCall/blob/c05473bea78a0197c639f7e82ab1c6f2e943e1cc/inst/julia/setup.jl#L165-L194

function docall(call1)
    try
        # ...
    catch e
        Rerror(e, stacktrace(catch_backtrace())).p;
    end;
end

2. on JuliaCall R side the following intercepts these R objects and raises them as R errors https://github.com/Non-Contradiction/JuliaCall/blob/c05473bea78a0197c639f7e82ab1c6f2e943e1cc/R/interface.R#L76-L78

    r <- .julia$do.call_(jcall)

    if (inherits(r, "error")) stop(r)

Both these steps would need to be replicated inside the conversion of a Julia Function to an R function.

[schlichtanders]

I found where the julia error object is transformed into a simpleError, skipping stacktrace entirely

RCall.jl/src/callback.jl

Line 61 in 69c22dd

err = rcall(reval("base::simpleError"), string(e))

this should be changed to the RCall equivalent of JuliaCall's

err = Rerror(e, stacktrace(catch_backtrace())).p;

[schlichtanders]

The second step needed is to surround the following with the if (inherits(r, "error")) stop(r) logic

RCall.jl/src/callback.jl

Lines 132 to 135 in 69c22dd

	body = protect(rlang_p(Symbol(".External"),
	juliaCallback,
	fptr,
	Const.DotsSymbol))

Unfortunately, I couldn't find a way yet how to add such if else logic from Julia.

[schlichtanders]

I fixed the one part for me now, but it seems juliaCallback cannot be fixed outside of RCall itself.

As soon as I call setup_callbacks() again and run a typical failing example

julia_eval('() = error("fail")')()

I get the following error (or sometimes also a Segmentation fault)

ERROR: UndefVarError: `error_msg` not defined

I bet this has something todo with creating C functions and would be solved if this is officially fixed in RCall.jl