Re: Difficulty integrating with Swift/Objective-C

[paul <paul@inktvis.org>]

Hey Chris,

On 2021-09-05 at 20:56 AEST, quoth Chris Vine 
<vine35792468@gmail.com>:
> You appear to want to run scheme code as an extension language, 
> via the
> guile VM, in a program written for MacOS in the Swift language.

This is exactly right.

> I know
> nothing about the Swift language and whether it has a sufficient 
> C-like
> FFI to achieve this, but you may get some inspiration from the
> following, which enables scheme code to be run as an extension 
> language
> within a C++ program:
> https://sourceforge.net/p/cxx-gtk-utils/git/ci/master/tree/c++-gtk-utils/extension.h
>
> However there are various issues.  First, guile's exceptions
> (based on C's longjmp()/setjmp() and cognates) are almost 
> certainly
> not implemented in the same way as Swift's exceptions (assuming 
> Swift
> has exceptions), which means amongst other things that, if Swift 
> has
> something akin to destructors/deinitialization, you should not 
> allow
> guile exceptions to leave Swift objects undestroyed were guile 
> to
> jump out of local scope on raising an exception 
> (scm_dynwind_begin(),
> scm_dynwind_end and scm_dynwind_unwind_handler can be useful 
> here if
> you are able to manage your Swift objects manually in C).  Nor 
> more
> generally should you allow Swift exceptions (if Swift has them) 
> to
> escape out of a guile scm_dynwind_begin()/scm_dynwind_end() 
> pair, or
> you will stop guile's resource management working correctly.
>
> Secondly, there are some issues relating to the fact that 
> guile's
> 'make-fresh-user-module' and 'load' procedures leak memory, 
> which can
> be problematic if all tasks run as scheme code are not to share 
> the
> same top level.
>
> Thirdly you will need to cater for translating guile return 
> values such
> as lists or vectors into containers that Swift can work with, 
> and vice
> versa.
>
> This is all pretty well documented in the C++ code mentioned 
> above, but
> it is not for the faint hearted.

I'll definitely take a look at the codebase you link; thank you. 
Although i am probably on the fainter-hearted side, i have a bit 
of hope it might be feasible, since my project actually already 
includes a "core" implemented in Rust and included as a 
statically-linked library.  I'm able to call out to Rust functions 
i've marked as `extern C`, and so while i don't fully understand 
the mechanism and the implications for threading and exceptions, 
so far it has worked well in that limited case.  I'm hoping that i 
can make something similar work in Guile.  I think my needs are 
fairly modest - initially my ambition is to allow the user to 
rebind shortcut keys of my app using a Guile script.  We'll see 
what else i can come up with later.

Thanks for the info though, i'll definitely refer back if i run 
into issues!

Kind regards,
p.