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Re: Difficulty integrating with Swift/Objective-C

From: paul
Subject: Re: Difficulty integrating with Swift/Objective-C
Date: Mon, 06 Sep 2021 20:26:53 +1000
User-agent: mu4e 1.4.15; emacs 27.2

Hey Chris,

On 2021-09-05 at 20:56 AEST, quoth Chris Vine <>:
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:

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

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,

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