[Ecls-list] Embedding ECL interpreter in a C++ application

Andrew Topp talmakion at yahoo.com.au
Sun Mar 7 05:23:00 UTC 2004

This is pretty much what I was doing in Universal Corporation. I've only 
just started rewriting the ECL plugin over the past couple of weeks, but 
a laptop HD crash made me loose a fair chunk of work. So not much has 
been committed.

worm wrote:
>>I have a follow-up question: is there a way of
>>determining if the parsing of the lisp code works ok?
>>Ideally I'd like to be able to call a function to tell
>>me if the interpreted lisp code contains errors and,
>>if so, what those errors are (but I'd like the
>>application to capture this information, without
>>requiring user interaction).
> Why don't you enclose your lisp code in some handler-bind or
> handler-case form? si_safe_eval(..) just outputs the symbol
> error if something bad happens. If you want something
> fancier, you can install some lisp function, or macro, that
> captures the error and outputs the useful information.

I was doing something in the style of the Python postmortem debugger. A 
function defined in Lisp code (loaded from UC's initialization .lisp 
file), which caught errors and saved as much state as it could, call 
stack, any info that I could get (reading through ECL's own debugger is 
where I learnt most of that), saving it to a special variable, and 
returning an error status to the C++ caller.

This code has been ripped out, it wasn't as complete in the CVS as it 
was for me, but a fair bit is still there in the UC CVS history.

> Sorry, I am taking a plane in 5 hours and cannot be more
> precise, but anybody with a knowledge of lisp will be able
> to help you.

That counts me out :).

>>I was guessing cl_safe_eval() supports this, but I
>>couldn't figure out how to make it do so.

This function just sets si::*ignore-errors* (which causes the ECL error 
handler to not trigger on an error), sets up the frame stack (look at 
the frs_push()) with a CATCHALL 'tag' so that any error will just drop 
out into that function, and returns an error status if something bad 

Think of this usage of frs_push() as something like a setjmp/longjmp 
used in C for stack unwinding, except it should also handle resetting 
the ECL state to something sane.

This function is a very simple C version of the error-catching I was 
doing with native Lisp. From Lisp, you have access to a great deal more, 
easily. It just sucks that I'm not that great at Lisp.

> My suggestion is that you implement a cleverer routine in lisp,
> which takes a form and produces a bytecodes object (for instance
> with (coerce form 'function)). Write also another routine that
> applies this function capturing errors. Give them meaninful
> names, and if you want (you do not really need it) compile them.
> Use cl_funcal(...) to invoke these cleverer functions.

I tried doing this myself, but my implementation was too kludgy. As I 
mentioned, I'm going for a complete ground-up rewrite.

> Best regards,
> Juanjo
> P.S.: I will be away in the Alps for a week. Probably without
> internet :-/

Have fun :)
(Goes for Michael too ;)

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