[Chicken-users] Some pointers on easyffi and interfacing with C code

[Mark Fredrickson]

Hello Chicken expertgenzia,

In attempt to broaden my knowledge of Chicken, I'm attempting to  
create an egg for the GEOS C library[1]. My first cut has been to use  
easyffi, and I'm pleased with the results. With minimal hacking at the  
GEOS API header[2] I've been able to see some results. Now I'm looking  
to refine.

Here are my questions:

1. Many of the GEOS functions return 'char', even though the actual  
data should be a bool. Is there a way to tell easyffi to treat these  
characters as #t/#f? I'm thinking there might be a __declare() for it,  
but I didn't see anything that made immediate sense to me.

2. Several of the functions return ints to represent success, errors,  
and take a "return" argument by reference. Is there a best practice  
for dealing with these kinds of functions? Here's an example if I'm  
not being clear (the goal is to compute the length of a "geometry"  
e.g. a line):

extern int GEOSLength(const GEOSGeometry* g1, double *length);

3. How do you get those cool <#TYPENAME> print outs in the the  
interpreter? E.g.
#;1> (make-hash-table)
#<hash-table>

Since geometries have literal represenstions in the form of Well-Known- 
Text (e.g. "POINT(100 100)") is there a way to get a call back and  
print out a literal value? Similarly, can I write a reader macro to  
turn a literal in source into a geometry? Something like:

(let (p #WKT(POINT 100 100)) .....

This last point is just for my edification right now. I've got plenty  
to do with the rest of the API first.

Finally, if anyone is feeling like going above and beyond, I could use  
suggestions how to handle callbacks for the initGEOS() function in  
that C header. It's a little too complex for easyffi. While I can  
certainly write a pair of handlers in C, it'd be nice to be able to  
pass in user lambdas to execute on error and warning conditions.

Thanks in advance,
-Mark

[1] http://geos.refractions.net/
[2] http://geos.refractions.net/ro/doxygen_docs/html/geos__c_8h-source.html