[Chicken-users] Update: Cairo binding

[Luke McCarthy]

I've got about 99% of the API for Cairo 1.4.14 complete. Notable additions are 
patterns and scaled fonts, and some missing drawing functions, but it's 
mostly getters and setters of which there are many! The main thing missing is 
glyph support, because I can't think of a good way of passing arrays of 
structs from Scheme, and a few other trivialities like getting the reference 
count and get/set "user data" which are not very useful for Schemers.

I want to change some aspects of the API for ease of use. Obviously I want to 
consult the list first since I don't want to break anyone's code. I don't 
imagine many (any?) people are using the Cairo binding yet, it is *alpha* 
after all.

Mainly I want to change things so that values which are returned from the C 
function through "out" args are instead returned as any Scheme programmer 
would expect. For example, change (cairo-text-extents) to return a text 
extents object instead of mutating an object passed as an argument.

; private
(define cairo-font-extents*
  (foreign-lambda void "cairo_font_extents" cairo_t cairo_font_extents_t))

; public
(define (cairo-font-extents cr)
  (let ([fe (make-cairo-font-extents-type)])
    (cairo-font-extents* cr fe)
    fe))

There are similar cases for floating-point arguments which currently expect 
f64vectors (of 1) which is a bit of a hack. It would be better to hide this 
and return doubles.

Some procedures return multiple arguments. Should I use (values) or (list) ? 
Currently I am using (values) in most cases but there is a case where I had 
to use (list) since there could be an error (return #f on error) e.g. 
(cairo-pattern-get-rgba) which may fail if the pattern is not an RGBA pattern 
(there are different types of pattern). Alternatively I could throw an 
exception but I'm not familiar with the Chicken way of doing this.

I'd like to get feedback on these issues before I release the code.

Cheers,

Luke McCarthy