Custom foreign types

[Ryan Raymond <rjraymond@oakland.edu>]

Hello, all.
I know we can create pointers to structs with make-c-struct, but I would
like to pass structs directly as arguments to foreign functions. I can do
that by using bytevector-uint-ref, and setting the size to that of the
struct, but when it is time to specify the type in pointer->procedure, I am
out of luck if the sizes don't match.

Here is how I am binding Raylib using FFI currently.

(use-modules
  (rnrs bytevectors)
  (system foreign))
(define rl (dynamic-link "libraylib"))
(define pp pointer->procedure)
(define df dynamic-func)

(define BeginDrawing (pp void (df "BeginDrawing" rl) '()))
(define ClearBackground_c (pp void (df "ClearBackground" rl) (list uint32)))
(define (ClearBackground color)
  (ClearBackground_c (make-c-struct (list uint8 uint8 uint8 uint8) color)))
(define CloseWindow (pp void (df "CloseWindow" rl) '()))
(define DrawGrid (pp void (df "DrawPlane" rl) (list int float)))
(define EndDrawing (pp void (df "EndDrawing" rl) '()))
(define GetFrameTime (pp float (df "GetFrameTime" rl) '()))
(define InitWindow_c (pp void (df "InitWindow" rl) (list int int '*)))
(define (InitWindow width height title) (InitWindow_c width height
(string->pointer title)))
(define SetTargetFPS (pp void (df "SetTargetFPS" rl) (list int)))
(define WindowShouldClose_c (pp int8 (df "WindowShouldClose" rl) '()))
(define (WindowShouldClose) (= 1 (WindowShouldClose_c)))

(define width 600)
(define height 400)
(define title "This is a Guile-bound thinga")

(define (make-Color r g b a)
  (bytevector-uint-ref
    (u8-list->bytevector (list r g b a)) 0 (native-endianness) 4))

(define (make-Vector3 x y z)
  (bytevector-uint-ref
    (f32vector x y z) 0 (native-endianness) 4))


Is there a cleaner way to do this?
Ryan