canvas coordinates in guile-gnome-platform 2.7.96

[Greg Troxel]

I have created a canvas and placed items.  I didn't set up any
canvas/world/pixel mappings explicitly.  I think that means that
world (0,0) is center of canvas, and 1 pixel/canvas unit.

I tried to use functions to convert world to canvas.  I was able to
find the generic world-to-window and ask for methods:

guile> (generic-function-methods world-to-window)
(#<<method> (<gnome-canvas> <top> <top> <top> <top>) 8437f50>)

I didn't grasp why this took 5, not 3, but I passsed 0

(world-to-window *canvas* 10 20 0 0)

and got wrong type arg.
#f doesn't error, but no value is returned.

Reading the gnomecanvas header files, it seems the c library procedure
takes two doubles and two 'double *', which are "obviously" output
parameters.

I tried passing variables with values, hoping they'd mutate, and then
the symbols for the variable, but all of these lost with type errors.

Does anyone know how can I use this function from Scheme?

-- 
        Greg Troxel <address@hidden>