Troubles with guide position

[Ofnuts]

“Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it?”  ― Brian Kernighan

Not a scheme expert. Code typically too clever to be debuggable (I don't even know how to print a value in Scheme...).  You obviously try to play a clever trick to get the guide position but it it obviously wrong: if you are layer or image-centric, this doesn't just change the sign of the position.

IMHO, the simplest way:

• If you work on an image, then offsets are 0,0, and W&H are the image dimension
  
• If you work on layer, then offset are the layer's offset and W&H are the layers dimension
  

Then your guides are at

• offsetX+position (left),
  
• offsetY+position (top),
  
• offsetX+width-position (right),
  
• offsetY+height-position (bottom).
  

Also, despite not being a Scheme person:

• I had to drop the get bit in  your gimp-drawable-get-{offset,width,height} (ie, gimp-drawable-height is what works for me, what version of Gimp are you using)?
  
•  (if (= in_toggle TRUE) ...) is better written (if in_toggle ...)
  
•  (* -1 position) is better written (- position)
  

If you have Python support in your Gimp and always put the guides at the same place relative to image or layers, you can try my ofn-guide-presets script (use it as is or steal the code).