let mut stitch_info: Option<StitchInfo> = None;
stitch_info.as_mut().map(|stitch_info| {
                // Update stitch values. Subtracting PerlinN before the multiplication and
                // adding it afterward simplifies to subtracting it once.
                stitch_info.width *= 2;
                stitch_info.wrap_x = 2 * stitch_info.wrap_x - PERLIN_N;
                stitch_info.height *= 2;
                stitch_info.wrap_y = 2 * stitch_info.wrap_y - PERLIN_N;
                stitch_info
            });

data Option a = Some a | None

map : (a -> b) -> Option a -> Option b
map f None = None
map f (Some a) = Some (f a)

if (optional_stitch_info != None) {
  let stitch_info = optional_stitch_info.get_value();
  // ...
}