Hi, With the patch I recently submitted, Yhc can now compile StackSet with the pattern guards. Using the latest Yhc (from darcs), and the latest Catch (0.1.1 from Hackage, recently updated): There are 5 incomplete patterns, 2 are proven safe. All the 3 unsafe ones are in "new". Taking a closer look, you are allowing any Integral method to be passed, which includes those who have really wacky semantics. Restricting this to Int there are 6 incomplete patterns, 5 are proven safe. The remaining one should be proven safe by Catch, if the initial transformation stage hadn't obfustcated something. I am confident I can fix Catch to fix this, and will do so for Catch 0.2. The exact problem is that let's are not floated inside case's currently, but should be. Thanks Neil