Hi Two patches. The first one deletes tests/Catch.hs. Using the darcs version of Catch, you can simply do: $ catch StackSet.hs And that fires off the test, without requiring driver scripts etc. The second one introduces abort, and uses it instead of error. This lets Catch know that an error is intentional, and won't be flagged. With these, I can't prove pattern match safety for two reasons: * new gives an error on compiling with Yhc: -- during after type inference/checking Error: Context for Prelude.Enum and Prelude.Num needed in left hand pattern at 1 67:13. Error: Context for Prelude.Enum and Prelude.Num needed in left hand pattern at 1 66:9-166:21. catch: Failed to compile, StackSet.hs I guess its defaulting issues - Malcolm may be able to suggest an appropriate work around. * view uses pattern guards, these don't work with Yhc. By commenting out those two bits, Catch is able to prove safety of the rest. Thanks Neil