"Every so often, when starting via runInGhci, you may get an error message like:
*Main> runInGhci happs tutorial running in ghci. exit :q ghci completely and reenter ghci, before restarting. *** Exception: _local/happs-tutorial_state/events-0000000006: openFile: resource busy (file is locked)
Don't worry about it. Every time I get this error I simply run runInGhci again, and the second time it always works. ".