The Snap debugging system has been reworked; before, to get debugging output from a Snap program, you needed to build snap-core with the -fdebug flag and rebuild your entire webapp stack. Now, in a stock Snap installation, you can set the environment variable DEBUG=1 to get Snap to produce debugging output to stderr.

There is a slight performance hit associated with this change (in that GHC can no longer inline calls to debug because we use unsafePerformIO to read the DEBUG environment variable to pick a debug implementation at runtime) — if you simply cannot stand the overhead of a no-op function call, pass the no-debug flag when installing snap-core :