thomas.dubuisson:
I could try GHC's new debugger. But my experiences with it so far have shown that for all but the most trivial programs possible, it becomes intractably difficult to figure out what the debugger is actually showing you.
At times I think of ghcid as the anti-gdb. If there's a series of let bindings, each mutating the predecessor, its enjoyable to see the debugger start at the bottom and crawl its way back up.
I'd like to relate a debugging effort this week. A colleague had an exception thrown from deep within a large body of code, we knew not where. A few trace statements didn't yield much information. The debugger was fired up on this >50 module program, and we got a few interesting hints and pieces, but it seemed as if the exception was floating away from its call site, to the point it was being demanded. Hmm. A puzzle. So instead we compiled the application with -fhpc, ran it till the exception occured, and then ran hpc markup on the result. (This marks up the program source in colours showing what code was actually executed during a given program run). Loading the colourised trace into firefox, we saw at a glance all the code that had been executed up to the point of the exception, and then there was the exception itself, staring at us amongst a chunk of bright yellow code, a lone streak of uncoloured code where it shouldn't have been. It took all of 5 seconds to find the bug with HPC. -- Don