I don't see anything hard to debug in this code. Honestly, if this kind of code makes debugging hard for one it means that one assumes too much about how code should behave instead of looking how it actually behaves.
I hate this "dynamic programming[0] = hard to debug" meme. A truly hard to debug code is one that's nonlocal (you have to read 20 files to follow the execution flow (hello Scala trait abusers)) or displays random behaviour (e.g. threading, dependence on external resources). This one? Every "tricky" thing is contained in a block of 10-20 lines. Just isolate the endpoints and follow the execution until it does something you think it shouldn't.
In a way, the biggest enemy of a bug hunter is their own assumptions.
[0] - examples here aren't really metaprogramming, and even the latter isn't that hard to debug if you actually sit down and read the code.
I hate this "dynamic programming[0] = hard to debug" meme. A truly hard to debug code is one that's nonlocal (you have to read 20 files to follow the execution flow (hello Scala trait abusers)) or displays random behaviour (e.g. threading, dependence on external resources). This one? Every "tricky" thing is contained in a block of 10-20 lines. Just isolate the endpoints and follow the execution until it does something you think it shouldn't.
In a way, the biggest enemy of a bug hunter is their own assumptions.
[0] - examples here aren't really metaprogramming, and even the latter isn't that hard to debug if you actually sit down and read the code.