Clever!

Here is still the issue (1 year old), as asQuirreL commented:
"The before as defined in Tagless conflicts with the one defined in Hspec. I fixed it in the Sample Test Cases by just hiding before when importing Test.Hspec, but I obviously can't do that in your tests, so I am somewhat at a loss when trying to submit."

Just write "import Test.Hspec hiding (before)" in the "Your Test Cases" section! Thank you!

This is my favorite kata! ~6 hours to solve (all night)!

Finally, I solved kata, but I haven't felt a lot of things. I'm confusing about deep theoretical background. In the end I just acurately copy-pasted some code from the hint and added some unimplemented parts. Also I think this "2 kyu" kata is harder to understand than "1 kyu" Tiny Three-Pass Compiler kata. In my opinion, all this stuff could be explained much simplier. For example:

1. "Tiny Three-Pass Compiler" kata (1 kyu) - parse expression into simple tree.
2. "Finally Tagless Interpreter" kata (2 kyu) - describe expressions by serializing of lambda calls.
3. "Data Types a la Carte" kata (3 kyu) - how to flexibly extend existing data type.

By the way, I fill these 3 katas are ranked by amount of possible copy-paste from the "hint" articles, really. 1. - no copy-paste, but very interesting to implement, 2. - middle amount of copy-paste from article, hard to understand, 3. - only copy-paste, even harder to understand what is going on.

The main positive moment: increasing awareness about types, extensions and deep mechanisms of Haskell when you're solving such kata, so it's very useful in any case. Goog luck!

Looks nice, really, but it doesn't handle one moment - when nim sum is 0 at the input (then there is error).

Genius

Try to mark your question as an issue, seems like it can get some reaction then. I also can't finish this kata.

I didn't hear about eta-conversion, I will study. About point-free I read again and again, but what I can't agree - someone says "Look, with points your code is more clear and nice", but in fact, it really leads to obfuscation.

Great explanation. Thanks! So, as I understand, it is just a partial application?

Cannot understand how does it really work. Can anyone explain? How "filter" gets correct filter function and compares every word in the list with some pattern (target word)...

Last few days I didn't have that problem any more. So I mark this issue as resolved.

Got it :)

Oh, thanks, my fault: I just mistakenly removed a function prototype, but "Run tests" works fine, thereby I thought the problem with "Submit". But now everything is fine. Thanks again.

Haskell: cannot submit, it gives me monstruous stacktrace, something wrong with tests. Please, check.

I think it's good to add a hint about specific Python features which help to solve a kata. At least it's a Python's kata, and it should help to learn a language, not just functional programming concepts.

P.S.: Also I think the difficulty of kata is not of "fundamentals": this is fundamental for any functional language, but not as excersise for Python.