Approved the fork.

At some point, the description must have been updated as it now specifies that the input consists solely of ASCII letters, which consists only of ['A'..'Z'] ++ ['a'..'z'].

Haskell fork which updates the tests to conform to this specification.

OP solved it, closing. (Your initial code will not work for odd number of chained calls)

Yes, and "\201", "\213", etc. are considered uppercase. They really shouldn't be.

I agree, such twisted logic don't belong to anything bigger than such self encapsulated function. If a code is meant to be mantained, it should be simple.

Not necessarily. Although I made it like that just for fun, I could totally use functions like that in bigger projects. They are like components or DLLs, just put them in a project and they do what they were made to. They are small, so once you wrote a function like that, you can forget about how it works and just use as compiled code. For example, I often use a function for stable sorting, so for me it is quite important for size of that function to be as small as possible.

Look at 140byt.es. There are many little, practical functions ready to use (some of them have some limitations, which are described). Also each of them is annotated, so if you want you can learn how they are made.

Such snippets are also compliant with the Small.JS manifesto.

Any special cases? Otherwise I don't see a reason to do so, 'cause it's only a 7 kyu kata.