Ok, I got it. I missed the glaringly obvious case :O

I keep getting errors for the "More valid inputs" category, and I'm running out of ideas what could be causing them (Haskell version).

Example:

Falsifiable (after 3 tests):
expected: Just "flux-capacitors-are-dangerous"
but got: Nothing

The only inputs I can think of that can generate this are:
changeCase "FluxCapacitorsAreDangerous" "kebab"
changeCase "fluxCapacitorsAreDangerous" "kebab"
changeCase "flux_capacitors_are_dangerous" "kebab"

All of these are handled correctly by my solution. So I guess that my assumption about the valid inputs is wrong? Are we supposed to accept inputs with leading / trailing blanks or similar?

I'd put it in 7kyu, as well - unless usage of sort() is explicitly prohibited.

It is obviously very wrong. And so are the tests which should be modified. Thanks for pointing it out!

This solution looks wrong (or I misunderstood the assignment). It returns true for

isMerge "hello world" "elloh " "world"

whereas it should return false, since the ordering of characters in part1 is not the same as in s.

The Haskell kata seems to be broken. I got a test error for input (-1, 1) - the test instructions clearly specify that we can assume non-negative integers as input.
Also, I submitted the identical solution in Ruby, and it passed - presumably because Haskell uses Quickcheck to generate random tests, and the properties for these tests are broken for negative input. Please fix this - it's quite frustrating if you get test errors and cannot figure out why.