What was it, please say it :)

Done.

Yes, you are correct. My intention in the first place was to force lazy people to use proper error handling, instead of only error, which is bad style almost always. ;)

mtthw123, after typing that bit a few times, and feeling like it was giving too much away, I ended up adding a couple more example test cases, one in which the vectors were not same-length, then mention that it holds a clue to the correct solution (or a correct solution).

(= (vector-affinity [6 6 6 6 6 6] [6]) (float (/ 1 6))

Just since here obviously the second vector is shorter, and the equality to (float (/ 1 6)) illustrates that the larger of the two is used as the denominator.

What are you thoughts on that?

I hadn't thought of that, but it makes perfect sense. I'll update the description with that. Thanks!

Added. Thanks, both of you!

And while we're at it, lets check arbitrary strings

property $ \xs -> parses xs `shouldBe` solution xs
where solution xs = -- ...

I struggled a lot with this whilst trying to edit the code but I thought I had ironed it all out.

It should be fixed now? This is a problem with the editor, automatic indentation and tabbing insert different characters.