Yes fitness[i] is the fitness of chromosome[i].
It looks like you are doing this in Ruby. In that case you should use an integer instead of a binary string.
The description states: "What the test will do is generate a random binary string of 35 digits (a random Integer with 35 bits for Ruby)".
So in Ruby we use Integers instead of strings (since we have Integers of arbitrarily large size already).

Updated. Some people are going to have to re-write their solutions, now... *hides in a corner*

I didn't ban any modules.
According to the list of packages posted in the comments to Lensmaker, parsec isn't installed in the test environment, but attoparsec is.

I'm not in control of the timeout messages, sadly. I will add a note clarifying the relation between Hughes' Lists and regular lists. Note: it's not that Hughes is abstract (in the context of this module, it's completely concrete) but it is intended to be treated as abstract. Truly, the best way to talk about it is to say that Hughes' Lists are simply isomorphic to regular lists (which is what mkHughes/runHughes demonstrate) but that's not an obvious way to think about it.