I can hear all the Haskell fanboys screaming that they didn't do Haskell right because a kleisli arrow didn't appear and ascend them into monadic heaven.
To be fair, I don’t think any Haskeller would actually write this project without attoparsec and maybe lens by choice. They’re basically base libraries for this sort of thing.
Agreed. The rule about not using any library that doesn't ship with the compiler creates a heavy bias in favor of "batteries included" languages like Python. I understand not allowing parsing libraries, given the nature of the assignment, but if even widely used libraries like lens are off-limits the result is not going to resemble idiomatic Haskell. As a matter of basic fairness, if some library or built-in language feature is to be permitted for one implementation then analogous libraries should be permitted for all the other implementations regardless of whether they happen to ship with the compiler.
The eval function leveraged heavily by the Python version should probably also have been off-limits, for the same reason that parsing libraries were prohibited. Using eval amounts to embedding an existing fully-developed parser and runtime environment into the project as a library.