Fix GHC warnings

[david-yz-liu]

If we compile our code with -Wall (this will be done by default in a pull request soon), we get lots of GHC warnings. We should eliminate these.

[Ian-Stewart-Binks]

@david-yz-liu What are your thoughts on non-exhaustive pattern matches?

[david-yz-liu]

Generally, the non-exhaustive pattern matches indicate some larger failure in the types. These are what we should be aiming to fix; but, I am open to the idea that this won't always be possible. We shall see.

[david-yz-liu]

@PkHutch @RyanDJLee @AilsaF this is a good issue to work on for getting to know Haskell as well. It also has the benefit that you can all work on this, since it's just a matter of fixing the errors for certain files, and there's plenty of Haskell files to go around.

I think you can try to do something like stack ghc <path-to-file> to try to compile a particular file, but I could be wrong. Worth investigating and documenting that!

[RyanDJLee]

Okay! I still have some reading to do I realized, but I will be on it shortly!