Eliminate compilation warnings

[avh4]

I'd like to fix all haskell compilation warnings, and then turn on the -Werror flag (to make all warnings act as errors) for compilation.

[avh4]

Note: There was a bug in stack < 1.4.0, causing ghc-options in the cabal file to not be honored. (commercialhaskell/stack#2289)

[ahstro]

Would it be okay if I looked into this? I don't have a ton (see close to none) of Haskell experience, but I'm interested to learn, and I'm a sucker for getting rid of warnings.

[avh4]

@ahstro sure! The most common warning is that there are local variables that shadow outer scope variables, so coming up with good distinct names will probably be the bulk of the work. One thing to note is that cabal won't rebuild files that haven't changed, so if you've already compiled everything, recompiling will only print out the warnings for files that need to be recompiled.

I'd suggest doing a PR for one file at a time, so there isn't a huge PR.

[ahstro]

I saw that, I'll do my best trying to figure out good names. Guess that gives me a good reason to read and figure out some of the code, so feels like a good starting point.

Thanks for the tip on Cabal!