Clean up warnings

[spencerjanssen]

hackage-server builds with lots of warnings. They mostly fall in a few categories:

• Derived Typeable instances
• Partial functions like head and tail which warn in recent versions of GHC and base
• Incomplete pattern matches

Is a PR to clean up all the warnings welcome?

[spencerjanssen]

I should have explained the value I see in addressing these warnings. I find -Wall to be very useful when hacking on Haskell code bases, the warnings often expose real bugs. However, the value of -Wall is undermined when there are many existing warnings -- it's too hard to find the newly introduced warnings. hackage-server has something like 60 warnings (excluding Typeable deriving) which is way too much noise to be useful.

Having said that, I know from experience that reviewing dozens of small refactors is not pleasant, and perhaps others don't share my outlook regarding warnings. Do we see value in aiming for near-zero warnings?