Syd's rules for sustainable Haskell

General Tips

Write tests first. If you can't do that, you've probably made things too complex.
Choose an automated linter and enforce its use.
Ban dangerous functions. Think twice before introducing exceptions.
Know about WATs
Choose an automated formatter, don't argue about which one, and enforce its use.
Remove every instance that you don't use.
Before writing an instance, write a function. Then reconsider whether you still need an instance. (You probably don't.)
Don't write an instance if you can't use it to write generic code.
If your instance is not law-abiding, remove it.
Have roundtrip tests for any serialisation.
Have golden tests for any external output you produce. (That means "for every ToJSON instance" as well.)
Write JSON instances manually until you have both roundtrip tests and golden tests.
Don't use lenses. You don't need them.
Use prefix naming: data Thing = { thingField :: FieldType }
Use as few languages extensions as you can get away with.
Don't try to break the laws of physics.
Don't try to prove "false".
Don't use any effect systems. Use IO or a ReaderT with a fixed Env over IO. No parametric environment and no parametric monad.
Never implement a custom Show and/or Read instance. If you want an instance, derive it.
Use monomorphic functions instead of type-class constraints.
Use polymorphic functions if you do not need type-class constraints.