- Reach for a deriving clause before an instance body, and let a newtype inherit what is underneath. A hand-written instance is one GHC stops keeping correct the moment the type changes.
- A class with one instance is a function you have not passed yet. Take the operation as an argument, and add the class when two callers genuinely need it chosen by type.
- Thread state through plain arguments until that actually hurts. A transformer stack costs a newtype, a run function, and a deriving list to buy back what one parameter was giving you.
- Put the bang on a fold accumulator while writing the fold, not after profiling. The strict version is the same length, and the lazy one is a thunk chain you will pay someone to find.
- Treat an incomplete-pattern warning as a failing test. Adding the missing equation is smaller than the unit test you would otherwise write to catch that case.