Advice on Haskell from that great philosopher, Bo Diddley

Don't let your instance write a check that your type can't cash.

Haskell's type checking is a glorious thing and saves one--let's be honest, saves me--from lots of stupid mistakes. However, there's one thing that it can't do for you. When you say a type is an instance of a type class, it has to trust that the operations you define on the type to gain admission to the type class behave the way they're supposed to. So be careful.