From the type of a polymorphic function we can derive a theorem that satisfies. Every function of the same type satisfies the same theorem. This provides a free source of useful theorems, courtesy of Reynolds' abstraction theorem for the polymorphic lambda calculus.
Some very good advice in this article. I especially agree on the fact that nick-picking during code review on minor coding styles is useless. The world would be a better place if auto-formating were the norm.