"Theorems for free"! By which I mean, when I'm coding something and it could be universally quantified on the type, then it's better to do so. That way, it's im

"Theorems for free"! By which I mean, when I'm coding something and it could be universally quantified on the type, then it's better to do so. That way, it's impossible to phrase certain errors. For example, a function `Set<a> -> Set<a>` must produce a subset of the input set; that's guaranteed by the types. I can't accidentally include the value 0 in my set, because 0 isn't of that generic type. The generic forces you to think in terms of the structure you're manipulating, rather than in terms of its contents.