Цитата из учебника:

So in conclusion, live fast, love hard and don't let anybody else use your comb!

Учебник по Haskell'у

UPD
Can a traffic light be a yes or no value? Sure. If it's red, you stop. If it's green, you go. If it's yellow? Eh, I usually run the yellows because I live for adrenaline.
Оттуда же.