I'm not sure I'd agree exactly that containers is well-documented. It's extremely common for beginners[1] to have unintentional space leaks in Data.Map due to the non-strict value, and the documentation doesn't say anything about this common pitfall. The only thing it says is "If you need value-strict maps, use Strict instead.", which is great if you already know what you're doing, but not so great if you don't. I don't have any problems with the implementation, design, or maintenance, but the documentation could be improved for people who are new to Haskell.