concurrent-output is a more meaty Haskell library than the ones I've covered so far. Its interface is simple, but there's a lot of complexity under the hood. Things like optimised console updates, ANSI escape sequence parsing, and transparent paging of buffers to disk.

It developed out of needing to display multiple progress bars on the console in git-annex, and also turned out to be useful in propellor. And since it solves a general problem, other haskell programs are moving toward using it, like shake and stack.

Next: twenty years of free software -- part 12 propellor