mtl, transformers, monads-fd, monadLib, and the paradox of choice

Question

Hackage has several packages for monad transformers:

• mtl: Monad transformer library
• transformers: Concrete functor and monad transformers
• mon

Answer 1

The factoring out Edward Kmett mentions in his answer was completed in late 2010. Its end result was monads-fd, built upon transformers, becoming version 2 of mtl. As a consequence of the ubiquitousness of mtl, monads-tf never really caught on. As of early 2017, mtl and transformers are the only monad transformer libraries that see widespread usage.