FYI: A paper on how Hackage packages use `mtl`.

[andreasabel]

Stumbled across this paper: https://pleger.cl/cv-pleger/papers/figueroaAl-SCP2020.pdf

Which Monads Haskell Developers Use: An Exploratory Study by Ismael Figueroa, Paul Leger, Hiroaki Fukuda

From the abstract:

Our results show that around 30.8% of the packages [on Hackage] depend on the mtl package, whereas only 1.2% depend on alternative, yet compatible implementations. ... the state monad is by far the most popular one [monad transformer], although all of them are used