mtl-style-example VS envy

Re: effects libraries, it's probably worth starting with e.g. https://www.fpcomplete.com/blog/2017/06/readert-design-pattern/ https://www.parsonsmatt.org/2018/03/22/three_layer_haskell_cake.html https://github.com/lexi-lambda/mtl-style-example if you're just getting familiar with the ecosystem. I'll add, anything written by the the people I linked to in this comment is probably worth reading as well.

Personally, it felt to me like, once I really understood monad transformers (https://hackage.haskell.org/package/transformers) and mtl (https://hackage.haskell.org/package/mtl), the reasons they both exist and how they are distinct, how you use them generally and what common patterns are around structuring your app (see e.g. https://github.com/lexi-lambda/mtl-style-example and the 3-layer-cake link someone else provided here for two), a lot of stuff in the ecosystem suddenly became comprehensible and useful to me, in a practical, "real world" way. In fact I'd go so far as to assert that getting comfortable with mtl in particular is the biggest single step you can take to being able to build arbitrarily useful real-world apps that are no longer toys.

when you already know how to compile and run single-module interactive console programs, it takes about a day to understand basics of Cabal, and about a week to learn about input parsing and output formatting. Do you need CLI args? Use optparse-applicative. Env vars? Use envy. JSON? Use aeson. Don't think about performance and/or API conventions, that's not what you should be concerned of at this point, as you are just learning to compose things together from indivdual parts.