mtl-style-example VS eff

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.

You can swap-out implementations for testing, avoid the crazy N^2 instances issues, etc. They're pretty cool. Currently there are many competing libraries. polysemy and eff both have good examples on their homepages.

Effect systems and Algebraic effects. ocaml has just released a stripped-down effect system. People are also working on Effect systems for Haskell (eff, fused-effects, effet). Koka is a language built with effects first and foremost and it’s rapidly gaining popularity. Unison also has effects.

There are also a few subtle issues that arise with delconts related to semantics of higher order effects (see here and here), but they might be solvable.

Details are in https://github.com/hasura/eff/issues/12 and https://www.reddit.com/r/haskell/comments/pywuqg/unresolved_challenges_of_scoped_effects_and_what/.

You'll get fearless parallel with STM in the mixture, and GHC is getting a work in progress effect system for Haskell, as Delimited continuation primops has been merged.

My last example is algebraic effects, some of which have been made possible in a both practical and efficient way thanks to extremely recent research, and that I can use to implement architectures like Ports and Adapters or Clean Architecture and have very maintainable code. (Extensible Effects — An Alternative to Monad Transformers was published in 2013, Effect Handlers in Scope was published in 2014 and they are behind Polysemy, while there is ongoing work on effects with even better performance, like Eff)

cleff's Eff monad is esentially implemented as a ReaderT IO. [...] This is first done by eff, [...]

Here is link number 1 - Previous text "eff"

Languages such as Koka only support algebraic effects, not scoping operations such as catch and listen. The Effect Handlers in Scope paper introduces scoping operations, which lead to the Haskell libraries fused-effects and polysemy, but they turned out to have some weird semantics. eff is her effort to fix that.

There are a few languages that have algebraic effect systems, most notably Haskell, but these systems are implemented as libraries, not baked into the language (which can have advantages and disadvantages).