[saityi]

I think it goes back to the neverending quest to find ways of representing computation that allows of ease of composition, changing implementation details, eliminating classes of errors by construction, etc. Monads have had some success in this arena, but they have notable issues with composition; monad transformers help, but can become unwieldy in their own ways.

An alternative are effects, hypothetically allowing for ease in building programs as separate but composeable components which can then be freely mixed in or swapped out. In practice I have found working with effect systems in Haskell via libraries stresses the type system so much you end up with scoped type variables and type applications everywhere. My understanding is that the theory behind using effects to structure computations comes from category theory's Lawvere theories (see e.g. Pretnar's 2010 dissertation on https://github.com/yallop/effects-bibliography). Lawvere theories give rise to many monads (see Bartosz Milewski's article on it -- https://bartoszmilewski.com/2017/08/26/lawvere-theories/), but with nicer compositional properties.

This is where languages like Effekt, Eff, Frank, and Koka come in -- by writing the entire language and type system to support the theories, a lot of the pain of expressing it in Haskell can be avoided.