Can someone give an elevator pitch of extensible effects? Monad transformers are very useful and rather easy once you get used to them, although not without their downsides. What advantages do extensible effects have?
Extensible effects are easier to compose in arbitrary ways. If you have a monad transformer stack you can only have each type of transformer once in each stack before you have to start using lift . lift . lift to get to the inner layers as the type class functions will only get you to the outermost instance.
