Hi! Regarding the "there can be no instance for which mapM is more efficient than traverse": There have been issues with Applicative functions leaking memory where Monad ones aren't in Polysemy - some of these have been fixed, but it's not clear that there are none left. There is also this claim in parser-combinators <https://hackage.haskell.org/package/parser-combinators-1.2.1/docs/Control-Applicative-Combinators.html> :
Due to the nature of the Applicative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Applicative> and Alternative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Alternative> abstractions, they are prone to memory leaks and not as efficient as their monadic counterparts. Although all the combinators we provide in this module are perfectly expressible in terms of Applicative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Applicative> and Alternative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Alternative>, please prefer Control.Monad.Combinators <https://hackage.haskell.org/package/parser-combinators-1.2.1/docs/Control-Monad-Combinators.html> instead when possible.
I have not verified it, but it is a bit worrying. Personally I would love to know of some kind of reasoning regarding these things, as I'm not aware of any! (efficiency of Applicative vs Monad based functions) ====== Georgi