"Option 3 avoids that problem but risks perplexity: if I make use of some cool package which introduces some Foo :: * -> *, I might notice that Foo is a monad and add a Monad Foo instance in my own code, expecting the Applicative Foo instance to be generated in concert; to my horror, I find my code has subtle bugs because the package introduced a different, non-monadic, Applicative Foo instance which I'm accidentally using instead."
talks about "subtle bugs". Could you give an example of such a bug?
I would expect that the non-monadic Applicative Foo instance is always somehow "compatible" with the monadic one. However I don't have a clear definition of "compatible" yet...
I think it's something like that. Module Foo defines list and make ZipList-like Applicative instance. Would you add standard list monad you have a bug. But if you add monad instance which is not compatible with existing applicative you have bug whether you use extension or not. module Foo where data [a] = a : [a] | [] instance Functor [] where fmap = map instamce Applicative [] where pure = repeat (<*>) = zipWith ($) module Main where instance Monad [] where return x = [x] (>>=) = concatMap