That's perfectly fine Haskell, but I wouldn't say that using Applicative is much of a Lean idiom. Especially in the mathlib, people prefer using custom notation

That's perfectly fine Haskell, but I wouldn't say that using Applicative is much of a Lean idiom. Especially in the mathlib, people prefer using custom notation that looks like math notation or using the underlying functions directly rather than by using generic point-free notations. For example, you'll see `⨆ i, s i` for a supremum across the values of `s`, vs a point-free version.