I didn't mean to imply that nobody else but PL researchers can use Haskell, clearly a lot of people do. What my post intended to convey was that the focus of the language seems a lot more focused on research than (say) Ruby or Javascript. This is not strange, given its history as a designed-by-committee language "to serve as a basis for future research in functional-language design". (see https://en.wikipedia.org/wiki/Haskell_(programming_language)... )

The point in your last sentence that "every now and then someone comes up with an abstraction which turns out to be incredibly useful in practice" is exactly what I meant in the OP: these abstractions seem to be developed way more often in Haskell first and then they leak to other programming languages later than the other way around.