I worked in a Haskell shop for a while. What I found was perhaps not precisely that they skewed smarter, but they definitely skewed more experienced. There were quite a few engineers who had been at it for two or more decades and very few whose first language was modern JavaScript. They were all
very happy to be working in Haskell, so the morale benefit was substantial.
This did not, perhaps, always result in better code. It did at times result in an excessive faith in the power of types to solve all problems easily and neatly. This sometimes got awkward, such as when the team had to learn what XSS was and how to prevent it.
