I've once worked on a large and old Scala-cats codebase (i.e. Haskel-style, the whole program as an IO monad). I'd say we (the team) hardly introduced any bugs.

I've once worked on a large and old Scala-cats codebase (i.e. Haskel-style, the whole program as an IO monad). I'd say we (the team) hardly introduced any bugs. The language made making whole classes of errors less likely, and the team's disciplined approach handled the rest.