If you have a large application written in Haskell, you're probably already using some sort of abstract or extensible monad for your "business logic", and that

If you have a large application written in Haskell, you're probably already using some sort of abstract or extensible monad for your "business logic", and that means it's usually not hard (in practice) to add a MonadLogger instance to your code. Also, when you've written Haskell for long enough, you start to write your code in such a way that it's astronomically unlikely that you need to add logging to a pure function. I haven't found myself wanting to do that in years. Haskell has a library to do logging in pure code, but it's unpopular for a reason.