[freekh]

Anecdoctally this happened to me the other day in my Haskell backend: had a recursive function (in the IO monad) that every ten second would take a connection of my connection pool (and since it was recursive) would not give it back to the pool. Of course it was a stupid bug, but it was hard to figure out because it would take a while before it got to that point and once it was there every call to the backend would block then timeout. I like functional programming, and yes: it was in the IO monad so not pure, but I think most applications ends up in some stateful monad even if your stack is built for functional stuff... This was 'not at scale' but it as a Haskell project it is fairly big. Point being: yes, pure apps will not havr this predicament, but even with a stack built for pureness I can see it happen quite often. Running software is messy...

[Jeff_Brown]

This makes another argument for Haskell, because the language allows you to isolate the non-pure code into a thin top layer. That makes debugging much easier than it would be if the complexity of IO extended all the way down through your app's subfunctions.