Jon Pretty

I don't consider anything ad-hoc that has the proof of billions of lines of code in working software. Also, I am certain that there are plenty of Scala and Haskell FP purests that write horrible code that few can reason about.

I think there's a tendency for FP purists to exaggerate the brokenness of software. Most people have lower standards. But perfect purity is the only position from which anyone can claim the absence of certain classes of bugs... and they do.

And they get confidence from the fact they write thousands of lines of code, and they never see certain kinds of bug. And from their perspective, amongst those thousands of lines of code, a single impure method call will look egregious.

Are there any Scala and Haskell bugs that programmers feel confident don't exist because the compiler assured them?

As one example, if you have no mutable state, there's no chance of a race condition caused by another thread mutating some state underneath you. In Haskell, you can't mutate state, though in Scala you rely on being careful not to use any mutable state.

I don't have to worry about var state = State...() being raced in an actor because actors by definition prevent that.