I write Haskell with Claude Code and it's got remarkably good recently. We have some code at work that uses STM to have what is essentially a mutable state machine. I needed to split a state transition apart, and it did an admirable job. I had to intervene once or twice when it was going down a valid, but undesirable approach. This almost one shot performance was already a productivity boost, but didn't quite build. What I find most impressive now is the "fix" here is to literally have Claude run the build and see the errors. While GHC errors are verbose and not always the best it got everything building in a few more iterations. When it later got a test failure, I suggested we add a bit more logging - so it logged all state transitions, and spotted the unexpected transition and got the test passing. We really are a LONG way away from 3.5 performance.