I write Haskell professionally and I can tell you that Opus 4.5 can do a great writing industrial Haskell code consistent with an existing code base.

I don't think it is capable of writing galaxy brain Haskell libraries, it absolutely missed the forest for the trees, but if you have an existing code base with consistent patterns it can emulate then it can do a surprisingly good job.

Here is an example side project I have done extremely heavily with Claude: https://github.com/solomon-b/kpbj.fm

I built a library (without Claude) that wraps in an opinionated way Servant and a handful of other common libraries used to build Haskell web-apps and then have let Claude use that to build this site. There is absolutely some hairy code and I have done a ton of manual refactors on what Claude produces, but claude has been highly effective for me here.