Mixed
I found it excellent at writing difficult TemplateHaskell and Generics implementations, such as oneshotting Postgres JsonPath queries derived from lens-like field accessors on Haskell types
nh2 on Hacker News
In the thread “Writing Parquet files using Haskell”, replying to this comment
Helped: Pinpointing difficult async exception bugs and writing complex TemplateHaskell/Generics.
Got in the way: Writing general high-quality engineering code with scope errors.
… But most of my experience is from Opus >= 4.8. I found it excellent at writing difficult TemplateHaskell and Generics implementations, such as oneshotting Postgres JsonPath queries derived from lens-like field accessors on Haskell types. Note it's still not great at writing general high-quality "engineering" stuff, e.g. …