eff has never been released to Hackage and as far as I know never used in production. I wouldn't call it "real-world code". For effect systems that people do actually use in production I suggest

* Polysemy: https://hackage.haskell.org/package/polysemy

* effectful: https://hackage.haskell.org/package/effectful

* Bluefin: https://hackage-content.haskell.org/package/bluefin/docs/Blu...

[Disclosure: Bluefin in my effect system]

Hey, those docs for Bluefin are a great introduction to the space. Very well written!