I don't know if this is the right forum for this, I apologise if I'm intruding... Are there any plans to use the type system to enforce safety in Core, via e.g. GADTs? This would replace much of core-lint with static checking. Conal Eliottt has done something similar in a blog post ( http://conal.net/blog/posts/overloading-lambda#:~:text=Haskell%20source%20la...) and it seems relatively straightforward. This would be especially beneficial to those working at the cutting edge of GHC features, statically ensuring their Core manipulations are correct. I would be surprised if existing compiler bugs wouldn't be found while implementing this. What would the performance impact be? would using GADTs incur extra overhead? I'd assume you'd save something by lugging around less type information in Core. Ari Fordsham