Hi Simon, As #18018 says, you need to use head.hackage to reproduce it. This page [1] contains a rather short tutorial on how to use it, but Ben is currently drafting a more detailed tutorial on its use here [2]. This is what I use whenever building any Hackage library with GHC HEAD. Ryan S. ----- [1] https://ghc.gitlab.haskell.org/head.hackage/ [2] https://gitlab.haskell.org/ghc/homepage/-/merge_requests/29