This is a heads up that D3609[1], the first step toward implementing Trees that Grow[2] for the hsSyn AST is about to land in master. The key change is that it is replacing the HsSyn type parameter with one that is an index into a set of type families, enabling the extension points (for a future diff). Any current work in progress branches will need a one-time search and replace of all type parameters as follows RdrName -> GhcPs Name -> GhcRs Var -> GhcTc Id -> GhcTcId Any code that makes use of the GHC API and needs to compile with earlier versions too can do something like #if MIN_VERSION_ghc(8,3,0) type ParseI = GhcPs type RenameI = GhcRn type TypecheckI = GhcTc #else type ParseI = RdrName type RenameI = Name type TypecheckI = Var #endif and then replace the RdrName / Name / Var type params with ParseI / RenameI / TypecheckI. A real world example is in ghc-exactprint[3] Regards Alan [1] https://phabricator.haskell.org/D3609 [2] https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow [3] https://github.com/alanz/ghc-exactprint/commit/6a6e3cdeb40f9623e9a7fc6f1be90...