Friends As you know * We are trying to use "Trees That Growhttps://gitlab.haskell.org/ghc/ghc/wikis/implementing-trees-that-grow" (TTG) to move HsSyn towards a situation in which GHC is merely a client of a generic HsSyn data type that can be used by clients other than GHC. * One sticking point has been the question of attaching source locations. We used to have a "ping-pong" style, in which very node is decorated with a source location, but that's a bit more awkward in TTG. * This wiki pagehttps://gitlab.haskell.org/ghc/ghc/wikis/implementing-trees-that-grow/handli... outlines some choices, while ticket #15495https://gitlab.haskell.org/ghc/ghc/issues/15495 has a lot of discussion. * HEAD embodies Solution A. But it has the disadvantage that the type system doesn't enforce locations to be present at all. That has undesirable consequences (eg ticket #17330https://gitlab.haskell.org/ghc/ghc/issues/17330) * The proposal is to move to Solution D on that page; you can see how it plays out in MR !1970https://gitlab.haskell.org/ghc/ghc/merge_requests/1970. * (I think Solutions B and C are non-starters by comparison.) If you care, please check out the design and the MR. We can change later, of course, but doing so changes a lot of code, including client code, so we'd prefer not to. Let's try to converge by the end of the week. Thanks Simon