Friends

As you know

• We are trying to use “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 page outlines some choices, while ticket #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 #17330)
• The proposal is to move to Solution D on that page; you can see how it plays out in MR !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