Re: Handling source locations in HsSyn via TTG

I care about this, and I maintain my viewpoint described in https://mail.haskell.org/pipermail/ghc-devs/2019-February/017080.html I’m willing to implement this. As to merge request !1970, it isn’t good to special-case GhcPass in a closed type family, making other tools second-class citizens. Let’s say I have `MyToolPass`, how would I write an instance of `WrapL` for it? - Vlad

On 28 Oct 2019, at 12:31, Simon Peyton Jones via ghc-devs wrote:

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

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