Use an Ordered Set for Project

[DanBurton]

Addresses #664

[Gabriella439]

Looks great! Just one nitpick about naming 🙂

[Gabriella439]

Would it be possible to call this module either Dhall.Set or Dhall.OrderedSet (and similarly rename the type to either Set or OrderedSet)? The main reason is that I try to avoid one-letter abbreviations when naming types and modules. I would slightly prefer Dhall.Set for consistency with Dhall.Map (the module for ordered Maps) and also because then if you wanted the single character name you could import Dhall.Set as O and use O.Set or whatever prefix you prefer.

[Gabriella439]

Oh, one more thing: we should probably add a formatting test to tests/Format.hs to verify that formatting now preserves order for field projection

[DanBurton]

No problem, I'll see if I can get around to making those changes this weekend.

[DanBurton]

Whilst attempting the rename, I'm encountering a baffling error with the mutually recursive modules Dhall.Core and Dhall.Pretty.Internal. The latter claims to not know about Expr constructors supplied by the former. (See the failed hydra job; I'm getting the same errors locally but pushed the commit to see if it was specific to my build process. It's not.) I'm not sure how to proceed.

[ocharles]

I'm not entirely sure why Dhall.Pretty.Internal is doing {-# SOURCE #-} import of Dhall.Core, but that doesn't seem necessary. Does it work if you remove that pragma? I actually can't see how it could ever have worked, as the .hs-boot doesn't have the constructors!

[DanBurton]

@ocharles ah, yes it does! And that makes sense. Only one of the two needs to do the SOURCE import, to break the cycle. I'm not sure why it worked in the first place, but I did find it odd that the most innocuous, unrelated changes seemed to cause it to break.

[Gabriella439]

@DanBurton: Thank you! 🙂