Build on older versions of GHC

[Gabriella439]

CI only tests GHC 8.4, so it missed the fact that Dhall.Util does not build
on GHC 8.0 or GHC 8.2 due to missing a Data.Monoid import, which this fixes.

I also plan to separately fix CI to test building on GHC 8.0 to prevent this from
recurring

[jneira]

Would be possible to test ghc 7.10.3? Eta is built with this version...

[Gabriella439]

@jneira: I just tried this yesterday and the problem I ran into is that at least one of Dhall's transitive dependencies (basement) only builds on GHC 8.0 or newer. I think that probably the more reliable way to verify this is to build with Eta directly if we get etaPackages working because that's a more accurate check than building with GHC 7.10.

[jneira]

mmm that would be nice too, but i am afraid that etlas are built with ghc 7.10.3 and now it has dhall as transitive dependency.
Eta itself is 7.10.3 but it has some extensions from ghc >= 8 and base <= 4.11.1. It would be possible then that eta could build dhall but ghc 7.10 could not build dhall for etlas. 🤔
So far we have been able to build etlas with dhall using the stack-lts-6 file in dhall-haskell, that uses basement-0.0.6.

[Gabriella439]

@jneira: I see what you mean now. Yeah, in that case, I'll try to get it to build against ghc-7.10 then and pin basement to an older version