TH Lift instances for time types
kokobd opened this issue · comments
Can we provide instances of Language.Haskell.TH.Syntax.Lift for types in time, like UTCTime, DiffTime, etc, so that time can be used in TH splices.
If there aren't major blockers (should time depend on template-haskell?), I'll be happy to submit a PR.
Probably better to put them in the template-haskell package? I'm not sure, though.
My gut instinct is that template-haskell is a domain-independent library, and so it seems weirder to for users of template-haskell to depend on time than vice-versa. template-haskell seems pretty locked to specific GHC releases, whereas adding new instances to time should be a bit easier to publish? (Sadly, th-compat will be unusable here due to dep footprint, so we'll probably have to CPP it like time-qq does.)
Can we use DeriveLift?
https://github.com/brendanhay/amazonka/pull/677/files
Looks like it, for everything except DiffTime, which does not export its constructor.
OK, checking with the CLC in case there's a problem with time depending on template-haskell.
I think the dependency is OK. You can make a PR, however, there may not be instance Lift Fixed, which should be added to template-haskell.
I've raised an issue in template-haskell: https://gitlab.haskell.org/ghc/ghc/-/issues/22953.
I think we can first release a version without relying on instance Lift Fixed. Instead, manually implement Lift for DiffTime using diffTimeToPicoseconds. This also makes time compatible with older versions of template-haskell.
The primary problem with lifting DiffTime is that it doesn't export its contstructor. The way Amazonka works around it is to call fromEnum, lift that, then call toEnum. Seems to work fine.