Switch from template-haskell to template-haskell-lift

[TeofilC]

This new boot library should be more stable than template-haskell and
should eventually allow us to remove much of the CPP around TH.

It will also make it easier for end-users to reinstall template-haskell
as it will no longer be used by any boot libraries

This GHC MR tests this PR against GHC-HEAD: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/14978

[RyanGlScott]

Let me know when you'd like a full review of this. (It looks like there is still some work to be done to make this pass CI.)

[TeofilC]

@RyanGlScott in the current version I have set things up to only use this library when GHC >=9.14.

template-haskell-lift itself is compatible with GHC>=8.10. But it is only a boot library with GHC>=9.14. So you wouldn't be able to backport new releases of exceptions to older GHCs as a boot lib (they would build fine from Hackage though).

Do you have a preference?
Either we can:

• raise the minimum GHC version to 8.10, use th-lift, cannot backport to minor versions of GHC before 9.14
• Add CPP and only use th-lift when GHC is >=9.14

[RyanGlScott]

Given that exceptions is a boot library, I would like for it to only depend on boot libraries as much as possible. As such, I would prefer the "Add CPP and only use th-lift when GHC is >=9.14" option.

[RyanGlScott]

Great!

When would be a good time for merging this? My understanding is that we want this to coincide with GHC 9.14 bundling template-haskell-lift as a boot library. Has this happened yet?

[TeofilC]

Has this happened yet?

Not yet. The MR still needs to be backported to the GHC-9.14 branch.

I think it would be best to wait until 9.14.1 is released and then we can add a proper CI job to test this. And then 9.14.2 could perhaps ship with a version of exception that has this patch.

[TeofilC]

This should be ready for review now, since GHC 9.14 is out

[RyanGlScott]

Thanks, @TeofilC!

[hasufell]

This broke MicroHS support.

@augustss

---

Suggested change

	else
	elif impl(ghc)