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Switch from template-haskell to template-haskell-lift #1162

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@TeofilC TeofilC commented Oct 10, 2025 •
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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

@TeofilC TeofilC force-pushed the wip/th-lift branch 2 times, most recently from 4ccd3a1 to ad2e782 Compare October 16, 2025 22:14
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treeowl commented Oct 16, 2025

Why doesn't the new package support older GHC versions? The CPP seems a bit blecherous.

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TeofilC commented Oct 16, 2025 •
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template-haskell-lift 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 containers to older GHCs as a boot lib (they would build fine from Hackage though).

Let me know what you'd prefer.
Either we can:

  • raise 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

Eventually though, template-haskell-lift should let us remove all the TH CPP since that should all in that package.

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treeowl commented Oct 16, 2025

That's only a potential issue for new releases on old GHC branches, right? I suppose it would be okay to have some CPP temporarily to support that as long as such branches are "supported" by GHC HQ. But wouldn't it be cleaner to just add template-haskell-lift as a boot package in those releases?

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TeofilC commented Oct 17, 2025

Yeah I agree that would be optimal. Unfortunately I don't think we can add new boot libs in minor releases (but I'll ask) since that can be disruptive.

I can commit to removing this CPP when it's no longer necessary though.

@TeofilC TeofilC force-pushed the wip/th-lift branch 2 times, most recently from 150bcfc to cb380cd Compare January 10, 2026 20:50
@TeofilC TeofilC marked this pull request as ready for review January 10, 2026 20:51
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TeofilC commented Jan 10, 2026

Now that GHC-9.14 is released we can test this.

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treeowl commented Jan 10, 2026

Any idea what's going wrong with MucroHs? It smells like a CPP formatting issue, but I can't find it.

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TeofilC commented Jan 10, 2026

Any idea what's going wrong with MucroHs? It smells like a CPP formatting issue, but I can't find it.

I think that it doesn't like the elif syntax in the .cabal file? I'm not sure though

@TeofilC TeofilC mentioned this pull request Jan 10, 2026
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The MicroHs version was updated in #1177. Hopefully the CI will pass after a rebase.

containers-tests/tests/seq-properties.hs Outdated
import Control.Monad.Fix (MonadFix (..))
import Test.Tasty.HUnit
import Test.ChasingBottoms.IsBottom (isBottom)
# if __GLASGOW_HASKELL__ >= 914
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@meooow25 meooow25 Jan 25, 2026

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This is minor, but let's not have spaces after #. I think it would be less surprising (I'm surprised to learn that this is acceptable syntax).

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I agree it doesn't belong here, but in nested conditions it's very helpful to see what's going on visually.

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@TeofilC TeofilC Jan 26, 2026

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Good spot. That was a copy-paste error

containers/src/Data/IntMap/Internal.hs Outdated
# if __GLASGOW_HASKELL__ >= 914
import Language.Haskell.TH.Lift (Lift(..))
# else
import Language.Haskell.TH.Syntax (Lift(..))
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@meooow25 meooow25 Jan 25, 2026

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Why the change from Lift to Lift(..)? Same for other files.

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@TeofilC TeofilC Jan 26, 2026

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I don't think it's needed yeah.

containers/src/Data/Set/Internal.hs Outdated
import Language.Haskell.TH ()
import Data.Coerce (coerce)
# endif
import Data.Coerce
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Why change this line?

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@TeofilC TeofilC Jan 26, 2026

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Typo. Fixed now

containers/src/Data/IntMap/Internal.hs
# else
import Language.Haskell.TH.Syntax (Lift(..))
-- See Note [ Template Haskell Dependencies ]
import Language.Haskell.TH ()
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So we don't need this any more if we depend on template-haskell-lift? The current reason is documented in note-template-haskell-deps.md

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@TeofilC TeofilC Jan 26, 2026

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Yep in fact we haven't needed it since GHC 9.12, I think, when these things moved to ghc-internal from template-haskell

@TeofilC
Use template-haskell-lift for GHC>=9.14
4206889 
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
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