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<h3 style="margin-top: 20px; margin-bottom: 10px;">
Simon Peyton Jones pushed to branch wip/T24359 at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong style="font-weight: 600;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/372955161d56dd61ca88da7da02dc1d4648016b3">37295516</a></strong>
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<span> by Simon Peyton Jones </span> <i> at 2024-12-23T14:08:56+00:00 </i>
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<pre class="commit-message" style='white-space: pre-wrap; display: block; font-size: 14px; color: #28272d; position: relative; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; border-radius: 2px; margin: 0; padding: 8px 12px; border: 1px solid #dcdcde;'>Specialising expressions -- at last
This MR addresses #24359, which implements the GHC proposal 493 on SPECIALISE pragmas.
* The old code path (using SpecSig and SpecPrag) still exists.
* The new code path (using SpecSigE and SpecPragE) runs alongside it.
* All SPECIALISE pragmas are routed through the new code path, except
if you give multiple type sigs, when the old code path is still used.
* Main documentation: Note [Handling new-form SPECIALISE pragmas] in
GHC.Tc.Gen.Sig`
Thanks to @sheaf for helping with this MR.
The Big Thing is to introduce
{-# SPECIALISE forall x. f @Int x True #-}
where you can give type arguments and value argument to specialise; and
you can quantify them with forall, just as in Rules.
I thought it was going to be pretty simple, but it was a Long, Long Saga.
Highlights
* Overview Note [Handling new-form SPECIALISE pragmas] in GHC.Tc.Gen.Sig
- New data constructor `SpecSigE` in data type `L.H.S.Binds.Sig`
- New data construtor `SpecPragE` in data type `GHC.Hs.Binds.TcSpecPrag`
- Renamer: uses `checkSpecESigShape` to decide which function to assocate the
SPECIALISE pragma with
- Some of the action is in `GHC.Tc.Gen.Sig.tcSpecPrag`
- The rest is in `GHC.HsToCore.Binds.dsSpec`
* All of GHC.Tc.Gen.Rule is moved into GHC.Tc.Gen.Sig, because the code is
very closely related.
* The forall'd binders for SPECIALISE are the same as those for a RULE, so I
refactored, introducing data type `L.H.S.Binds.RuleBndrs`, with functions
to rename, zonk, typecheck it. I refactored this data type a bit; nicer now.
* On the LHS of RULES, or SPECIALISE, we want to disable the tricky mechanims
described in Note [Desugaring non-canonical evidence] in GHC.HsToCore.Expr.
Previously it wasn't fully disabled (just set to the empty set), and that
didn't quite work in the new regime.
* There are knock-on changes to Template Haskell.
* For the LHS of a RULE and a SPECIALISE expression, I wanted to simplify
it /without/ inlining the let-bindings for evidence variables. I added
a flag `so_inline` to the SimpleOpt optimiser to support this. The
entry point is `GHC.Core.SimpleOpt.simpleOptExprNoInline`
* Since forever we have had a hack for type variables on the LHS of
RULES. I took the opportunity to tidy this up. The main action is
in the zonker. See GHC.Tc.Zonk.Type Note [Free tyvars on rule LHS],
and especially data construtor `SkolemiseFlexi`
in data type `GHC.Tc.Zonk.Env.ZonkFlexi`
* Move `scopedSort` from GHC.Core.TyCo.FVs to GHC.Core.Predicate
Reason: it now works for Ids as well, and I wanted to use isEvVar,
which is defined in GHC.Core.Predicate
Avoiding module loops meant that instead of exporting GHC.Core.TyCo.Tidy
from GHC.Core.Type, modules now import the former directly.
I also took the opportunity to remove unused exports
from GHC.Core.Type.hs-boot
* Flag stuff:
- Add flag `-Wdeprecated-pragmas` and use it to control the warning when
using old-style SPECIALISE pragmas with multiple type ascriptions,
- Add flag `-Wuseless-specialisations` and use it to control the warning emitted
when GHC determines that a SPECIALISE pragma would have no effect. Don't
want if the SPECIALISE is SPECIALISE INLINE (#4444)
In response to #25389, we continue to generate these seemingly code for these
seemingly useless SPECIALISE pragmas
- Adds deprecations to Template Haskell `pragSpecD` and `pracSpecInlD`,
* Split up old-style SPECIALISE pragmas in GHC.Internal.Float,
GHC.Internal.Numeric, GHC.Internal.Real
* Remove useless SPECIALISE pragmas in Data.Array (updating the array submodule)
Smaller things:
- Update the Users Guide
- Add mention of the changes to the 9.14 release notes as well as
the Template Haskell changelog,
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compiler/GHC/Builtin/Names/TH.hs
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compiler/GHC/Builtin/PrimOps/Ids.hs
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compiler/GHC/Core/FamInstEnv.hs
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compiler/GHC/Core/InstEnv.hs
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compiler/GHC/Core/Make.hs
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compiler/GHC/Core/Opt/CSE.hs
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compiler/GHC/Core/Opt/Simplify/Iteration.hs
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compiler/GHC/Core/Opt/Simplify/Utils.hs
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compiler/GHC/Core/Opt/SpecConstr.hs
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compiler/GHC/Core/Predicate.hs
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compiler/GHC/Core/Tidy.hs
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compiler/GHC/Core/TyCo/FVs.hs
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compiler/GHC/Core/TyCo/Ppr.hs
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compiler/GHC/Core/TyCo/Tidy.hs
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compiler/GHC/Core/Type.hs
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compiler/GHC/Driver/Config.hs
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compiler/GHC/Driver/Flags.hs
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compiler/GHC/Driver/Session.hs
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compiler/GHC/Hs/Binds.hs
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compiler/GHC/Hs/Decls.hs
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compiler/GHC/HsToCore.hs
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compiler/GHC/HsToCore/Binds.hs
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compiler/GHC/HsToCore/Errors/Ppr.hs
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