Simon Peyton Jones authored Feb 05, 2024 and Marge Bot committed Mar 18, 2025

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`

* We use a new TcS mode, TcSFullySolve, when simplifying the Wanteds
  that arise from the specialise expression. The mechanism is explained
  in Note [TcSFullySolve] in GHC.Tc.Solver.Monad. The reason why we need
  to do this is explained in Note [Fully solving constraints for specialisation]
  in GHC.Tc.Gen.Sig.

* 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,