Simon Peyton Jones pushed to branch wip/substitution-via-mapper at Glasgow Haskell Compiler / GHC

Commits:

• 966da29b
  by Simon Peyton Jones at 2026-10-09T15:44:32+01:00

  Remove the Eq and Ord instances for ForAllTyFlag
  
  In Note [Comparing visibility] in GHC.Core.TyCo.Compare we read that
  "we always compare ForAllTyFlags with `eqForAllVis` or `cmpForAllVis`"
  But alas there was an Eq and Ord instance for ForAllTyFlag, which made it
  impossible to check this claim.   So I made these changes:
  
  * Move the definition of ForAllTyFlag and its related functions from
    L.H.S.Specificity (where it did not belong, since L.H.S didn't use it)
    into GHC.Types.Var.
  
  * Remove the Eq and Ord instances for ForAllTyFlag
  
  * Deal with the fallout, by using `eqForAllVis` or related functions.
  
  * In interface files we use IfaceType as a key for a de-deduplication map
    See Note [Deduplication during iface binary serialisation]
    in GHC.Iface.Binary.  For this we need /exact/ comparison, so we use a
    newtype wrapper for IfaceForAllTyFlag, and give use `cmpForAllTyFlagExactly`
    to implement its Ord instance.  Tiresome but easy.
  
  * Rename `toIfaceForAllBndrs` to `toIfaceForAllTyBndrs`
  

• 18828859
  by Simon Peyton Jones at 2026-10-09T15:44:32+01:00

  Refactor type and coercion substitution
  
  This MR refactors type and coercion substitution so that they use
  TyCoMapper, rather than hand-rolled code.  Fundamentally it's a simple
  idea, but I had to do quite a bit of refactoring to make it fly.
  
  I got Claude to help me this this refactoring, but I paid careful
  attention to every line of code.
  
  Main changes:
  
  * New module GHC.Core.TyCo.Make which defines TyCoMapper, and a bunch of other
    simple "smart constructors" for types and coercions.  See
    GHC.Core.TyCo.Rep Note [The Type-related module hierarchy], esp (TRM1)
  
  * The new TyCoMapper has new fields `tcm_tcapp_ty` and `tcm_tcapp_co` to handle
    type-constructor applications.  These replace `tcm_tycon`.
  
  * Rewrite GHC.Core.TyCo.Subst to use TyCoMapper
  
  * Rewrite GHC.Core.Type.expandTypeSynonyms to use TyCoMapper.
  
  * Implement `seqType` and `seqCo` with TyCoFolder. To to this I had to add
    a `tcf_lit` field to TyCoFolder.
  
  * A new module GHC.Utils.StrictIdentity to define a strict identity monad,
    which itself is used for
        substTyCoMapper :: TyCoMapper Subst StrictIdentity
  
  * I removed an apparently-unused Binary instance for Levity
  
  Refactoring `seq` functions
  
  * In GHC.Core.Seq, redefine `seqOneShot`, `seqRuleInfo`, and friends to return
    `SeqResult`, a new type defined in GHC.Types.Basic, rather than `()`.  We
    can define a Monoid instance for `SeqResult` that does the right thing.
  
  * This has knock-on effects across the compiler.
  

58 changed files:

• compiler/GHC/Builtin/WiredIn/Prim.hs
• compiler/GHC/Core/Coercion.hs
• compiler/GHC/Core/Coercion.hs-boot
• compiler/GHC/Core/Coercion/Opt.hs
• compiler/GHC/Core/DataCon.hs
• compiler/GHC/Core/Lint.hs
• compiler/GHC/Core/Map/Type.hs
• compiler/GHC/Core/Opt/Arity.hs
• compiler/GHC/Core/Reduction.hs
• compiler/GHC/Core/Seq.hs
• compiler/GHC/Core/TyCo/Compare.hs
• compiler/GHC/Core/TyCo/FVs.hs
• + compiler/GHC/Core/TyCo/Make.hs
• + compiler/GHC/Core/TyCo/Make.hs-boot
• compiler/GHC/Core/TyCo/Ppr.hs
• compiler/GHC/Core/TyCo/Rep.hs
• compiler/GHC/Core/TyCo/Rep.hs-boot
• compiler/GHC/Core/TyCo/Subst.hs
• compiler/GHC/Core/Type.hs
• compiler/GHC/Core/Type.hs-boot
• compiler/GHC/Core/Unify.hs
• compiler/GHC/CoreToIface.hs
• compiler/GHC/CoreToIface.hs-boot
• compiler/GHC/Driver/Main/Compile.hs
• compiler/GHC/Iface/Decl.hs
• compiler/GHC/Iface/Ext/Types.hs
• compiler/GHC/Iface/Ext/Utils.hs
• compiler/GHC/Iface/Syntax.hs
• compiler/GHC/Iface/Type.hs
• compiler/GHC/Iface/Type.hs-boot
• compiler/GHC/IfaceToCore.hs
• compiler/GHC/Stg/EnforceEpt/TagSig.hs
• compiler/GHC/Tc/Errors/Ppr.hs
• compiler/GHC/Tc/Gen/App.hs
• compiler/GHC/Tc/TyCl.hs
• compiler/GHC/Tc/TyCl/Instance.hs
• compiler/GHC/Tc/Utils/Unify.hs
• compiler/GHC/Tc/Zonk/TcType.hs
• compiler/GHC/Tc/Zonk/Type.hs
• compiler/GHC/Types/Arity.hs
• compiler/GHC/Types/Basic.hs
• compiler/GHC/Types/Cpr.hs
• compiler/GHC/Types/Demand.hs
• compiler/GHC/Types/Name/Env.hs
• compiler/GHC/Types/Name/Occurrence.hs
• compiler/GHC/Types/Unique/FM.hs
• compiler/GHC/Types/Unique/Map.hs
• compiler/GHC/Types/Unique/Set.hs
• compiler/GHC/Types/Var.hs
• compiler/GHC/Types/Var.hs-boot
• compiler/GHC/Types/Var/Set.hs
• compiler/GHC/Utils/Binary.hs
• compiler/GHC/Utils/Outputable.hs
• + compiler/GHC/Utils/StrictIdentity.hs
• compiler/Language/Haskell/Syntax/Specificity.hs
• compiler/ghc.cabal.in
• testsuite/tests/count-deps/CountDepsParser.stdout
• utils/haddock/haddock-api/src/Haddock/Backends/Hyperlinker/Utils.hs

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