Simon Peyton Jones authored Nov 09, 2022 and Simon Peyton Jones committed Nov 11, 2022

This big patch addresses the rats-nest of issues that have plagued
us for years, about the relationship between Type and Constraint.
See #11715/#21623.

The main payload of the patch is:
* To introduce CONSTRAINT :: RuntimeRep -> Type
* To make TYPE and CONSTRAINT distinct throughout the compiler

Two overview Notes in GHC.Builtin.Types.Prim

* Note [TYPE and CONSTRAINT]

* Note [Type and Constraint are not apart]
  This is the main complication.

The specifics

* New primitive types (GHC.Builtin.Types.Prim)
  - CONSTRAINT
  - ctArrowTyCon (=>)
  - tcArrowTyCon (-=>)
  - ccArrowTyCon (==>)
  - funTyCon     FUN     -- Not new
  See Note [Function type constructors and FunTy]
  and Note [TYPE and CONSTRAINT]

* GHC.Builtin.Types:
  - New type Constraint = CONSTRAINT LiftedRep
  - I also stopped nonEmptyTyCon being built-in; it only needs to be wired-in

* Exploit the fact that Type and Constraint are distinct throughout GHC
  - Get rid of tcView in favour of coreView.
  - Many tcXX functions become XX functions.
    e.g. tcGetCastedTyVar --> getCastedTyVar

* Kill off Note [ForAllTy and typechecker equality], in (old)
  GHC.Tc.Solver.Canonical.  It said that typechecker-equality should ignore
  the specified/inferred distinction when comparein two ForAllTys.  But
  that wsa only weakly supported and (worse) implies that we need a separate
  typechecker equality, different from core equality. No no no.

* GHC.Core.TyCon: kill off FunTyCon in data TyCon.  There was no need for it,
  and anyway now we have four of them!

* GHC.Core.TyCo.Rep: add two FunTyFlags to FunCo
  See Note [FunCo] in that module.

* GHC.Core.Type.  Lots and lots of changes driven by adding CONSTRAINT.
  The key new function is sORTKind_maybe; most other changes are built
  on top of that.

  See also `funTyConAppTy_maybe` and `tyConAppFun_maybe`.

* Fix a longstanding bug in GHC.Core.Type.typeKind, and Core Lint, in
  kinding ForAllTys.  See new tules (FORALL1) and (FORALL2) in GHC.Core.Type.
  (The bug was that before (forall (cv::t1 ~# t2). blah), where
  blah::TYPE IntRep, would get kind (TYPE IntRep), but it should be
  (TYPE LiftedRep).  See Note [Kinding rules for types] in GHC.Core.Type.

* GHC.Core.TyCo.Compare is a new module in which we do eqType and cmpType.
  Of course, no tcEqType any more.

* GHC.Core.TyCo.FVs. I moved some free-var-like function into this module:
  tyConsOfType, visVarsOfType, and occCheckExpand.  Refactoring only.

* GHC.Builtin.Types.  Compiletely re-engineer boxingDataCon_maybe to
  have one for each /RuntimeRep/, rather than one for each /Type/.
  This dramatically widens the range of types we can auto-box.
  See Note [Boxing constructors] in GHC.Builtin.Types
  The boxing types themselves are declared in library ghc-prim:GHC.Types.

  GHC.Core.Make.  Re-engineer the treatment of "big" tuples (mkBigCoreVarTup
  etc) GHC.Core.Make, so that it auto-boxes unboxed values and (crucially)
  types of kind Constraint. That allows the desugaring for arrows to work;
  it gathers up free variables (including dictionaries) into tuples.
  See  Note [Big tuples] in GHC.Core.Make.

  There is still work to do here: #22336. But things are better than
  before.

* GHC.Core.Make.  We need two absent-error Ids, aBSENT_ERROR_ID for types of
  kind Type, and aBSENT_CONSTRAINT_ERROR_ID for vaues of kind Constraint.
  Ditto noInlineId vs noInlieConstraintId in GHC.Types.Id.Make;
  see Note [inlineId magic].

* GHC.Core.TyCo.Rep. Completely refactor the NthCo coercion.  It is now called
  SelCo, and its fields are much more descriptive than the single Int we used to
  have.  A great improvement.  See Note [SelCo] in GHC.Core.TyCo.Rep.

* GHC.Core.RoughMap.roughMatchTyConName.  Collapse TYPE and CONSTRAINT to
  a single TyCon, so that the rough-map does not distinguish them.

* GHC.Core.DataCon
  - Mainly just improve documentation

* Some significant renamings:
  GHC.Core.Multiplicity: Many -->  ManyTy (easier to grep for)
                         One  -->  OneTy
  GHC.Core.TyCo.Rep TyCoBinder      -->   GHC.Core.Var.PiTyBinder
  GHC.Core.Var      TyCoVarBinder   -->   ForAllTyBinder
                    AnonArgFlag     -->   FunTyFlag
                    ArgFlag         -->   ForAllTyFlag
  GHC.Core.TyCon    TyConTyCoBinder --> TyConPiTyBinder
  Many functions are renamed in consequence
  e.g. isinvisibleArgFlag becomes isInvisibleForAllTyFlag, etc

* I refactored FunTyFlag (was AnonArgFlag) into a simple, flat data type
    data FunTyFlag
      = FTF_T_T           -- (->)  Type -> Type
      | FTF_T_C           -- (-=>) Type -> Constraint
      | FTF_C_T           -- (=>)  Constraint -> Type
      | FTF_C_C           -- (==>) Constraint -> Constraint

* GHC.Tc.Errors.Ppr.  Some significant refactoring in the TypeEqMisMatch case
  of pprMismatchMsg.

* I made the tyConUnique field of TyCon strict, because I
  saw code with lots of silly eval's.  That revealed that
  GHC.Settings.Constants.mAX_SUM_SIZE can only be 63, because
  we pack the sum tag into a 6-bit field.  (Lurking bug squashed.)

Fixes
* #21530

Updates haddock submodule slightly.

Performance changes
~~~~~~~~~~~~~~~~~~~
I was worried that compile times would get worse, but after
some careful profiling we are down to a geometric mean 0.1%
increase in allocation (in perf/compiler).  That seems fine.

There is a big runtime improvement in T10359

Metric Decrease:
    LargeRecord
    MultiLayerModulesTH_OneShot
    T13386
    T13719
Metric Increase:
    T8095