• Simon Peyton Jones's avatar
    Make a smart mkAppTyM · 68278382
    Simon Peyton Jones authored
    This patch finally delivers on Trac #15952.  Specifically
    * Completely remove Note [The tcType invariant], along with
      its complicated consequences (IT1-IT6).
    * Replace Note [The well-kinded type invariant] with:
          Note [The Purely Kinded Type Invariant (PKTI)]
    * Instead, establish the (PKTI) in TcHsType.tcInferApps,
      by using a new function mkAppTyM when building a type
      application.  See Note [mkAppTyM].
    * As a result we can remove the delicate mkNakedXX functions
      entirely.  Specifically, mkNakedCastTy retained lots of
      extremly delicate Refl coercions which just cluttered
      everything up, and(worse) were very vulnerable to being
      silently eliminated by (say) substTy. This led to a
      succession of bug reports.
    The result is noticeably simpler to explain, simpler
    to code, and Richard and I are much more confident that
    it is correct.
    It does not actually fix any bugs, but it brings us closer.
    E.g. I hoped it'd fix #15918 and #15799, but it doesn't quite
    do so.  However, it makes it much easier to fix.
    I also did a raft of other minor refactorings:
    * Use tcTypeKind consistently in the type checker
    * Rename tcInstTyBinders to tcInvisibleTyBinders,
      and refactor it a bit
    * Refactor tcEqType, pickyEqType, tcEqTypeVis
      Simpler, probably more efficient.
    * Make zonkTcType zonk TcTyCons, at least if they have
      any free unification variables -- see zonk_tc_tycon
      in TcMType.zonkTcTypeMapper.
      Not zonking these TcTyCons was actually a bug before.
    * Simplify try_to_reduce_no_cache in TcFlatten (a lot)
    * Combine checkExpectedKind and checkExpectedKindX.
      And then combine the invisible-binder instantation code
      Much simpler now.
    * Fix a little bug in TcMType.skolemiseQuantifiedTyVar.
      I'm not sure how I came across this originally.
    * Fix a little bug in TyCoRep.isUnliftedRuntimeRep
      (the ASSERT was over-zealous).  Again I'm not certain
      how I encountered this.
    * Add a missing solveLocalEqualities in
      I came across this when trying to get level numbers
Code owners : Simon Peyton Jones