[Git][ghc/ghc][wip/T18126] 5 commits: Implement Quick Look impredicativity

[Ben Gamari]

Ben Gamari pushed to branch wip/T18126 at Glasgow Haskell Compiler / GHC


Commits:
2351f490 by Simon Peyton Jones at 2020-09-14T12:41:08-04:00
Implement Quick Look impredicativity

This patch implements Quick Look impredicativity (#18126), sticking
very closely to the design in
    A quick look at impredicativity, Serrano et al, ICFP 2020

The main change is that a big chunk of GHC.Tc.Gen.Expr has been
extracted to two new modules
    GHC.Tc.Gen.App
    GHC.Tc.Gen.Head
which deal with typechecking n-ary applications, and the head of
such applications, respectively.  Both contain a good deal of
documentation.

Three other loosely-related changes are in this patch:

* I implemented (partly by accident) points (2,3)) of the accepted GHC
  proposal "Clean up printing of foralls", namely
  https://github.com/ghc-proposals/ghc-proposals/blob/
        master/proposals/0179-printing-foralls.rst
  (see #16320).

  In particular, see Note [TcRnExprMode] in GHC.Tc.Module
  - :type instantiates /inferred/, but not /specified/, quantifiers
  - :type +d instantiates /all/ quantifiers
  - :type +v is killed off

  That completes the implementation of the proposal,
  since point (1) was done in
    commit df08468113ab46832b7ac0a7311b608d1b418c4d
    Author: Krzysztof Gogolewski <krzysztof.gogolewski at tweag.io>
    Date:   Mon Feb 3 21:17:11 2020 +0100
    Always display inferred variables using braces

* HsRecFld (which the renamer introduces for record field selectors),
  is now preserved by the typechecker, rather than being rewritten
  back to HsVar.  This is more uniform, and turned out to be more
  convenient in the new scheme of things.

* The GHCi debugger uses a non-standard unification that allows the
  unification variables to unify with polytypes.  We used to hack
  this by using ImpredicativeTypes, but that doesn't work anymore
  so I introduces RuntimeUnkTv.  See Note [RuntimeUnkTv] in
  GHC.Runtime.Heap.Inspect

WARNING: this patch won't validate on its own.  It was too
hard to fully disentangle it from the following patch, on
type errors and kind generalisation.

Changes to tests

* Fixes #9730 (test added)

* Fixes #7026 (test added)

* Fixes most of #8808, except function `g2'` which uses a
  section (which doesn't play with QL yet -- see #18126)
  Test added

* Fixes #1330. NB Church1.hs subsumes Church2.hs, which is now deleted

* Fixes #17332 (test added)

* Fixes #4295

* This patch makes typecheck/should_run/T7861 fail.
  But that turns out to be a pre-existing bug: #18467.
  So I have just made T7861 into expect_broken(18467)

- - - - -
d297b1b5 by Simon Peyton Jones at 2020-09-14T12:41:09-04:00
Improve kind generalisation, error messages

This patch does two things:

* It refactors GHC.Tc.Errors a bit.  In debugging Quick Look I was
  forced to look in detail at error messages, and ended up doing a bit
  of refactoring, esp in mkTyVarEqErr'.  It's still quite a mess, but
  a bit better, I think.

* It makes a significant improvement to the kind checking of type and
  class declarations. Specifically, we now ensure that if kind
  checking fails with an unsolved constraint, all the skolems are in
  scope.  That wasn't the case before, which led to some obscure error
  messages; and occasional failures with "no skolem info" (eg #16245).

Both of these, and the main Quick Look patch itself, affect a /lot/ of
error messages, as you can see from the number of files changed.  I've
checked them all; I think they are as good or better than before.

Smaller things

* I documented the various instances of VarBndr better.
  See Note [The VarBndr tyep and its uses] in GHC.Types.Var

* Renamed GHC.Tc.Solver.simpl_top to simplifyTopWanteds

* A bit of refactoring in bindExplicitTKTele, to avoid the
  footwork with Either.  Simpler now.

* Move promoteTyVar from GHC.Tc.Solver to GHC.Tc.Utils.TcMType

Fixes #16245 (comment 211369), memorialised as
  typecheck/polykinds/T16245a
Also fixes the three bugs in #18640

- - - - -
6dd91ef1 by Simon Peyton Jones at 2020-09-14T12:41:09-04:00
wibbles ql

- - - - -
fda9dc7f by Simon Peyton Jones at 2020-09-14T12:41:09-04:00
wibbles generalisation patch

- - - - -
d2a6cc9a by Ben Gamari at 2020-09-14T12:41:09-04:00
Use UniqSet for FieldLabelString instead of Data.Set

FieldLabelString, which is a FastString, no longer has an Ord instance.

- - - - -


19 changed files:

- compiler/GHC/Core/TyCon.hs
- compiler/GHC/Hs/Expr.hs
- compiler/GHC/Hs/Pat.hs
- compiler/GHC/Hs/Type.hs
- compiler/GHC/HsToCore/Coverage.hs
- compiler/GHC/HsToCore/Expr.hs
- compiler/GHC/Iface/Ext/Ast.hs
- compiler/GHC/Parser/PostProcess.hs
- compiler/GHC/Rename/Pat.hs
- compiler/GHC/Runtime/Eval.hs
- compiler/GHC/Runtime/Heap/Inspect.hs
- compiler/GHC/Tc/Deriv/Generate.hs
- compiler/GHC/Tc/Errors.hs
- compiler/GHC/Tc/Errors/Hole.hs
- + compiler/GHC/Tc/Gen/App.hs
- compiler/GHC/Tc/Gen/Arrow.hs
- compiler/GHC/Tc/Gen/Default.hs
- compiler/GHC/Tc/Gen/Expr.hs
- compiler/GHC/Tc/Gen/Expr.hs-boot


The diff was not included because it is too large.


View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/92e20da38f8a802470f6f172835c214ec78e7216...d2a6cc9a067d7570221e319e523b11de969287ac

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/92e20da38f8a802470f6f172835c214ec78e7216...d2a6cc9a067d7570221e319e523b11de969287ac
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