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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
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The testsuite output now contains diagnostic codes, so many tests need
to be updated at once.
We decided it was best to keep the diagnostic codes in the testsuite
output, so that contributors don't inadvertently make changes to the
diagnostic codes.
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Co-authored by: Sam Derbyshire
Previously, GHC had three flavours of constraint:
Wanted, Given, and Derived. This removes Derived constraints.
Though serving a number of purposes, the most important role
of Derived constraints was to enable better error messages.
This job has been taken over by the new RewriterSets, as explained
in Note [Wanteds rewrite wanteds] in GHC.Tc.Types.Constraint.
Other knock-on effects:
- Various new Notes as I learned about under-described bits of GHC
- A reshuffling around the AST for implicit-parameter bindings,
with better integration with TTG.
- Various improvements around fundeps. These were caused by the
fact that, previously, fundep constraints were all Derived,
and Derived constraints would get dropped. Thus, an unsolved
Derived didn't stop compilation. Without Derived, this is no
longer possible, and so we have to be considerably more careful
around fundeps.
- A nice little refactoring in GHC.Tc.Errors to center the work
on a new datatype called ErrorItem. Constraints are converted
into ErrorItems at the start of processing, and this allows for
a little preprocessing before the main classification.
- This commit also cleans up the behavior in generalisation around
functional dependencies. Now, if a variable is determined by
functional dependencies, it will not be quantified. This change
is user facing, but it should trim down GHC's strange behavior
around fundeps.
- Previously, reportWanteds did quite a bit of work, even on an empty
WantedConstraints. This commit adds a fast path.
- Now, GHC will unconditionally re-simplify constraints during
quantification. See Note [Unconditionally resimplify constraints when
quantifying], in GHC.Tc.Solver.
Close #18398.
Close #18406.
Solve the fundep-related non-confluence in #18851.
Close #19131.
Close #19137.
Close #20922.
Close #20668.
Close #19665.
-------------------------
Metric Decrease:
LargeRecord
T9872b
T9872b_defer
T9872d
TcPlugin_RewritePerf
-------------------------
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This patch simplifies GHC to use simple subsumption.
Ticket #17775
Implements GHC proposal #287
https://github.com/ghc-proposals/ghc-proposals/blob/master/
proposals/0287-simplify-subsumption.rst
All the motivation is described there; I will not repeat it here.
The implementation payload:
* tcSubType and friends become noticably simpler, because it no
longer uses eta-expansion when checking subsumption.
* No deeplyInstantiate or deeplySkolemise
That in turn means that some tests fail, by design; they can all
be fixed by eta expansion. There is a list of such changes below.
Implementing the patch led me into a variety of sticky corners, so
the patch includes several othe changes, some quite significant:
* I made String wired-in, so that
"foo" :: String rather than
"foo" :: [Char]
This improves error messages, and fixes #15679
* The pattern match checker relies on knowing about in-scope equality
constraints, andd adds them to the desugarer's environment using
addTyCsDs. But the co_fn in a FunBind was missed, and for some reason
simple-subsumption ends up with dictionaries there. So I added a
call to addTyCsDs. This is really part of #18049.
* I moved the ic_telescope field out of Implication and into
ForAllSkol instead. This is a nice win; just expresses the code
much better.
* There was a bug in GHC.Tc.TyCl.Instance.tcDataFamInstHeader.
We called checkDataKindSig inside tc_kind_sig, /before/
solveEqualities and zonking. Obviously wrong, easily fixed.
* solveLocalEqualitiesX: there was a whole mess in here, around
failing fast enough. I discovered a bad latent bug where we
could successfully kind-check a type signature, and use it,
but have unsolved constraints that could fill in coercion
holes in that signature -- aargh.
It's all explained in Note [Failure in local type signatures]
in GHC.Tc.Solver. Much better now.
* I fixed a serious bug in anonymous type holes. IN
f :: Int -> (forall a. a -> _) -> Int
that "_" should be a unification variable at the /outer/
level; it cannot be instantiated to 'a'. This was plain
wrong. New fields mode_lvl and mode_holes in TcTyMode,
and auxiliary data type GHC.Tc.Gen.HsType.HoleMode.
This fixes #16292, but makes no progress towards the more
ambitious #16082
* I got sucked into an enormous refactoring of the reporting of
equality errors in GHC.Tc.Errors, especially in
mkEqErr1
mkTyVarEqErr
misMatchMsg
misMatchMsgOrCND
In particular, the very tricky mkExpectedActualMsg function
is gone.
It took me a full day. But the result is far easier to understand.
(Still not easy!) This led to various minor improvements in error
output, and an enormous number of test-case error wibbles.
One particular point: for occurs-check errors I now just say
Can't match 'a' against '[a]'
rather than using the intimidating language of "occurs check".
* Pretty-printing AbsBinds
Tests review
* Eta expansions
T11305: one eta expansion
T12082: one eta expansion (undefined)
T13585a: one eta expansion
T3102: one eta expansion
T3692: two eta expansions (tricky)
T2239: two eta expansions
T16473: one eta
determ004: two eta expansions (undefined)
annfail06: two eta (undefined)
T17923: four eta expansions (a strange program indeed!)
tcrun035: one eta expansion
* Ambiguity check at higher rank. Now that we have simple
subsumption, a type like
f :: (forall a. Eq a => Int) -> Int
is no longer ambiguous, because we could write
g :: (forall a. Eq a => Int) -> Int
g = f
and it'd typecheck just fine. But f's type is a bit
suspicious, and we might want to consider making the
ambiguity check do a check on each sub-term. Meanwhile,
these tests are accepted, whereas they were previously
rejected as ambiguous:
T7220a
T15438
T10503
T9222
* Some more interesting error message wibbles
T13381: Fine: one error (Int ~ Exp Int)
rather than two (Int ~ Exp Int, Exp Int ~ Int)
T9834: Small change in error (improvement)
T10619: Improved
T2414: Small change, due to order of unification, fine
T2534: A very simple case in which a change of unification order
means we get tow unsolved constraints instead of one
tc211: bizarre impredicative tests; just accept this for now
Updates Cabal and haddock submodules.
Metric Increase:
T12150
T12234
T5837
haddock.base
Metric Decrease:
haddock.compiler
haddock.Cabal
haddock.base
Merge note: This appears to break the
`UnliftedNewtypesDifficultUnification` test. It has been marked as
broken in the interest of merging.
(cherry picked from commit 66b7b195cb3dce93ed5078b80bf568efae904cc5)
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