| Commit message (Collapse) | Author | Age | Files | Lines |
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When implementing Quick Look I'd failed to remember that overloaded
labels, like #foo, should be treated as a "head", so that they can be
instantiated with Visible Type Application. This caused #19154.
A very similar ticket covers overloaded literals: #19167.
This patch fixes both problems, but (annoyingly, albeit temporarily)
in two different ways.
Overloaded labels
I dealt with overloaded labels by buying fully into the
Rebindable Syntax approach described in GHC.Hs.Expr
Note [Rebindable syntax and HsExpansion].
There is a good overview in GHC.Rename.Expr
Note [Handling overloaded and rebindable constructs].
That module contains much of the payload for this patch.
Specifically:
* Overloaded labels are expanded in the renamer, fixing #19154.
See Note [Overloaded labels] in GHC.Rename.Expr.
* Left and right sections used to have special code paths in the
typechecker and desugarer. Now we just expand them in the
renamer. This is harder than it sounds. See GHC.Rename.Expr
Note [Left and right sections].
* Infix operator applications are expanded in the typechecker,
specifically in GHC.Tc.Gen.App.splitHsApps. See
Note [Desugar OpApp in the typechecker] in that module
* ExplicitLists are expanded in the renamer, when (and only when)
OverloadedLists is on.
* HsIf is expanded in the renamer when (and only when) RebindableSyntax
is on. Reason: the coverage checker treats HsIf specially. Maybe
we could instead expand it unconditionally, and fix up the coverage
checker, but I did not attempt that.
Overloaded literals
Overloaded literals, like numbers (3, 4.2) and strings with
OverloadedStrings, were not working correctly with explicit type
applications (see #19167). Ideally I'd also expand them in the
renamer, like the stuff above, but I drew back on that because they
can occur in HsPat as well, and I did not want to to do the HsExpanded
thing for patterns.
But they *can* now be the "head" of an application in the typechecker,
and hence something like ("foo" @T) works now. See
GHC.Tc.Gen.Head.tcInferOverLit. It's also done a bit more elegantly,
rather than by constructing a new HsExpr and re-invoking the
typechecker. There is some refactoring around tcShortCutLit.
Ultimately there is more to do here, following the Rebindable Syntax
story.
There are a lot of knock-on effects:
* HsOverLabel and ExplicitList no longer need funny (Maybe SyntaxExpr)
fields to support rebindable syntax -- good!
* HsOverLabel, OpApp, SectionL, SectionR all become impossible in the
output of the typecheker, GhcTc; so we set their extension fields to
Void. See GHC.Hs.Expr Note [Constructor cannot occur]
* Template Haskell quotes for HsExpanded is a bit tricky. See
Note [Quotation and rebindable syntax] in GHC.HsToCore.Quote.
* In GHC.HsToCore.Match.viewLExprEq, which groups equal HsExprs for the
purpose of pattern-match overlap checking, I found that dictionary
evidence for the same type could have two different names. Easily
fixed by comparing types not names.
* I did quite a bit of annoying fiddling around in GHC.Tc.Gen.Head and
GHC.Tc.Gen.App to get error message locations and contexts right,
esp in splitHsApps, and the HsExprArg type. Tiresome and not very
illuminating. But at least the tricky, higher order, Rebuilder
function is gone.
* Some refactoring in GHC.Tc.Utils.Monad around contexts and locations
for rebindable syntax.
* Incidentally fixes #19346, because we now print renamed, rather than
typechecked, syntax in error mesages about applications.
The commit removes the vestigial module GHC.Builtin.RebindableNames,
and thus triggers a 2.4% metric decrease for test MultiLayerModules
(#19293).
Metric Decrease:
MultiLayerModules
T12545
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This commit introduces a DecoratedSDoc type which replaces the old
ErrDoc, and hopefully better reflects the intent.
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Updates Haddock submodule
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This commit boldly removes the ErrDoc and the MsgDoc from the codebase.
The former was introduced with the only purpose of classifying errors
according to their importance, but a similar result can be obtained just
by having a simple [SDoc], and placing bullets after each of them.
On top of that I have taken the perhaps controversial decision to also
banish MsgDoc, as it was merely a type alias over an SDoc and as such it wasn't
offering any extra type safety. Granted, it was perhaps making type
signatures slightly more "focused", but at the expense of cognitive
burden: if it's really just an SDoc, let's call it with its proper name.
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Provoked by #19074, this patch makes GHC.Core.PatSyn.PatSyn
immutable, by recording only the *Name* of the matcher and
builder rather than (as currently) the *Id*.
See Note [Keep Ids out of PatSyn] in GHC.Core.PatSyn.
Updates haddock submodule.
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Related to a future change in Data.List,
https://downloads.haskell.org/ghc/8.10.3/docs/html/users_guide/using-warnings.html?highlight=wcompat#ghc-flag--Wcompat-unqualified-imports
Companion pull&merge requests:
- https://github.com/judah/haskeline/pull/153
- https://github.com/haskell/containers/pull/762
- https://gitlab.haskell.org/ghc/packages/hpc/-/merge_requests/9
After these the actual change in Data.List should be easy to do.
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----------------
What:
There are two splits.
The first spit is:
- `Language.Haskell.Syntax.Extension`
- `GHC.Hs.Extension`
where the former now just contains helpers like `NoExtCon` and all the
families, and the latter is everything having to do with `GhcPass`.
The second split is:
- `Language.Haskell.Syntax.<mod>`
- `GHC.Hs.<mod>`
Where the former contains all the data definitions, and the few helpers
that don't use `GhcPass`, and the latter contains everything else. The
second modules also reexport the former.
----------------
Why:
See the issue for more details, but in short answer is we're trying to
grasp at the modularity TTG is supposed to offer, after a long time of
mainly just getting the safety benefits of more complete pattern
matching on the AST.
Now, we have an AST datatype which, without `GhcPass` is decently
stripped of GHC-specific concerns. Whereas before, not was it
GHC-specific, it was aware of all the GHC phases despite the
parameterization, with the instances and parametric data structure
side-by-side.
For what it's worth there are also some smaller, imminent benefits:
- The latter change also splits a strongly connected component in two,
since none of the `Language.Haskell.Syntax.*` modules import the older
ones.
- A few TTG violations (Using GhcPass directly in the AST) in `Expr` are
now more explicitly accounted for with new type families to provide the
necessary indirection.
-----------------
Future work:
- I don't see why all the type families should live in
`Language.Haskell.Syntax.Extension`. That seems anti-modular for
little benefit. All the ones used just once can be moved next to the
AST type they serve as an extension point for.
- Decide what to do with the `Outputable` instances. Some of these are
no orphans because they referred to `GhcPass`, and had to be moved. I
think the types could be generalized so they don't refer to `GhcPass`
and therefore can be moved back, but having gotten flak for increasing
the size and complexity types when generalizing before, I did *not*
want to do this.
- We should triage the remaining contents of `GHC.Hs.<mod>`. The
renaming helpers are somewhat odd for needing `GhcPass`. We might
consider if they are a) in fact only needed by one phase b) can be
generalized to be non-GhcPass-specific (e.g. take a callback rather
than GADT-match with `IsPass`) and then they can live in
`Language.Haskell.Syntax.<mod>`.
For more details, see
https://gitlab.haskell.org/ghc/ghc/-/wikis/implementing-trees-that-grow
Bumps Haddock submodule
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This commit paves the way to a richer and more structured representation
of GHC error messages, as per GHC proposal #306. More specifically
'Messages' from 'GHC.Types.Error' now gains an extra type parameter,
that we instantiate to 'ErrDoc' for now. Later, this will allow us to
replace ErrDoc with something more structure (for example messages
coming from the parser, the typechecker etc).
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This commit removes the errShortString field from the ErrMsg type,
allowing us to cleanup a lot of dynflag-dependent error functions, and
move them in a more specialised 'GHC.Driver.Errors' closer to the
driver, where they are actually used.
Metric Increase:
T4801
T9961
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This patch significantly refactors key renamer datastructures (primarily Avail
and GlobalRdrElt) in order to treat DuplicateRecordFields in a more robust way.
In particular it allows the extension to be used with pattern synonyms (fixes
where mangled record selector names could be printed instead of field labels
(e.g. with -Wpartial-fields or hole fits, see new tests).
The key idea is the introduction of a new type GreName for names that may
represent either normal entities or field labels. This is then used in
GlobalRdrElt and AvailInfo, in place of the old way of representing fields
using FldParent (yuck) and an extra list in AvailTC.
Updates the haddock submodule.
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Fix #19082, #17045
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This commit renames parser's Error and Warning types (and their
constructors) to have a 'Ps' prefix, so that this would play nicely
when more errors and warnings for other phases of the pipeline will
be added. This will make more explicit which is the particular type
of error and warning we are dealing with, and will be more informative
for users to see in the generated Haddock.
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The haddock submodule is also updated so that it understands the changes
to patterns.
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Move code unrelated to runtime evaluation out of GHC.Runtime.Eval
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This refactors the GHC AST to remove `HsImplicitBndrs` and replace it with
`HsOuterTyVarBndrs`, a type which records whether the outermost quantification
in a type is explicit (i.e., with an outermost, invisible `forall`) or
implicit. As a result of this refactoring, it is now evident in the AST where
the `forall`-or-nothing rule applies: it's all the places that use
`HsOuterTyVarBndrs`. See the revamped `Note [forall-or-nothing rule]` in
`GHC.Hs.Type` (previously in `GHC.Rename.HsType`).
Moreover, the places where `ScopedTypeVariables` brings lexically scoped type
variables into scope are a subset of the places that adhere to the
`forall`-or-nothing rule, so this also makes places that interact with
`ScopedTypeVariables` easier to find. See the revamped
`Note [Lexically scoped type variables]` in `GHC.Hs.Type` (previously in
`GHC.Tc.Gen.Sig`).
`HsOuterTyVarBndrs` are used in type signatures (see `HsOuterSigTyVarBndrs`)
and type family equations (see `HsOuterFamEqnTyVarBndrs`). The main difference
between the former and the latter is that the former cares about specificity
but the latter does not.
There are a number of knock-on consequences:
* There is now a dedicated `HsSigType` type, which is the combination of
`HsOuterSigTyVarBndrs` and `HsType`. `LHsSigType` is now an alias for an
`XRec` of `HsSigType`.
* Working out the details led us to a substantial refactoring of
the handling of explicit (user-written) and implicit type-variable
bindings in `GHC.Tc.Gen.HsType`.
Instead of a confusing family of higher order functions, we now
have a local data type, `SkolemInfo`, that controls how these
binders are kind-checked.
It remains very fiddly, not fully satisfying. But it's better
than it was.
Fixes #16762. Bumps the Haddock submodule.
Co-authored-by: Simon Peyton Jones <simonpj@microsoft.com>
Co-authored-by: Richard Eisenberg <rae@richarde.dev>
Co-authored-by: Zubin Duggal <zubin@cmi.ac.in>
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This disallows `a %001 -> b`, and makes sure the type literal is
printed from its SourceText so it is clear why.
Closes #18888
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Haskell98 and GADT constructors both use `HsConDeclDetails`, which includes
`InfixCon`. But `InfixCon` is never used for GADT constructors, which results
in an awkward unrepresentable state. This removes the unrepresentable state by:
* Renaming the existing `HsConDeclDetails` synonym to `HsConDeclH98Details`,
which emphasizes the fact that it is now only used for Haskell98-style data
constructors, and
* Creating a new `HsConDeclGADTDetails` data type with `PrefixConGADT` and
`RecConGADT` constructors that closely resemble `PrefixCon` and `InfixCon`
in `HsConDeclH98Details`. The key difference is that `HsConDeclGADTDetails`
lacks any way to represent infix constructors.
The rest of the patch is refactoring to accommodate the new structure of
`HsConDecl{H98,GADT}Details`. Some highlights:
* The `getConArgs` and `hsConDeclArgTys` functions have been removed, as
there is no way to implement these functions uniformly for all
`ConDecl`s. For the most part, their previous call sites now
pattern match on the `ConDecl`s directly and do different things for
`ConDeclH98`s and `ConDeclGADT`s.
I did introduce one new function to make the transition easier:
`getRecConArgs_maybe`, which extracts the arguments from a `RecCon(GADT)`.
This is still possible since `RecCon(GADT)`s still use the same representation
in both `HsConDeclH98Details` and `HsConDeclGADTDetails`, and since the
pattern that `getRecConArgs_maybe` implements is used in several places,
I thought it worthwhile to factor it out into its own function.
* Previously, the `con_args` fields in `ConDeclH98` and `ConDeclGADT` were
both of type `HsConDeclDetails`. Now, the former is of type
`HsConDeclH98Details`, and the latter is of type `HsConDeclGADTDetails`,
which are distinct types. As a result, I had to rename the `con_args` field
in `ConDeclGADT` to `con_g_args` to make it typecheck.
A consequence of all this is that the `con_args` field is now partial, so
using `con_args` as a top-level field selector is dangerous. (Indeed, Haddock
was using `con_args` at the top-level, which caused it to crash at runtime
before I noticed what was wrong!) I decided to add a disclaimer in the 9.2.1
release notes to advertise this pitfall.
Fixes #18844. Bumps the `haddock` submodule.
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While, say, alternating "he" and "she" in sequential writing
may be nicer than always using "they", reading code/documentation
is almost never sequential. If this small change makes individuals
feel more welcome in GHC's codebase, that's a good thing.
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I was working on making DynFlags stateless (#17957), especially by
storing loaded plugins into HscEnv instead of DynFlags. It turned out to
be complicated because HscEnv is in GHC.Driver.Types but LoadedPlugin
isn't: it is in GHC.Driver.Plugins which depends on GHC.Driver.Types. I
didn't feel like introducing yet another hs-boot file to break the loop.
Additionally I remember that while we introduced the module hierarchy
(#13009) we talked about splitting GHC.Driver.Types because it contained
various unrelated types and functions, but we never executed. I didn't
feel like making GHC.Driver.Types bigger with more unrelated Plugins
related types, so finally I bit the bullet and split GHC.Driver.Types.
As a consequence this patch moves a lot of things. I've tried to put
them into appropriate modules but nothing is set in stone.
Several other things moved to avoid loops.
* Removed Binary instances from GHC.Utils.Binary for random compiler
things
* Moved Typeable Binary instances into GHC.Utils.Binary.Typeable: they
import a lot of things that users of GHC.Utils.Binary don't want to
depend on.
* put everything related to Units/Modules under GHC.Unit:
GHC.Unit.Finder, GHC.Unit.Module.{ModGuts,ModIface,Deps,etc.}
* Created several modules under GHC.Types: GHC.Types.Fixity, SourceText,
etc.
* Split GHC.Utils.Error (into GHC.Types.Error)
* Finally removed GHC.Driver.Types
Note that this patch doesn't put loaded plugins into HscEnv. It's left
for another patch.
Bump haddock submodule
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For the case
foo :: a %p -> b
The location of the '%' is captured, separate from the 'p'
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The linear arrow can be parsed as `%1 ->` or a direct single token unicode
equivalent.
Make sure that this distinction is captured in the parsed AST by using
IsUnicodeSyntax where it appears, and introduce a new API Annotation,
AnnMult to represent its location when unicode is not used.
Updated haddock submodule
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This patch implements two related warnings:
-Woperator-whitespace-ext-conflict
warns on uses of infix operators that would be parsed
differently were a particular GHC extension enabled
-Woperator-whitespace
warns on prefix, suffix, and tight infix uses of infix
operators
Updates submodules: haddock, containers.
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The HomeUnitId is only used by the Cmm parser and this one has access to
the DynFlags, so it can grab the UnitId of the HomeUnit from them.
Bump haddock submodule
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A cleanup in 7f418acf61e accidentally discarded some parens in
ConDeclGADT.
Make sure these stay in the AST in a usable format.
Also ensure the AnnLolly does not get lost in a GADT.
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When linear types are disabled, HsUnrestrictedArrow is treated as
HslinearArrow.
Move this adjustment into the type checking phase, so that the parsed
source accurately represents the source as parsed.
Closes #18791
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Haskell and Cmm parsers/lexers now report errors and warnings using ADTs
defined in GHC.Parser.Errors. They can be printed using functions in
GHC.Parser.Errors.Ppr.
Some of the errors provide hints with a separate ADT (e.g. to suggest to
turn on some extension). For now, however, hints are not consistent
across all messages. For example some errors contain the hints in the
main message. I didn't want to change any message with this patch. I
expect these changes to be discussed and implemented later.
Surprisingly, this patch enhances performance. On CI
(x86_64/deb9/hadrian, ghc/alloc):
parsing001 -11.5%
T13719 -2.7%
MultiLayerModules -3.5%
Naperian -3.1%
Bump haddock submodule
Metric Decrease:
MultiLayerModules
Naperian
T13719
parsing001
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Bump haddock submodule
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Implements GHC Proposal #356
Updates the haddock submodule.
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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@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
Updates haddock submodule.
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)
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This switches `deriv_clause_tys` so that instead of using a list of
`LHsSigType`s to represent the types in a `deriving` clause, it now
uses a sum type. `DctSingle` represents a `deriving` clause with no
enclosing parentheses, while `DctMulti` represents a clause with
enclosing parentheses. This makes pretty-printing easier and avoids
confusion between `HsParTy` and the enclosing parentheses in
`deriving` clauses, which are different semantically.
Fixes #18662.
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@alanz pointed out on ghc-devs that the payload of this pragma does
not appear to be used anywhere.
I (@bgamari) did some digging and traced the pragma's addition back to
d386e0d2 (way back in 2006!).
It appears that it was intended to be used by code generators for use
in informing the code coveraging checker about generated code
provenance. When it was added it used the pragma's "payload" fields as
source location information to build an "ExternalBox". However, it
looks like this was dropped a year later in 55a5d8d9. At this point
it seems like the pragma serves no useful purpose.
Given that it also is not documented, I think we should remove it.
Updates haddock submodule
Closes #18639
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Since Backpack the "home unit" is much more involved than what it was
before (just an identifier obtained with `-this-unit-id`). Now it is
used in conjunction with `-component-id` and `-instantiated-with` to
configure module instantiations and to detect if we are type-checking an
indefinite unit or compiling a definite one.
This patch introduces a new HomeUnit datatype which is much easier to
understand. Moreover to make GHC support several packages in the same
instances, we will need to handle several HomeUnits so having a
dedicated (documented) type is helpful.
Finally in #14335 we will also need to handle the case where we have no
HomeUnit at all because we are only loading existing interfaces for
plugins which live in a different space compared to units used to
produce target code. Several functions will have to be refactored to
accept "Maybe HomeUnit" parameters instead of implicitly querying the
HomeUnit fields in DynFlags. Having a dedicated type will make this
easier.
Bump haddock submodule
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As suggested in #18545.
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- put panic related functions into GHC.Utils.Panic
- put trace related functions using DynFlags in GHC.Driver.Ppr
One step closer making Outputable fully independent of DynFlags.
Bump haddock submodule
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And prefix ~
(cherry picked from commit 8dbee2c578b1f642d45561be3f416119863e01eb)
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Remove unused ApiAnns, add one for linear arrow.
Include API Annotations for trailing comma in export list.
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This patch started as a small documentation change, an attempt to make
Note [Parser-Validator] and Note [Ambiguous syntactic categories]
more clear and up-to-date.
But it turned out that runECP_P/runECP_PV are weakly motivated,
and it's easier to remove them than to find a good rationale/explanation
for their existence.
As the result, there's a bit of refactoring in addition to
a documentation update.
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Before this patch, we parsed types into a reversed sequence
of operators and operands. For example, (F x y + G a b * X)
would be parsed as [X, *, b, a, G, +, y, x, F],
using a simple grammar:
tyapps
: tyapp
| tyapps tyapp
tyapp
: atype
| PREFIX_AT atype
| tyop
| unpackedness
Then we used a hand-written state machine to assemble this
either into a type, using 'mergeOps',
or into a constructor, using 'mergeDataCon'.
This is due to a syntactic ambiguity:
data T1 a = MkT1 a
data T2 a = Ord a => MkT2 a
In T1, what follows after the = sign is a data/newtype constructor
declaration. However, in T2, what follows is a type (of kind
Constraint). We don't know which of the two we are parsing until we
encounter =>, and we cannot check for => without unlimited lookahead.
This poses a few issues when it comes to e.g. infix operators:
data I1 = Int :+ Bool :+ Char -- bad
data I2 = Int :+ Bool :+ Char => MkI2 -- fine
By this issue alone we are forced into parsing into an intermediate
representation and doing a separate validation pass.
However, should that intermediate representation be as low-level as a
flat sequence of operators and operands?
Before GHC Proposal #229, the answer was Yes, due to some particularly
nasty corner cases:
data T = ! A :+ ! B -- used to be fine, hard to parse
data T = ! A :+ ! B => MkT -- bad
However, now the answer is No, as this corner case is gone:
data T = ! A :+ ! B -- bad
data T = ! A :+ ! B => MkT -- bad
This means we can write a proper grammar for types, overloading it in
the DisambECP style, see Note [Ambiguous syntactic categories].
With this patch, we introduce a new class, DisambTD. Just like
DisambECP is used to disambiguate between expressions, commands, and patterns,
DisambTD is used to disambiguate between types and data/newtype constructors.
This way, we get a proper, declarative grammar for constructors and
types:
infixtype
: ftype
| ftype tyop infixtype
| unpackedness infixtype
ftype
: atype
| tyop
| ftype tyarg
| ftype PREFIX_AT tyarg
tyarg
: atype
| unpackedness atype
And having a grammar for types means we are a step closer to using a
single grammar for types and expressions.
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This removes the `ConDeclGADTPrefixPs` per the discussion in #18517.
Most of this patch simply removes code, although the code in the
`rnConDecl` case for `ConDeclGADTPrefixPs` had to be moved around a
bit:
* The nested `forall`s check now lives in the `rnConDecl` case for
`ConDeclGADT`.
* The `LinearTypes`-specific code that used to live in the
`rnConDecl` case for `ConDeclGADTPrefixPs` now lives in
`GHC.Parser.PostProcess.mkGadtDecl`, which is now monadic so that
it can check if `-XLinearTypes` is enabled.
Fixes #18157.
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* Create a dedicated production for type operators
* Create a dedicated type for the UNPACK pragma
* Remove an outdated part of Note [Parsing data constructors is hard]
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Before this patch, NegativeLiterals used to parse x-1 as x (-1).
This may not be what the user expects, and now it is fixed:
x-1 is parsed as (-) x 1.
We achieve this by the following requirement:
* When lexing a negative literal,
it must not be preceded by a 'closing token'.
This also applies to unboxed literals, e.g. -1#.
See GHC Proposal #229 for the definition of a closing token.
A nice consequence of this change is that -XNegativeLiterals becomes a
subset of -XLexicalNegation. In other words, enabling both of those
extensions has the same effect as enabling -XLexicalNegation alone.
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Previously it was in ghc-boot so that ghc-pkg could use it. However it
wasn't necessary because ghc-pkg only uses a subset of it: reading
target arch and OS from the settings file. This is now done via
GHC.Platform.ArchOS (was called PlatformMini before).
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