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Summary:
Make HsLit and OverLitVal have original source strings, for source to
source conversions using the GHC API

This is part of the ongoing AST Annotations work, as captured in
https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations and
https://ghc.haskell.org/trac/ghc/ticket/9628#comment:28

The motivations for the literals is as follows

```lang=haskell
x,y :: Int
x = 0003
y = 0x04

s :: String
s = "\x20"

c :: Char
c = '\x20'

d :: Double
d = 0.00

blah = x
  where
    charH = '\x41'#
    intH = 0004#
    wordH = 005##
    floatH = 3.20#
    doubleH = 04.16##
    x = 1
```

Test Plan: ./sh validate

Reviewers: simonpj, austin

Reviewed By: simonpj, austin

Subscribers: thomie, goldfire, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D412

GHC Trac Issues: #9628

This adds the module `Data.Void` (formerly provided by Edward Kmett's `void`
package) to `base`.

The original Haskell98 compatible implementation has been modified to use
modern GHC features (among others this makes use of `EmptyCase` as
motivated by #2431), and `vacuousM` was dropped since it's redundant now
with the AMP in place.  Instances for classes not part of `base` had to be
dropped as well.

TODO: Documentation could be improved

Reviewed By: ekmett, austin

Differential Revision: https://phabricator.haskell.org/D506

Summary:
This patch exports functions for finding the active package
databases and their locations from the Packages module. This
allows GHC API clients to use other tools, like Cabal, to gather
package information that's not directly available from the
binary package db.

Reviewers: duncan, austin

Reviewed By: austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D514

Summary: ghci unset could not reverse language extensions.

Reviewers: hvr, thomie, austin

Reviewed By: hvr, thomie, austin

Subscribers: goldfire, hvr, thomie, carter

Differential Revision: https://phabricator.haskell.org/D516

GHC Trac Issues: #9293

Summary:
The final design and discussion is captured at
https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations

This is a proof of concept implementation of a completely
separate annotation structure, populated in the parser,and tied to the
AST by means of a virtual "node-key" comprising the surrounding
SrcSpan and a value derived from the specific constructor used for the
node.

The key parts of the design are the following.

== The Annotations ==

In `hsSyn/ApiAnnotation.hs`

```lang=haskell
type ApiAnns = (Map.Map ApiAnnKey SrcSpan, Map.Map SrcSpan [Located Token])

type ApiAnnKey = (SrcSpan,AnnKeywordId)

-- ---------------------------------------------------------------------

-- | Retrieve an annotation based on the @SrcSpan@ of the annotated AST
-- element, and the known type of the annotation.
getAnnotation :: ApiAnns -> SrcSpan -> AnnKeywordId -> Maybe SrcSpan
getAnnotation (anns,_) span ann = Map.lookup (span,ann) anns

-- |Retrieve the comments allocated to the current @SrcSpan@
getAnnotationComments :: ApiAnns -> SrcSpan -> [Located Token]
getAnnotationComments (_,anns) span =
  case Map.lookup span anns of
    Just cs -> cs
    Nothing -> []

-- | Note: in general the names of these are taken from the
-- corresponding token, unless otherwise noted
data AnnKeywordId
         = AnnAs
         | AnnBang
         | AnnClass
         | AnnClose -- ^ } or ] or ) or #) etc
         | AnnComma
         | AnnDarrow
         | AnnData
         | AnnDcolon
         ....
```

== Capturing in the lexer/parser ==

The annotations are captured in the lexer / parser by extending PState to include a field

In `parser/Lexer.x`

```lang=haskell
data PState = PState {
        ....
        annotations :: [(ApiAnnKey,SrcSpan)]
        -- Annotations giving the locations of 'noise' tokens in the
        -- source, so that users of the GHC API can do source to
        -- source conversions.
     }
```

The lexer exposes a helper function to add an annotation

```lang=haskell
addAnnotation :: SrcSpan -> Ann -> SrcSpan -> P ()
addAnnotation l a v = P $ \s -> POk s {
  annotations = ((AK l a), v) : annotations s
  } ()

```

The parser also has some helper functions of the form

```lang=haskell
type MaybeAnn = Maybe (SrcSpan -> P ())

gl = getLoc
gj x = Just (gl x)

ams :: Located a -> [MaybeAnn] -> P (Located a)
ams a@(L l _) bs = (mapM_ (\a -> a l) $ catMaybes bs) >> return a
```

This allows annotations to be captured in the parser by means of

```
ctypedoc :: { LHsType RdrName }
        : 'forall' tv_bndrs '.' ctypedoc {% hintExplicitForall (getLoc $1) >>
                                            ams (LL $ mkExplicitHsForAllTy $2 (noLoc []) $4)
                                                [mj AnnForall $1,mj AnnDot $3] }
        | context '=>' ctypedoc         {% ams (LL $ mkQualifiedHsForAllTy   $1 $3)
                                               [mj AnnDarrow $2] }
        | ipvar '::' type               {% ams (LL (HsIParamTy (unLoc $1) $3))
                                               [mj AnnDcolon $2] }
        | typedoc                       { $1 }
```

== Parse result ==

```lang-haskell
data HsParsedModule = HsParsedModule {
    hpm_module    :: Located (HsModule RdrName),
    hpm_src_files :: [FilePath],
       -- ^ extra source files (e.g. from #includes).  The lexer collects
       -- these from '# <file> <line>' pragmas, which the C preprocessor
       -- leaves behind.  These files and their timestamps are stored in
       -- the .hi file, so that we can force recompilation if any of
       -- them change (#3589)
    hpm_annotations :: ApiAnns
  }

-- | The result of successful parsing.
data ParsedModule =
  ParsedModule { pm_mod_summary   :: ModSummary
               , pm_parsed_source :: ParsedSource
               , pm_extra_src_files :: [FilePath]
               , pm_annotations :: ApiAnns }
```

This diff depends on D426

Test Plan: sh ./validate

Reviewers: austin, simonpj, Mikolaj

Reviewed By: simonpj, Mikolaj

Subscribers: Mikolaj, goldfire, thomie, carter

Differential Revision: https://phabricator.haskell.org/D438

GHC Trac Issues: #9628

Summary:
AST changes to prepare for API annotations

Add locations to parts of the AST so that API annotations can
then be added.

The outline of the whole process is captured here
https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations

This change updates the haddock submodule.

Test Plan: sh ./validate

Reviewers: austin, simonpj, Mikolaj

Reviewed By: simonpj, Mikolaj

Subscribers: thomie, goldfire, carter

Differential Revision: https://phabricator.haskell.org/D426

GHC Trac Issues: #9628

Unfortunately, splice patterns in brackets still do not work
because we don't run splices in brackets. Without running a pattern
splice, we can't know what variables it binds, so we're stuck.

This is still a substantial improvement, and it may be the best
we can do. Still must document new behavior.

Test case: th/T1476b.

This commit also refactors a bunch of lexeme-oriented code into
a new module Lexeme, and includes a submodule update for haddock.

Dot-dot record-wildcard notation is simply illegal for constructors
without any named fields, but that was neither documented nor checked.
This patch does so

- Make the check in RnPat
- Add test T9815
- Fix CmmLayoutStack which was using the illegal form (!)
- Document in user manual

Test T2239 actually succeeds without impredicativity, because of the new co/contra subsumption check

I can't say I really understand this DLL-split thing, but the build fails
if I don't remove TcMType here.

I've put this in a commit on its own, but it's a knock-on effect of the
immediately preceding wave of typechecker changes

We weren't promoting enough type variables, with unpredictable consequences.
The new code is, if anything, simpler.

This has to be done by the full constraint solver anyway, and it's
rare, so there's really no point in doing it twice.  This change
just deletes some (tricky) code.

This was being doing independently in two places. Now it's done in one
place, TcType.canUnifyWithPolyType

This is a pretty big patch, but which substantially iproves the subsumption
check.  Trac #9569 was the presenting example, showing how type inference could
depend rather delicately on eta expansion.  But there are other less exotic
examples; see Note [Co/contra-variance of subsumption checking] in TcUnify.

The driving change is to TcUnify.tcSubType.  But also

* HsWrapper gets a new constructor WpFun, which behaves very like CoFun:
       if     wrap1 :: exp_arg <= act_arg
              wrap2 :: act_res <= exp_res
       then   WpFun wrap1 wrap2 : (act_arg -> arg_res) <= (exp_arg -> exp_res)

* I generalised TcExp.tcApp to call tcSubType on the result,
  rather than tcUnifyType.  I think this just makes it consistent
  with everything else, notably tcWrapResult.

As usual I ended up doing some follow-on refactoring

* AmbigOrigin is gone (in favour of TypeEqOrigin)
* Combined BindPatSigCtxt and PatSigCxt into one
* Improved a bit of error message generation

When a pattern synonym is for an unlifted pattern, its "builder" would
naturally be a top-level unlifted binding, which isn't allowed.  So we
give it an extra Void# argument.

Our Plan A involved then making *two* Ids for these builders, with
some consequential fuss in the desugarer.  This was more pain than I
liked, so I've re-jigged it.

 * There is just one builder for a pattern synonym.

 * It may have an extra Void# arg, but this decision is signalled
   by the Bool in the psBuilder field.

   I did the same for the psMatcher field.

   Both Bools are serialised into interface files, so there is
   absolutely no doubt whether that extra Void# argument is required.

 * I renamed "wrapper" to "builder".  We have too may "wrappers"

 * In order to deal with typecchecking occurrences of P in expressions,
   I refactored the tcInferId code in TcExpr.

All of this allowed me to revert 5fe872
   "Apply compulsory unfoldings during desugaring, except for `seq` which is special."
which turned out to be a rather messy hack in DsBinds

Phrases like “Currently, you can...” are going to sound strange in a few
years; I rephrased the note to sound less like a proposal and more like
an explanation.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Signed-off-by: Austin Seipp <austin@well-typed.com>

Summary: (this has been submitted on behalf on @dreixel)

Reviewers: simonpj, hvr, austin

Reviewed By: simonpj, austin

Subscribers: goldfire, thomie, carter, dreixel

Differential Revision: https://phabricator.haskell.org/D476

GHC Trac Issues: #5462

Summary: add `-fwarn-missing-exported-sigs` to only warn about missing signatures if the name is exported

Test Plan: validate, see testsuite/tests/warnings/should_compile/T2526.hs

Reviewers: ezyang, austin, thomie

Reviewed By: austin, thomie

Subscribers: ezyang, thomie, carter

Differential Revision: https://phabricator.haskell.org/D482

GHC Trac Issues: #2526

Conflicts:
	docs/users_guide/7.10.1-notes.xml

Summary: Fixes #9368

Reviewers: nomeata, hvr, ekmett, austin

Reviewed By: ekmett, austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D498

GHC Trac Issues: #9368

Summary:
The `Data.OldList` module was originally created in
3daf0023 to provide a way to access the
original list-specialised functions from `Data.List`. It was also made
an exposed module in order to facilitate adapting the
`haskell2010`/`haskell98` packages. However, since the
`haskell2010`/`haskell98` packages were dropped, we no longer need to
expose `Data.OldList`.

Depends on D511

Reviewers: ekmett, austin

Reviewed By: ekmett, austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D513